Part C
Natural ecosystems do not hold exclusive rights to ecology, and ecological study is not the prerogative of those who gain pleasure from working in the grandeur of Sumatra's wild areas. Disturbed, agricultural and urban areas have biological components which interact, change in abundance, adapt to physical constraints and impinge upon human life according to the same principles described in the preceding chapters. There is no reason, for example, for people in Lampung, one of the most disturbed provinces in Indonesia, to regard ecology as an irrelevant science. Thus the Government of Indonesia now employs ecologists to work in transmigration areas.
As will be described in chapter 11, a characteristic of man-made ecosystems is their relative simplicity. This in itself should encourage those people who have been surprised at the complexity of the ecology of Sumatra's natural ecosystems to gain an understanding of how a simple ecosystem works. Valuable studies can be made of a pond, a street, a rice field or a cattle pasture. Although these may not appear at first sight to be very exciting, a short acquaintance with the areas and the first attempts to understand what eats what, when, how, in what quantities and why, are likely to produce enthusiasm to learn more and to apply the knowledge gained to increasingly complex systems.
Few of the present or planned industrial developments in Sumatra have major impacts on entirely natural ecosystems because they tend to be sited near road or rail networks and near centres of population which can provide the necessary manpower. Large-scale developments are now required by law to be preceded by an environmental impact assessment. This cannot be effectively conducted until a data base exists concerning the distribution and movements of species, their yearly (or longer) population cycles, species food and reproductive requirements, competition, predators, prey, etc. Possible impacts can then be judged against the status quo. If monitoring starts when an industrial project begins to be built, many ecological 'impacts' might be detected. However, these changes may be nothing more than normal fluctuations. The time to start some limited form of ecological monitoring is now.
Consider the efforts of the Meteorological Department. The actual recording of one day's weather details is not in itself particularly useful - no one needs to be told that his town has just experienced floods or extremely high temperatures. The value of the information derives from the data being recorded regularly. Yearly patterns can be established, and frequency of extreme records and trends can be calculated. Agriculture benefits most from such information because planting times can be geared to maximum productivity. Ecological monitoring should be seen in the same light and should be undertaken as normal practice by departments concerned with different aspects of the biological and physical environment.
Chapter Eleven
INTRODUCTION
Natural ecosystems are not static. The plants and animals within them die and others are born to replace them. Energy and nutrients pass through the organisms and are removed from the system in water, by emigradon or by 'visitors'. If part of an ecosystem is destroyed in some way, the biotic community and the physical features of the soil will generally be rebuilt. This can take decades or even a thousand years depending on the ecosystem and the extent of the disturbance. The disturbed area is colonised by pioneer species which are gradually replaced by others until an ecosystem closely resembling the original is formed which is itself subject to disturbance. This sequence of changes is called succession. In areas where repeated disturbance occurs or where the process of succession is halted, an ecosystem may never have a chance to recover.
Where plants colonise a newly formed substrate, primary succession is said to occur. This has already been described for coastal mud (p. 84) and sandy beaches (p. 115), and the primary secession of plants on the remains of Mt. Krakatau is described on page 343.
The return of an area to its natural state following major disturbances, such as tree falls and landslides on steep, unstable slopes, forms part of the natural pattern of ecosystem functioning. These disturbances are the major cause of the mosaic of gap, building and mature phases in forests described on page 193. The disturbances envisaged in this chapter are of a large scale, probably caused by man, possibly repeated and relatively unselective in their effects.
Four basic ecological principles can be formulated from the study of succession (Ricklefs 1979).
1) Succession proceeds in only one direction. That is, fast-growing, tolerant colonisers are replaced by slower-growing species with more specific requirements but with great competitive ability where those requirements are met.
2) As new species colonise an area, they will inevitably alter the environment by their presence. The new conditions are generally less suitable for their own seedlings but more suitable for those of other species; hence the succession continues.
3) The understanding of a climax community should not be too rigid. A particular 'climax' vegetation is probably just one of a continuous range of possible 'climaxes' for that area. The actual climax which is formed is determined by local climate, soil, topography, the nature and duration of the preceding disturbance and the completeness of the community of herbivores and seed-dispersing animals.
4) A climax is itself a mosaic of successional stages (p. 193).
In the following sections disturbance is discussed with reference to all the natural ecosystems discussed in previous chapters. For some, it has been difficult to find any relevant information; for others, such as lowland forest, there is a great deal known, albeit not enough. No study of the effects of disturbance to a tropical lowland forest can even be compared to the detailed work at Hubbard Brook in the U.S.A. (Bormann and Likens 1981; Likens et al. 1977) but many of the principles are common to both tropical and non-tropical forests (Bazzaz and Picket 1980; Whitmore 1984). An equivalent study in the Sunda Region is not imminent, and because of this, research must be aimed at understanding particular problems. There is no shortage of problems.
GENERAL EFFECTS OF DISTURBANCE ON FORESTS
Introduction
In brief, the effects of large-scale disturbance on forested ecosystems are:
1) the creation of open, hot, simple habitats containing relatively few, small, widespread species with broad niches and great reproductive potential. These species are rarely, if ever, found in mature ecosystems;
2) a huge decrease in biomass (± 30 kg dry weight/m2 in lowland forests to 0.2 kg dry weight/m2 in alang-alang plains);
3) the temporary or permanent impoverishment of the soil;
4) the creation of increasingly small and isolated patches of natural vegetation whose animal and plant diversity also progressively declines;
5) less rain water enters the ground water and more flows in the surface runoff because virtually all the rainfall reaches the soil surface, and far more rapidly than in a forest (p. 296). This can lead to a loss of soil, a decrease in ground water supplies and a increased propensity to flooding.
These ecological effects can be observed whether the disturbed area becomes wasteland or valuable agricultural land.
The major effect of less intense disturbance (where the forest retains at least some of its former structure) is a simplification of the ecosystem caused by deleterious changes to the soil, hydrology and microclimate as well as by the actual removal of plant material such as logs. As the taller trees are removed, so the volume of living space available to the forest biota is considerably reduced (Ng 1983). There is also less substrate available for use as nest sites, aerial pathways or growing sites for epiphytes and climbing plants. In addition, there is obviously also the loss of resources, particularly food.
The Relevance of Island Biogeographic Theory
As the disturbance of natural ecosystems proceeds, so smaller and smaller pockets of pristine vegetation are left, and these contain reduced populations of animals and plants. The study of island biogeography (p. 46) is not yet 20 years old and academic arguments continue over what predictions the mathematical models can actually make about the extinction and colonisation of species in different-sized areas of land, and therefore what shapes are theoretically best for nature reserves (Cole 1981; McCoy 1982; Wright 1981; Eright and Biehl 1982). Despite this, a number of important principles concerning the remaining 'islands' of natural habitats can be agreed.
1) The smaller the area of a particular habitat type, the higher will be the rate of extinction.
2) Even the largest nature reserves (± 10, 000 km2) will lose about half their larger mammal species and many of their bird species over the next 1, 000 years or so, through natural extinction.
3) 'Corridors' of suitable habitat linking reserves or the close proximity of two or more reserves will increase the effective population for only a proportion of species. Others are distinctly averse to travelling through or over open or disturbed areas.
Corridors may also permit the transmission of diseases, and the risk of epidemics should not be underestimated. If a fatal disease were to be introduced to the wild orangutan population through orangutans released from a rehabilitation station, it could reduce the population to such a low level that extinction would soon follow. Virtually all the areas of forest occupied by Sumatran orangutans are connected and any areas unaffected by the disease would not contain enough animals for a viable breeding population. It is conceivable that the road to Blangkejeren which is splitting the Mt. Leuser National Park in two might one day be seen as a blessing if an epidemic were to occur - but this is no argument for dividing up Sumatra's reserves! As domestic plants and animals - both major reservoirs of contagious diseases - press ever more heavily against natural ecosystems, the risk of epidemics becomes ever greater. The extinction of certain species which play key roles - such as dipterocarp trees, fig wasps, and animals which are the major dispersers of particular fruit - would have disproportionate effects on the remaining biota (Frankel and Soulé 1981).
As far as Sumatra is concerned, the subject of nature reserve design is largely one for academic debate. The question of what percentage of species will become extinct in different-shaped reserves in 50, 500 or 5, 000 years is not of great relevance when it is by no means certain how much land will still be covered by natural vegetation in even 25 years. The shapes of Sumatran reserves do not generally conform to the neat models of the theories (see Gorman 1979) because the rationale behind the establishment of reserve boundaries has been set primarily by human settlement patterns. In Sumatra today the major priority is simply maintaining the integrity of reserves against the pressure of legal and illegal forms of habitat disturbance. Every day the natural ecosystems are being destroyed at a vastly faster rate than they are being added to by succession. Anyone acquainted with the major Sumatran reserves knows that not only non-reserved areas are being destroyed, but that incursions into reserved areas occur also, in the form of logging, settlement, and wood cutting.
Genetic Erosion and Conservation
Destruction of tropical forests, and particularly lowland forest, represents an extremely serious global loss of plant and animal genetic resources. These play a vital role in the improvement of crop species, and in the development of industrial and medicinal products, and as such play an essential role in world economic productivity. If their availability and diversity are reduced or lost, the effects on man and his many and growing needs will be severely felt. Anyone who has the slightest doubt that conservation of Sumatra's and other tropical genetic resources is one of the highest environmental priorities should consult some of the many papers and books on the subject (Adisoemarto and Sastrapradja 1977; Anon. 1980b, 1981; Djajasasmita and Sastraatmadja 1981; Ehrlich and Ehrlich 1981; Frankel and Soulé 1981; Jacobs 1982; Myers 1979, 1980; Oldfield 1981; Sastrapradja 1977, 1978a, b; Sastrapradja and Rifai 1975).
As stated in the previous section, reduced forest area leads to a reduced number of species as well as a reduced number of individuals. The resultant inbreeding of organisms often reduces their 'fitness' (adaptability, genetic stability and variation) and can fix deleterious traits in a population which can lead to extinction. Many social mechanisms exist in man and other animals to prevent inbreeding but for animals 'trapped' in a small population within a patch of forest there may be no choice.
How large should a population of animals be to prevent disastrous inbreeding? There is no hard and fast rule but it has been suggested that an 'effective population' of 50 may be the absolute minimum possible before deleterious characters begin to become fixed and that a minimum of 500 individuals should be the goal. An 'effective population' of 50 is not the same as 50 individuals nor necessarily the same as 25 breeding animals of each sex. As explained on page 237, many animals live in social groups which are not simply pairs of adults. The effective population number, Ne, can be calculated as follows: where Nm and Nf are the number of breeding males and breeding females, respectively (Frankel and Soulé 1981). Thus for tigers, in which one breeding male may reproduce with three females, 17 males and 51 females are required for an effective population of 50. These 68 adult animals will require 3, 400 km2 or 340, 000 ha. Thus each of the three largest reserve areas in Sumatra - the Kerinci-Seblat National Park (± 1, 000, 000ha), Mt. Leuser National Park (± 800, 000 ha) and South Sumatra I (356, 800 ha, much destroyed) - are large enough to retain a fully functioning population of tigers. At present the areas of habitat used by tigers extends outside the reserves, and those at Kerinci could conceivably walk to South Sumatra I. How much longer it will be before the tigers are confined to reserves, and disturbed reserves at that, is a matter for those in positions of authority.
Atmospheric and Climatic Changes
The destruction of forests and changes in land use contribute to the rise in atmospheric carbon dioxide levels since forests fix a great deal more carbon dioxide than grasslands or other secondary growth. Levels of carbon dioxide have been increasing over the last 100 years or so and if present trends persist the concentration will have doubled from 0.03% in the middle of last century to 0.06% in the middle of next century. Although still a small proportion of the atmosphere, the increase is likely to cause major climatic changes.
PRIMARY SUCCESSION — THE CASE OF KRAKATAU
Krakatau is the name of a group of four islands in the Sunda Straits halfway between Tanjung Karang and Labuan (fig. 11.1). These islands represent the visible remains of Mt. Krakatau, the whole cone of which is thought to have once been above the sea surface. Periods of volcanic activity have altered the shape of the islands and it was only in 1930 that Anak Krakatau Island emerged from beneath the sea after three years of activity (fig. 11.2) (Hehuwat 1982; Partomihardjo 1982; Richards 1982).
Figure 11.1. The Krakatau Islands showing contours every 100 m and (dotted line) the coastlines in 1883.
After Richards 1982
It was the massive eruption of 1883 that made Krakatau famous. The force of the explosion was equivalent to 2, 000 Hiroshima bombs, and it was heard as far away as Sri Lanka, the Philippines and Australia. Pumice and ash shot up 80 km above the earth and this dust darkened the sky so much that lamps had to be used during the day in Jakarta and Bandung. Enormous tidal waves claimed 36, 000 lives along the Sunda Strait and were detected in Alaska, San Francisco and South Africa (Hehuwat 1982).
Figure 11.2. The geological history of the Krakatau Islands, a - the original volcanic cone of Mt. Krakatau in prehistoric times; b - the three small islands left after a prehistoric eruption; c - the growth of Rakata as a result of prehistoric volcanic activity; d - Krakatau before 1883; e - Krakatau in 1930 (it was not until this year that Anak Krakatau emerged).
After Richards 1982
The explosion of 1883 made Krakatau very important ecologically. Not only was all the vegetation burned, but it is almost certain that the entire land surface was sterilised (Whittacker and Flenley 1982). Thus it is possible to observe the ways in which plants and animals colonise virgin land - a natural laboratory was formed in which to study primary succession.
The first botanical expedition to visit Krakatau after the explosion was in 1886 and irregular surveys of varying thoroughness continued up to 1951. It was in 1979 that the latest, major botanical survey was conducted. The total number of plant species found in 1979 was about 200 and this was similar to the total found by the previous comprehensive survey in 1934. This does not, however, indicate a static equilibrium because the species are changing (fig. 11.3).
That is, not all species found in the past appear to be present now, suggesting a turnover of species. It has been calculated that over the last 50 years new species have been appearing on the islands at an average of 2.28 to 2.60 species per year, representing an immigration rate of 1.14%-1.30% per year. At the same time, however, extinctions have been occurring at 1 % per year and so the net increase in plant species actually present on the islands is between 0.14% and 0.30% per year. Thus, very roughly, a new plant species arrives every two years (Whittacker and Flenley 1982).
So, a dynamic equilibrium appears to have been reached of about 200 species. This is, perhaps, surprising because within about 150 km of the islands there are areas of little-disturbed forests containing thousands of plant species. A problem with interpreting the data may be the time scale we are imposing. There is a nagging feeling that more species should have arrived in the second period of 50 years since the major eruption. In fact, as was stated above, the number of species is not at a perfect equilibrium because one new species appears every two years or so. That is equivalent to about 50 species per century. The flattening of the curve in figure 11.3 may just be because the r-selected (p. 190) species of the pioneer and building phases (p. 193) are coming to the end of their domination. We may now be witnessing the slow arrival of K-selected species which will contribute to the mature phase of forest development. Only one tree typical of primary forest has so far been recorded on Krakatau (Whittacker and Flenley 1982). It may well take a thousand years or more before the forest on Krakatau becomes properly mature but, at the present rates of destruction, there is no evidence to suggest that a pool of 'parent' primary lowland forest trees will exist anywhere in Lampung, South Sumatra or West Java in even 100 years time.
Figure 11.3. The cumulative total number of plant species recorded on Krakatau since the eruption of 1883 and the total of plant species actually present on the islands.
After Whittacker and Flenley 1982
MANGROVES
The management of mangrove forest for sustained yield forestry is possible but there is very little known about natural regeneration rates or processes (Aksornkoae 1981). Unless great care is taken, the result of large-scale disturbance is not the regeneration of productive forest but rather the appearance of a degraded type of vegetation. This is typically dominated by nipah (DKDPI Riau 1979) or Avicennia, or the fern Acrostichum aureum and sea holly Acanthus ilicifolius. Mangrove soils cannot be used for successful agriculture except when careful land management practices have been followed (Soegiarto and Polunin 1980; van Beers 1962; Hanson and Koesoebiono 1977; Koesoebiono et al. 1982).
Large-scale disturbance of mangroves can lead to coastal erosion because the shoreline is no longer protected by trees. The shore may be reduced to a narrow sandy beach or to inhospitable salt pans. The coastal population centres are then more susceptible to the effects of storms, such as flooding.
The replacement of mangrove with tambak ponds (for prawn and fish production) is increasing along the northeastern coast of Sumatra. As described on page 77, mangroves are vital for the continuance of coastal fisheries (Koesoebiono and Chairul 1980; Martosubroto 1979), but the establishment of tambak necessitates the removal of mangrove. In moderation this can be successful, but in some areas a situation is developing which has been called the 'Tragedy of the Commons' (Hardin 1968). The 'commons', or unowned useful resource, in this case is a mangrove forest. One person establishes an area of tambak in order to provide an income. Other people are attracted to the idea of owning tambak and more mangrove is felled. Yields stay more or less the same for a while but then they begin to drop. Tambak owners increase the area of their tambak to compensate and to increase their income, and this is achieved by clearing more mangrove. The first tambak is now some way from the source of the fish fry and young prawns (i.e., the mangroves) and the first owner may try to move towards the fringes of the tambak area (nearer the mangroves) where yields are higher. The first tambak is taken over by ferns and other scrub. Other owners experience the same drop in yields and so the process continues. Avoidance of this situation requires either a strong environmental ethic among the tambak owners, or strong enforcement of land use regulations. In fact, some tambak owners have learned, and others are discovering, that the area suitable for tambak is limited by certain conditions of tidal range, shore elevation and soil characteristics (Soegiarto and Polunin 1980).
The Showa Maru oil tanker accident of 1975 showed just how sensitive mangroves are to oil pollution (Soegiarto and Polunin 1980; Wisaksono, N.D.; Baker 1982). The oil probably acts by clogging the trees' pneu-matophores which act as organs of gas exchange, and by raising the water temperature and lowering the dissolved oxygen levels; leaves may also be easily damaged by oil (Lugo et al. 1978; Mathias 1977). Mangroves that survive exhibit signs of chronic stress such as reduced productivity and gradual leaf loss (Lugo et al. 1978; Saenger et al. 1981). A covering of oil on the mangrove mud would obviously have extremely adverse effects on the mangrove trees. The susceptibility of mangroves to other industrial and domestic pollutants requires more study (Bunt 1980), and although many mangrove species may be resistant to particular types of pollution, the equilibrium of the system will probably be upset (Saenger et al. 1981).
Some palm oil and rubber factories discharge their effluents into rivers near the coastal zone. One study of this was across the Malacca Straits along the Puloh River near the estuary of the Kelang River, the river that flows through Kuala Lumpur. The head of the Puloh River was the site of a plantation factory outfall and Seow (1976) monitored the macrofauna composition and various water parameters at different locations along the river through the mangrove. The water quality near the outfall was obviously the least conducive to aquatic life because it had hardly been diluted by the river water. The effluent had caused several species of fish to disappear from the estuary and others had moved downstream from the outfall. Unfortunately, no observations were made of the effects of the effluent on the surrounding vegetation.
OTHER COASTAL ECOSYSTEMS
Beach Vegetation
Destruction of fringing coral reefs (see p. 350) and the commercial exploitation of sand (such as near Banda Aceh) can almost totally destroy beach vegetation. By their pioneer nature, however, the smaller plants can recover quickly, particularly in the pes-caprae association (p. 115). The Barringtonia association (p. 119) has been replaced in most areas with coconut plantations and other types of agricultural areas. Where she-oak forest has been felled near Singkil, South Aceh, the vegetation has become dominated by the sensitive plant Mimosa pudica, Melastoma and various rushes, sedges and grasses.
The removal of beach vegetation may not be regarded as a particularly serious loss in itself, but its ability to hold together a loose, sandy substrate means that in its absence more or less continuous coastal erosion occurs. The erosion and the impact on human settlements are particularly serious during bad storms, because the power of the wind and waves is no longer countered by deep-rooted vegetation.
Oil pollution of sandy beaches in Sumatra is not generally a serious problem but it does occur. Even on the remote western coast of Siberut Island, patches of tar have been found (Whitten 1982b). Organic pollution by, for example, domestic waste can reduce the depth of the aerated layer of sand and so affect the zonation of beach biota. Serious pollution by oil or other organic wastes effectively removes the fauna, and the processes of soil formation in which they play an important role are greatly slowed.
The overexploitation of turtle eggs and turtles for food makes survival for the rest of the turtle population progressively less likely because the mass nesting behaviour of turtles is ecologically akin to the gregarious fruiting behaviour of dipterocarps (p. 221). Both act to satiate the appetites of their predators with the result that at least some of their eggs or seeds will get the chance to develop. The smaller the population of turtles, however, the larger the proportion of eggs destroyed.
Brackishwater Forest
The disturbance of this forest generally results in a great simplification and effective domination by nipah palms. No account of other effects specifically concerning brackishwater forest has been found.
Rocky Shores
Rocky shores are not usually subject to disturbance because the waves, strong currents and ruggedness and steepness of terrain are not compatible with agriculture or other forms of development.
Coral Reefs
Coral reefs experience different scales of disturbance. One of the most common forms of serious disturbance is caused by the mining of reefs for limestone either for lime or for road fill (Soegiarto and Polunin 1980). Mining wipes out the entire ecosystem and the fisheries that depend on it because all the components in this ecosystem depend on the biological and physical features of the coral for their existence (p. 127). Natural disturbances may be equally devastating; corals were more or less wiped out around Krakatau following the eruption of 1883.
Destruction of reefs also occurs when fishermen 'cheat' and catch fish using explosives. Explosions are totally unselective in the species and size of fish they kill and can reduce live coral to mere rubble (Odum 1976). Recovery of the coral, if it occurs at all, is very slow (Parrish 1980). The rubble is, of course, a 'habitat' of sorts but the biomass of plankton in such areas is little greater than over plain sand (Porter et al. 1977). When considering different impacts on coral it is worth remembering that most coral has a growth rate of between only 1 and 10 mm/yr (Soegiarto and Polunin 1980).
Waves generally break over a fringing reef, thereby dissipating the wave energy and leaving the sea between the reef and the shore relatively calm. Destruction of the reefs allows waves to break with their full force against the shore (fig. 11.4) and the land erosion this causes has been described for the north coast of Java (Praseno and Sukarno 1977).
Land use which increases the sediment load of rivers and thus coastal waters can have a negative impact on corals because the polyps are suffocated by a rain of silt. This distribution of dead and live coral around a log-loading pier on Siberut Island is shown in figure 11.5. Many hectares of muddy logging roads and log-ponds caused extremely muddy water to flow into the sea. The disturbance of reef communities by changes in patterns of sediment deposition caused by the construction of a pier near Padang is described by Umbgrove (1947). The land uses which increase sediment load in rivers also increase the seasonality of water flow (p. 132). Thus, at times of flood, large quantities of fresh water are discharged into the sea and the salinity level may fall, causing the death of certain corals.
Figure 11.4. The consequences of clearing coastal forest and of destroying living coral reefs which protect the coast are that the land suffers from erosion, the coral no longer supports fish and other animals, the water is turbid, and fishing becomes less successful.
After Odum 1976
Changes in the community structure of coral reef fishes caused by different types of fishing methods can affect the composition and growth of corals. This subject is reviewed by Parrish (1980).
Figure 11.5. The distribution of living and dead coral around a logging company pier on Siberut Island.
After Anon. 1980
The detailed effects of domestic and industrial pollution on corals have not been investigated but they are clearly deleterious (Soegiarto and Polunin 1980). Corals have been observed to exhibit a 'shutdown reaction'. When they are stressed in some way - for example, by the effects of reduced water quality or explosions - they are weakened to such an extent that they may die if another stress is imposed, even though the second stress alone would not have caused their deaths. This means that the total cumulative disturbances on coral should be assessed in a study of potential environmental impact, not just the likely stresses caused by one particular development (Antonius 1977).
Rivers and Lakes
Introduction
Disturbance of aquatic ecosystems can, as with clearance of forest, be deleterious to some species but also extend the habitat of other more opportunistic species. Disturbance can occur in a variety of different ways. Industrial and domestic pollution reduce the water quality with consequent effects on the whole biota: fish poisons and bombs cause indiscriminate fish death; overexploitation of fish stocks to supply the aquarium trade can have devastating effects; introduced fish can cause the demise of indigenous species; and forest clearance causes changes in temperature, turbidity, insolation, stream flow, water input, etc., thereby altering the habitat of many species.
Industrial and Domestic Pollution
Pollutants of water have been divided into four categories—pathogens, toxins, deoxygenators and nutrient enrichers (Prowse 1968). Pathogens include a wide range of bacteria, protozoa and parasitic worms harmful to man and other organisms. These are all associated with untreated sewage. Toxins derive from industrial waste and from agricultural chemicals. Their effects can be both dramatic and cumulative and affect not only aquatic animals but also plants and, of course, man. Deoxygenation is caused by bacterial and fungal decay of organic matter and by animal and plant respiration. It can also be related to certain weather conditions (Johnson 1961a). Where large quantities of organic wastes are disposed of, deoxygenation and the subsequent death of animals by suffocation can be expected.
Nutrient enrichment is known as eutrophication. Eutrophic habitats are excellent for fisheries because of their high productivity. Indeed, lakes and ponds are often artificially fertilised to increase fish production. In the Philippines, for example, yields of over 1, 000 kg/ha/yr can be expected from such habitats, whereas a normal, medium-sized oligotrophic lake might produce only one-hundredth of this yield (Ricklefs 1979). Primary production of the phytoplankton is similarly enhanced.
Eutrophication is not, of itself, a major problem for aquatic ecosystems. Naturally eutrophic systems are usually well balanced, but the addition of artificial nutrients can upset this balance and cause devastating results. Algal 'blooms' are the most spectacular of these effects where the combination of high nutrient levels, and favourable conditions of temperature and light, stimulate rapid algal growth. These blooms are a natural response of algae to the environment, but when the environment changes and can no longer support such high algae populations, the algae that accumulate during the bloom then die and decay. The ensuing rapid decomposition of organic debris by bacteria robs the water of its oxygen, sometimes to the extent that fish and other aquatic organisms suffocate.
Various studies (e.g., Schindler 1974) have shown that phosphorus is usually the limiting factor in eutrophication (rather than nitrogen or potassium which might have been suspected) and 0.01 mg phosphorus per litre of water has been suggested as the figure above which algal blooms are likely to occur in North America. Research is needed to determine the appropriate level for Sumatran lakes, so that it can be used for the establishment of rational effluent control standards.
In spite of the disturbing effects of artificially enriching lakes to such high levels, these lakes can recover their original condition if the artificial inputs cease. Sediments at the bottom of most lakes have a high affinity for phosphates which are 'locked' into the sediment even if there is an overturn (p. 145). The classic example of this is Lake Washington which in 1963 was receiving 75, 600 m3 of phosphate-rich human waste from the city of Seattle each day; it stank, algal blooms occurred and the fish died. Six years after the human waste was diverted, the phosphorus levels had fallen to those measured before Seattle became a city (Edmondson 1970).
Sumatra has yet to experience serious, large-scale, artificial eutrophication of lakes but it has been reported from a Malaysian reservoir (Aru-mugan and Furtado 1981). It is important to know and understand the processes involved in order to prevent any such occurrence in Sumatra. Lake Kerinci is perhaps the most likely place to experience such eutrophication given its area, and relatively dense surrounding human population. Saravanamuthu and Lim (1982) describe how algae can be used as indicator plants to determine the trophic status of fresh water.
Chemical, physical and microbiological parameters were studied at points along the Kelang River (which passes through Kuala Lumpur) and the results were examined against fish catches at the same locations (Law and Mohsin 1980; Mohsin and Law 1980). The species diversity, richness and the number of fish per m2 were much higher in stations above Kuala Lumpur than in the city itself or downstream. In the worst polluted places only the guppy Poecilia reticulata (p. 412) was found. Downstream of Kuala Lumpur the water was unsuitable for most freshwater fishes but a few air-breathing fish species were found. The primary causes of the change in fish abundance and variety were heavy siltation, low pH and high biological oxygen demand. Heavy silt load kills certain fish by clogging their gills, thereby causing suffocation.
Lakes and rivers in North America and Europe are experiencing the serious environmental effects of acid rain. Under completely natural conditions rain is in fact slightly acid, about .pH 5.7, because atmospheric carbon dioxide dissolves to form carbonic acid. Acid rain is understood to be rain which is more acid than this. The increased acidity of rain has been caused primarily by gaseous nitrogen and sulphur oxides which form nitric and sulphuric acids. These gases are waste products from the combustion of oil and coal, from metal smelting and from other industrial processes. Most water bodies have some buffering capacity but these are limited and so eventually their acidity will increase. At pHs below 5.0 most fish will die, and almost all other aquatic organisms are affected in some way. The problem is worsened because when the acid rain percolates through soil it releases certain metals, particularly aluminium, which are toxic to most organisms, even at very low concentrations. Acid rain is not currently an environmental problem in Sumatra and is unlikely to become one in the near future. However, acid rain is a transnational problem - that is, industrial processes in one country can produce waste gases which fall as acid rain in another. Sumatran environmental scientists should be aware of the problems of acid rain and it would be useful to begin a low-key, long-term monitoring program of water bodies.
Poisons and Bombs
The most common traditional fish poison is 'tuba', made from the roots of the climber Derris (various species) in which the active, identifiable chemical is rotenone (Sinnappa and Chang 1972). Other poisons, typically commercial insecticides, are also commonly used. Poisons and bombs are very effective and kill or debilitate large numbers of fish (the very reasons they are used), and because of this, many urban rivers such as the Deli and Babura in Medan, have virtually no fish left in them. Bombs are even used in rivers within reserves such as the rivers Alas and Bohorok and their tributaries around the Mt. Leuser National Park. Apart from any other arguments against the use of poisons and bombs, these forms of fishing are selfish, needlessly destructive and, in the case of poisonous insecticides, can cause sickness in humans.
Aquarium and Food Trade
The keeping of freshwater fish in aquaria is a popular, low-cost hobby the world over. Most of these fish originate from the tropics, and Southeast Asia is a source of many popular species. Most of the indigenous species exported from Sumatra are caught in the wild rather than bred in captivity. The scale of the trade is hard to gauge but in Peninsular Malaysia and in Singapore uncontrolled collecting has completely robbed small rivers of some of their species (Johnson 1961a; Johnson et al. 1969), with unknown effects on the river ecosystem.
Similarly, the effects of catching certain species of fish for the food trade has not been investigated. In Medan, large featherbacks Chitala lapis (fig. 11.6) can be seen being sold by roadside traders. They are especially esteemed for making fish krupuk (Adisoemarto and Sastrapradja 1977). Featherback females lay their eggs on submerged timber in slow-moving water and these are then jealously guarded by the male. During this time he is extremely easy to catch and without his protective presence the eggs will be eaten by smaller fish and other animals.
Figure 11.6. Featherback Chitala lopis.
After Tweedie and Harrison 1970
Introduced Species
Introduced fish can have significant and detrimental effects on the natural ecosystems of rivers and lakes, but this does not seem to be documented for Sumatra. Tilapia, Oreochromis mossambica, are extremely productive pond fish but where stock is uncontrolled they can enter waterways. They are aggressive fish and compete very successfully (both by exploitation and interference competition - p. 159) to the detriment of the indigenous fish fauna (Prowse 1968).
Forest Clearance
Forest clearance is by far the most serious threat to natural river and lake ecosystems. Many aquatic animals depend on allochthonous material (material falling into the river - p. 160) for their existence, for they feed directly or indirectly on dead leaves and other vegetable matter. If forest around the river is cleared, plant matter fails to accumulate regularly in sufficient quantities and many organisms such as loaches, catfish, prawns, crabs and dragonfly nymphs die even if other conditions are still suitable (Johnson 1973).
The species that are least resistant to such changes tend to be the specifically Sumatran or Sunda species with restricted ranges, and they are ousted by species with wider ranges more typical of northern Southeast Asia where open habitats occur naturally.
No systematic explanation of the effects of forest clearance on the aquatic ecosystems of Sumatra appears to have been published but it is certain that the experience in Singapore (Alfred 1966) and Peninsular Malaysia (Crowther 1982b; Johnson 1973) is extremely relevant. Unforested streams may indeed have considerable numbers of fish but few species are represented and these tend to be widely-distributed or introduced species. With loss of forest those species which are exclusive to Sumatra will generally be lost.
PEATSWAMP FOREST
In areas where this forest is disturbed on a small scale, such as where careful controlled logging occurs, regeneration of timber trees is successful and weed trees (list in Whitmore 1984) dominate the regrowth along the heavily disturbed roads.
The patterns of secondary growth on peat soils after large-scale disturbance in Sumatra and Kalimantan are described by Kostermans (1958). Fire often prevents the natural succession and instead paperbark Melaleuca cajuputih develops extensive and virtually single-species stands in areas where its roots can reach the mineral soil beneath the peat. Fire occurs most years in southern Sumatra where peat has been drained for transmigration projects. Paperbark occurs naturally in peatswamp forest but generally in the drier, better-drained parts. In areas where the peat is too thick for paperbark, Macaranga maingayi (p. 376) forms extensive, almost pure stands (Wyatt-Smith 1963) such as observed on the CRES expedition to peatswamp areas of Labuhan Batu. In the areas where the peat itself has burned, small, shallow lakes have formed. Some of these are almost completely covered with floating or semi-floating islands of grasses and herbs. This has been paralleled by farmers who grow young rice on floating rafts (Vaas et al. 1953).
In the areas of deeper peat, plants of the early stages of succession commonly include sedges and grasses and occasional trees of Combreto-carpus rotundatus and Campnosperma macrophylla amongst others. In the inner regions of the swamp where nutrients are poor and the tree growth stunted, the predominant tree is Tristania obovata (p. 173) and this persists in the secondary growth as smaller, thinner trees. The secondary growth is also characterized by pitcher plants Nepenthes being even more abundant.
FRESHWATER SWAMP FOREST
The only source of information concerning disturbance in freshwater swamp is the management plan for Way Kambas National Park (Wind et al. 1979). Apart from large-scale logging (especially from 1968 to 1974) there have been several large fires (1972, 1974, 1976, 1982). These disturbances and the wide yearly fluctuations in water level hinder regeneration of the original vegetation types, and areas of swamp grass and bush grow in their place. If these were left undisturbed, it would still take several centuries for a vegetation similar to the primary forest to form. The alang-alang grass and bush would probably first give way to a forest dominated by legumes and oaks with Commersonia bartramia (Sterculiaceae), a common 10 m tree of secondary forest. Dipterocarp seedlings are not light-tolerant and so can only grow once a secondary forest has grown up. The seeds cannot last many years in or on the soil, so dipterocarps may have to spread from parent trees. Also, since dipterocarp trees may be 100 years old before they fruit, and seeds rarely fall more than 100 m from the parent, the replacement of these economically important trees would be painfully slow.
The opening up of the swamp forests has allowed weeds such as the aggressive, shade-intolerant creeper Mikania micrantha to form dense mats over grass and bush. In addition, the small fern Salvinia molesta and water hyacinth Eichhornia crassipes form thick mats on some of the waterways.
In Peninsular Malaysia, and possibly in parts of Sumatra, paperbark is a common secondary growth tree in freshwater swamps (Whitmore 1984; Wyatt-Smith 1963). The papery bark peels off and, with the resin-rich leaf litter, forms a highly inflammable soil cover. When burning occurs the paperbark tree itself is relatively resistant, but not so the other trees of the secondary growth. Grass will burn down to the soil, but the rhizomes will remain alive.
Lowland Forest
Introduction
It is often stated that after forest clearance the loss of soil fertility and the loss of nutrients in plant biomass pose a serious threat to the integrity of most natural ecosystems in the Sunda Region. While this is certainly true for heath forest, mountain vegetation and certain peat swamps, there are no descriptions of disturbance to other types of forest resulting in succession which is not directed towards the re-establishment of 'primary' forest where continued disturbance is absent. Unfortunately, once disturbed, most forests in Sumatra experience a long series of destructive disturbances which never allow the succession to progress. Effects which are sometimes attributed to the loss of nutrients from soils, may possibly be due to competitive interaction between plants. Studies have shown that the fertility of soil does indeed decline following forest removal, but the evidence of short-term loss in soil fertility is usually accompanied by evidence that the successional vegetation is remarkably good at regenerating soil fertility (von Baren 1975; Harcombe 1980; Soerianegara 1970). This is not an argument against all possible precautions to reduce nutrient loss, but it indicates that further studies are required to elucidate the situation.
A detailed comparison of soil chemistry, soil respiration, seed storage and plant growth between soils of primary forest, cut-over forest, and cut-over and burned forest both before and after rain, has produced many interesting results (Ewel et al. 1981). Amongst these it was noted that after the disturbed vegetation had been burned, 57% of the initial amount of nitrogen and 39% of the initial amount of carbon remained in the soil.
The number of viable seeds in the forest soil (the seed bank) was 8, 000 seeds per m2 (67 species) but 3, 000 seeds per m2 (37 species) after the burn. Thus vigorous and relatively diverse growth followed the burn because only a proportion of the seeds were killed. Mycorrhizal fungi (p. 206) survived, and nutrients were released from the burnt material. The 5, 000 seeds that did not survive the fire may have either been killed in the heat of the fire or, as might be expected for primary forest trees, be dependent on high humidity for germination (Ng 1983). If humidity remains low for a certain length of time, the seeds will die.
A 1 ha plot of Queensland lowland forest (which contains many genera found in Sumatran lowland forest) was felled and then burned shortly afterwards. Two years afterwards the regeneration of the tree species which had reappeared were studied (Stocker 1981). Many of the 82 tree species present had regenerated in more than one way: 74 had formed shoots from the base of the old trunk, 10 had formed shoots from roots, and 34 had germinated from seed. This last category appeared to have the greatest growth rate, but the frequency of shoots from old trunks shows that this is an extremely important mode of regeneration. The ability of these trees to form shoots is probably limited, however, and repeated cutting or repeated burning may give smothering, light-demanding creepers the advantage and prevent regeneration in this form.
When forest is cut selectively for timber, recovery is relatively rapid because the seed bank has many types of viable seeds and many tree species have the ability to sprout from stumps. The silvigenetic cycle has been described on page 193. However, when forest is cut, dried and burned and then abandoned, the succession proceeds more slowly because part of the seed bank has been destroyed and because some of the sprouting species are not resistant to fire (Uhl et al. 1981).
Figure 11.7. Land-use and forest succession.
After Tanimoto 1981; A.J. Whitten, pers obs., and Wyatt-Smith 1963
Thus, when forests are cut, burned, farmed, weeded or burned and farmed again, recovery of the forest (if allowed) is extremely slow. In places where the only remaining stands of primary forest (source of seeds) are some kilometres away, full regeneration would take hundreds if not thousands of years (Uhl et al. 1981). Even if a few mature trees have been left to stand there is no guarantee that these will regenerate successfully because some species have seeds which have to pass through an animal intestine before they will germinate (Ng 1983). Repeatedly burned and farmed areas should not, however, be regarded as ecological deserts or as being without interest, particularly since their rehabilitation to useful land requires an ecological approach.
The vegetation that grew up during the seven-year fallow period between tobacco crops in the Deli area has been described in detail by Jochems (1928). It contained those species which were able to fruit within seven years and whose seeds could remain viable for at least one year. The shorter fallow period allowed nowadays has presumably reduced the diversity of the secondary vegetation but it still avoids invasion by the pernicious alang-alang grass (BIOTROP 1976).
A common strategy in reforestation programs for wastelands is to plant pines or other plantation trees. Where there is a local human population these programs do not always succeed. It has been suggested that assisted natural regeneration might, in the long term, produce a larger yearly biomass increment than plantations, particularly in areas where financial resources were limited (Jordan and Farnworth 1982). Assisted natural regeneration would require strips or other shaped areas to be planted with a range of good, light-tolerant timber trees and other species. One likely candidate is Anthocephalus chinensis. Regeneration of neighbouring areas would then occur outwards from these centres.
The typical (but not inevitable) vegetation types resulting from different forms of disturbance are summarised in figure 11.7.
Shifting Cultivation
Shifting cultivation, in its purest sense, is the repeated use of a patch of forested land for agriculture, characterised by long fallow periods between short periods of intensive production. An area of forest is cleared, the remains are left to dry and are burned. Crops are subsequently sown in the ashes between the tree trunks which were too large to burn.1 Nutrients are lost in the smoke, and rain washes ash into rivers before it has had a chance to become incorporated into the soil (Geertz 1963). After one or two crops, yields decrease, weeds become a serious problem (Chapman 1975) and the area is abandoned for 15-30 years to allow the soil to recover (von Baren 1975; Soerianegara 1970). The farmer then moves to another area of forest. This is usually one of mature secondary forest on a former cultivated area because it is easier to clear than primary forest. This secondary forest may contain mature fruit trees such as durian which would have been visited in the intervening years. This form of shifting cultivation can support 30-40 individuals per km2 and depends on subsistence agriculture being the predominant use of the cleared areas, and on cultural restraints which had evolved as part of the traditions bound up in animistic religions. The environmental care implicit in Christianity and Islam (Achmad 1979; Schaeffer 1970) does not seem to find its way into the lives of new generations of converts. True shifting cultivation has virtually vanished from Sumatra and only a minuscule percentage of the population is now involved. Some publications still quote figures for areas under shifting cultivation which were determined some decades ago. It should be remembered that Sumatra's population has more than doubled since the 1940s.
Figure 11.8. The distinction between traditional shifting agriculture (a) and the now prevalent slash-and-burn agriculture (b) which is endangering so many of Sumatra's natural ecosystems.
After Rijksen 1978
Use of cleared and burned land for shifting cultivation alters the early stages of succession (p. 340). Weeding removes shoots growing from tree stumps and although herbaceous plants are removed, their relative and absolute abundance increases. These herbs dominate the area as soon as the land is abandoned, but after a year, fast-growing pioneer trees dominate these cultivated areas because they have many viable seeds in the soil and/or the seeds are easily dispersed.
It is clear that when the shifting cultivation cycle is long enough (e.g., 30 years), species of herbaceous weeds are not present in the secondary forest felled prior to cultivation. When the cycle is reduced to, say, just five years and succession has not been allowed to progress very far, domination by herbaceous weeds such as Chromolaena (formerly Eupatorium) odoratum can become a serious problem (Kushwaha et al. 1981).
Shifting cultivation is not, despite what one might be led to believe, the relentless slash-and-burn cultivation practised in many areas. The distinction is illustrated in figure 11.8. Slash-and-burn is practised with almost no cultural or social restraint and the cultivation of cash rather than subsistence crops is the major use of cleared land. The change from shifting cultivation to slash-and-burn is caused largely by population increase (Geertz 1963), whereby rested land is virtually nonexistent and primary forest is cleared when the former cultivated area is exhausted or when alang-alang grass can be kept at bay no longer by simple (cheap) weeding. Regeneration of forest in such areas would take many centuries.
Effects of Logging on the Soil
The damage that logging causes to soil has barely been studied in Indonesia but the general subject has been reviewed by Kartawinata (1980). The major problems are soil removal by bulldozers when making roads, soil compaction caused by heavy vehicles, and soil loss due to rain striking the ground with its full force and the greater quantity of rain reaching the soil. The removal or destruction of trees also reduces litterfall and hence the organic inputs to the soil, but in areas where the soil has not been too greatly damaged, soil nutrient levels and microorganisms can recover within a few years. Since only the larger portions of the tree trunks are removed, and most of a tree's nutrients are in its leaves and roots, the loss of biomass after logging is proportionally greater than the loss of nutrients.
Effects of Logging on Hydrology
The study of the effects of logging and other forms of forest clearance on hydrology is also beset by inadequate data and differences in interpretation (Nordin and Meade 1982). It is obvious, however, from a brief visit to a logging camp or a flight over areas where logging occurs, that a great deal of water runs off the roads, carrying soil with it. This has been quantified by Hamzah (1978) who measured the silt content:
a) of a river near, but not influenced by, a particular logging area,
b) from a river in the logging area, and
c) from a ditch by a logging road.
Figure 11.9. Traditional swidden plot on Siberut Island, West Sumatra.
A.J. Whitten
The silt contents were 0.01%, 0.05% and 0.1%, respectively - a tenfold increase. The increased silting of Sumatra's main rivers could probably be traced to logging as well as other forms of unprotective land use.
In the lower stretches of major rivers the actual volume of water is probably not greatly affected by forest clearance, but it is in the higher river regions, before high water levels can be dampened, that dramatic effects are noticed. For example, the consequence of clearing just 10 ha of forest for fields on slopes above the Mengkudu River near the edge of the Mt. Leuser National Park in May 1981 was that 13 people died (Robertson and Soetrisno 1981, 1982). The Mengkudu River is normally only 1 m wide and 8-15 cm deep but landslides on the steep, deforested slopes and the increased surface runoff, both caused by the 10 ha ladang, turned the Mengkudu into a river capable of moving 1-3 m3 boulders. A nearby river also flooded and destroyed new bridges and roads (which took months to repair), rice fields and most of a town, and by luck claimed only one more life. Few floods in Sumatra have been so well documented as these but they are commonly reported in the newspapers. The floods are generally attributed to illegal or irresponsible logging or other forest clearance.
Figure 11.10. Even selective logging causes damage beyond simply the felling of trees, but if done carefully and if given time, the forest will recover.
Effects of Logging on the Forest
The felling of trees and extraction of logs are clearly the primary causes of disturbance during a logging operation. Felling damages tree crowns, boles and saplings, exposes wood, making it susceptible to fungus damage, and also covers seedlings. Extraction of the logs exposes bare soil and damages large areas of the forest floor (Kartawinata 1980b).
Selective logging sounds a very mild and benign activity to many people who have not visited or worked in a logging concession and it comes as a surprise to learn how much damage is caused to the forest as a whole. One estimate is that five times more timber is destroyed or badly damaged than is extracted (Burgess 1971). It has also been reported that for every large tree felled, 17 similar- or smaller-sized ones are destroyed (Abdulhadi et al. 1981).
Most of the destruction is caused by the access roads which remove all cover from the soil and form channels which are further scoured by water during storms. It has been estimated that 20%-30% of a logged area is completely bare, being composed of roads and log yards (Kartawinata 1980; Meijer 1970). Some of the roads cut across small rivers, acting as dams, and the subsequent flooding kills most of the inundated trees and other plants (Anon. 1980a; Kartawinata 1980b). The lorries used to drag logs out of the forest cause compaction of the soil surface, and this disturbance is traceable in some forests by the occurrence of the pioneer tree Anthocephalus chinensis even 40 years after the logging. By this time other pioneer trees such as Trema, Macaranga and Homolanthus have died or become rare as different genera take over in the succession. The soil compaction caused by lorries is, however, only a fraction of that caused by the bulldozers used to make the roads. Additional disturbance is caused when trees are felled on both sides of the main logging roads to hasten the drying of the road surface after rain. These 'daylighting' areas have been estimated as occupying 8 ha/km of road (Hamzah 1978).
A very thorough study of the effects of logging on the forest was conducted on South Pagai Island in the Mentawai Islands, West Sumatra (Alrasjid and Effendi 1979). A total of 2, 416 trees (20 cm diameter and over) originally stood in the 15 1-ha plots, half of which were commercial species and half non-commercial species. A total of 194 trees (13/ha) were felled and extracted. On average, about half of the remaining trees had been noticeably damaged, broken or knocked down, and the other half had escaped damage (fig. 11.11). This proportion seems to be relatively consistent between studies (Abdulhadi et al. 1981; Burgess 1971; Tinal and Palinewan 1978). In areas of South Pagai Island where many trees were extracted, over 70% of the remaining trees had been damaged or killed. Damage to saplings and seedlings was also extremely high.
Damage to a forest during logging can be reduced if all climbing plant stems are cut. When a tree is felled the tangle of these stems often pulls other trees over and the climbers also compete with the trees for light, water and nutrients. This cutting may be sound forestry but the death of climbers must have a significant effect on frugivorous animals. For example, 25% of the food plants of orangutans are climbers (Rijksen 1978), as are 42% of the food plants of Mentawai gibbons (Whitten 1982e).
The cutting of climbers is obviously disastrous for the famous Rafflesia and other members of its family (p. 217), all of which are rare enough as it is, because they live only as non-deleterious parasites on the stems and roots of Tetrastigma climbers. In addition, the climbers of the genus Aristolochia have spectacular flowers and their leaves are the sole food of some of the larger (and saleable) butterflies and moths. It is difficult to resolve the conundrum of whether to cut or not to cut climbers. The answer will probably have to be total and enforced protection of such plants in reserves since they are unlikely to survive otherwise.
Selective logging is, in theory, a repeatable exercise in any given area. After the largest commercial trees have been extracted the area should be left for about 70 years before the next timber crop is harvested. The average volume of dipterocarp timber in a forest logged forty years earlier is only half the volume found in adjacent unlogged forest (Meijer 1970). Logging of a forest at intervals of less than about 70 years is unsound forestry practice but as more species become economically worth exploiting and when timber prices increase, and when smaller and smaller trees are permitted to be cut, repeated logging occurs. Over a hundred species of Sumatran trees are probably commercially exploited but a tree species which is non-commercial today can easily become commercial tomorrow if a particular use is found for its timber properties or if supplies of better species are insufficient to meet the demand. Each time logging occurs plant succession is set back a step, the forest becomes progressively poorer in desirable species and progressively richer in 'weed' species (Whitmore 1984; Wyatt-Smith 1963).
Figure 11.11. The effects of extracting 8% of the trees by selective logging on an area of forest on South Pagai Island (Mentawai). The other categories are (clockwise) totally destroyed, badly damaged crowns and stripped bark, seriously broken, and undamaged forest.
From data in Airasjid and Effendi 1979
The microclimatic changes caused by logging are obvious - it is hotter, lighter and drier in logged-over forest. These changes result in the dieback of crowns, scalding of intolerant trunks and branches, water stress and even an increased likelihood of insect attack, any one of which might lead to the death of a tree (Kartawinata 1980b).
While selective logging clearly has many advantages, its also removes the best individual trees, leaving only inferior ones to produce seeds for future 'crops'. This genetic erosion is potentially extremely serious for future forestry (Ashton 1980; Kartawinata 1980b; Sastrapradja et al. 1980; Whit-inore 1984) but in many logged areas it is doubtful whether in fact a full cycle of regeneration will ever be allowed to occur.
The regeneration of lowland forests in natural gaps has been described on page 193. Regeneration after selective logging takes a similar course and has been studied in parts of West Sumatra, Jambi, South Sumatra and Lampung (Geollegue 1979; Geollegue et al. 1981; Huc 1981; Huc and Rosalina 1981a). Plots studied one, three, four, and ten years after logging each had distinctly different structure and species composition (table 11.1). After 10 years it was generally found that the secondary forest was forming a layered canopy and entering the building phase (Geollegue 1979; Geollegue and Huc 1981). It seems unlikely that regenerated forests will grow to their original heights and Ng (1983) has suggested that a height decrease of 25%-50% would not be surprising.
The daylighting areas at the sides of the roads are quickly colonised by wild bananas (although in some areas these appear to be quite rare), smaller weed trees and various members of the ginger family (Zingiber-aceae). This early community plays an important role in stabilising otherwise vulnerable soil.
Effects of Disturbance on the Fauna
About 70% of the mammal and bird species found in the Sunda Region are dependent on more or less intact primary lowland forest (Medway 1971; Wells 1971). Considering the very large number of insects which are restricted to a single species of plant (p. 192), the percentage of the region's insects dependent on undisturbed forest is probably even greater. In view of this, there is clearly a need to know what effects different types of disturbance have on Sumatra's fauna. The trends and general patterns, at least for mammals and birds, however, are now quite well understood. The forest fauna is affected by disturbance in at least three ways:
(1) the noise and shock of disturbance may cause immediate changes in behaviour;
(2) the actual removal of parts of the forest canopy will alter ranging patterns and diet which may in turn affect social behaviour and population dynamics;
(3) the slow regeneration rate may cause permanent changes in population density (Johns 1983).
After Geollegue and Hue 1981
Figure 11.12. The total number of non-flying mammal species in six types of vegetation and the percentage of introduced species.
From data in Harrison 1968
Different species show different degrees of tolerance to disturbance and some animals will usually be found even in the most disturbed areas. A long-term study of small mammals in different habitats ranging from primary forest to an area of alang-alang grass showed, not surprisingly, that the total number of species decreased. It also demonstrated that the proportion of introduced species (rats) increased from 0%-100% (Harrisson 1968) (fig. 11.12). Similar results were reported by Yong (1978). Surveys for noticeable animals along transects in forest with different levels of disturbance in the Sekundur Reserve (Langkat) also showed a dramatic decrease in species present with increasing disturbance (Rijksen 1978) (table 11.2).
Figure 11.13. Logging roads cause serious damage to the soil, the forest and hydrology.
Properly executed selective logging is not disastrous for much of the forest fauna, although some squirrels and birds fare badly (Johns 1981, 1983; Marsh and Wilson 1981; Marsh et al. 1984; Wilson and Wilson 1975; Wilson and Johns 1982). 'Properly executed' in the ecological sense used here means an average of 8-10 and an absolute maximum of 15 trunks removed per ha, no relogging for at least 50 and preferably 70 years (p. 365), no replanting with foreign tree species, and an adjacent area of primary forest from which fruits can be dispersed into the logged area. If such practices are adhered to, there is no reason why nature conservation and forestry should conflict. Logged forest supports a lower species diversity but is able to maintain viable populations of many species.
The initial effects of selective logging are probably the most serious, and Johns (1981) found a lower birth rate and a greater infant mortality amongst primates in such areas. These effects were temporary and the populations eventually began to return to normal. It is likely, however, that the effects of disturbance will last several decades because it takes that long for the lost resources such as food sources and nest sites to be replaced and for microclimatic conditions to recover.
After Rijksen 1978
Figure 11.14. Changes in population density of leaf monkeys (non-territorial) and gibbons (territorial) after selective logging.
After Marsh and Wilson 1981
The effects of disturbance depend in part on the social system and diet of the species concerned. Territorial species are worst affected because they are unable, even temporarily, to move out of the disturbed area because they will probably be surrounded by defended territories of the same species. Non-territorial species may be able to withstand some temporary crowding before moving back into the disturbed forest (fig. 11.14). Species with relatively specialised diets (e.g., p. 240) would be expected to fare rather worse in disturbed forest than unspecialised species.
Some indigenous animals, such as bulbuls Pycnonotus, tapirs and, to some extent, elephants, actually seem to benefit from selective logging because they favour the r-selected foods (p. 190) which are abundant in disturbed areas.
When forest is completely cleared for large-scale agriculture or other purposes the animals that once lived in it will eventually die. The territorial species are very unlikely to be able to find unoccupied areas in the neighbouring undisturbed areas, and the non-territorial species may be able to move into adjacent forests (assuming there are any) but sooner or later the food resources will limit the population to more or less its original level.
It is worthwhile to quantify roughly the losses in order to make the effects more understandable. If 10, 000 ha of lowland forest were felled for, say, an oil palm plantation, the following would be among the subsequent deaths:
|
30, 000 |
squirrels |
|
5, 000 |
monkeys |
|
15, 000 |
hornbills |
|
900 |
siamang |
|
600 |
gibbons |
|
20 |
tigers |
|
10 |
elephants |
More than these would be affected by the disturbance, but some may later be able to exploit the plantation in some way.
Those people involved with logging or forest clearance and those who remain behind after the initial disturbance occasionally see the animals whose habitat has been disturbed. Some of these are shot by hunters and others are caught (often illegally) and sold. The trade in young orangutans has decreased enormously because of the efforts of the Directorate-General of Nature Conservation but the problem of what to do with these displaced animals is not easily solved.
Rehabilitation of animals displaced by forest clearance or caught and sold as pets is not really an answer because most of them found themselves homeless because of forest destruction. Rehabilitation centres for orangutans, such as at Bohorok (Langkat), are wonderfully valuable as sites for conservation education (Aveling and Mitchell 1982) but in terms of long-term conservation of orangutans their role is doubtful for two main reasons2:
(a) The surrounding forests have their own resident population of orangutans, and their birth-rates in areas close to logging operations (that is, near the forest edge) are lower than average, indicating that they are suffering from stress (MacKinnon 1974). Intro-ducing rehabilitant orangutans into this already disturbed population could have any number of consequences.
(b) There is the danger that diseases contracted from humans during the time the orangutans were in captivity could spread to the wild population with possibly disastrous effects. The orangutans are kept in quarantine for a while but this will not necessarily reveal those diseases with a long incubation period or animals which are 'carriers' of the disease but do not show any symptoms.
Orangutans are particularly amenable to rehabilitation because they do not live in social groups. Other Sumatran primates are social, however, and attempts to introduce a monkey into an established group are quite likely to lead to the new animal being killed, probably by the adult male. There is no reason why the adult male should bother to defend his group or territory for any individual except his mate(s) and offspring, because the introduced animal will not increase the adult male's genetic legacy.
Bamboo
Repeated disturbance of an area of lowland forest allows various species of bamboo to gain a toehold and they can form a more or less permanent vegetation cover. Many species are shade-tolerant and so persist even when forest has grown around and above them. Patches of bamboo in the forest almost invariably indicate sites disturbed intensively by man at some time in the past. Bamboos are giant members of the large grass family, Poaceae (formerly Graminae), which includes rice, sugar cane, alang-alang and maize. Like rice, bamboo dies after flowering and fruiting, but it can take over a century for flowering to occur (Janzen 1976b; Soderstrom and Calderon 1979). Stranger than this, however, is that under natural conditions most species flower simultaneously over wide areas. This is similar to the phenomenon of gregarious flowering found in dipterocarp trees (p. 221) and pigeon orchids (p. 403) but in these cases flowering is mainly governed by environmental factors. The flowering of bamboo is not. Even when individual plants of some bamboo species are grown in different parts of the world with different climates, the species will still flower simultaneously (Janzen 1976b).
Why should bamboo behave in this way and how are the cycles maintained? The reasons are in essence the same as for the dipterocarps. That is, long gaps between fruiting periods allow reserves to be accumulated so that very heavy fruiting occurs, and this satiates the available seed predators so that at least some seeds are left to form the next generation. If the plants did not synchronise the fruiting, heavy fruiting would simply result in large populations of seed predators. Bamboos provide very attractive food:
1) Bamboo seeds are extremely nutritious, having a nutrient quality slightly greater than either rice or wheat (Janzen 1976b; Rao et al. 1969).
2) There is no evidence to suggest that bamboo seeds are defended chemically against predation.
3) The fallen seeds are easily found beneath the clumps of bamboo because the dense shade produced by the bamboo stems and leaves precludes most other plants from growing beneath them.
Bamboo seeds therefore form an easily harvested, nutritious but periodic food source. The seeds are eaten by a wide range of opportunistic vertebrates such as rats, pigs, porcupines, peasants, jungle fowl, doves and parrots. None of these animals form defended territories and so can range widely around forests in search of food. They also have r-selected reproductive strategies; that is, they can increase dramatically in numbers in response to an abundant source of food which, in the case of bamboos, may last for two years. It has been hypothesised by Janzen (1976b) that the above animals would have been effective at devouring the majority of seeds produced by clumps of bamboo fruiting out of phase of, at the very beginning or very end of, a fruiting period. Conversely, when bamboo fruit was most abundant, the animals would eat proportionately less of the crop. Since the determination of the inter-fruiting period appears to be largely genetic rather than environmental, such 'weeding out' of those plants behaving independently would favour increasingly synchronous behaviour. When the fruiting period is over, the swollen population of certain animals, such as rats, often turn their attention to growing or stored crops.
Man disrupts this synchrony in three ways:
1) He removes (by hunting or habitat change) the animals which are the agents of selection.
2) He moves bamboo around deliberately, mixing different species or cohorts of the same species (Soderstrom and Calderon 1979). He does this because of the many and varied uses of bamboo (Sharma 1980; Soderstrom and Calderon 1979; Widjaya 1980).
3) He harvests (or harvested) the larger bamboo seeds when they are most abundant and so his impact is most when the other animals' impact is least. This is largely because it is not worth his while putting effort into collecting the seeds unless there are considerable quantities available. Conversely, a foraging animal would be able to occupy a bamboo area and gradually increase its intake of bamboo seeds.
Bamboo deserves more attention, particularly now that it is being grown as a minor plantation crop in Sumatra for use in the oil-palm industry (Widjaya 1980).
Macaranga
One of the most common genera of pioneer trees found in disturbed lowland forest (and peat swamp - p. 357) is Macaranga (Euphorbiaceae). This genus contains over 20 species found in Sumatra and the vast majority are shade intolerant and are only found in open conditions. Keys to the species have been prepared by Corner (1952) and Whitmore (1967), of which the second is the more useful.
About one-third of the species of Macaranga are peculiar in having hollow twigs in which live colonies of ants. Winged females bite their way into the twigs of young seedlings and subsequent holes are made by other members of the growing colony. It is usually the case that every branch of a tree will be occupied and only rarely is an 'ant-species' of Macaranga found without ants. Only one species of ant, Cremastogaster borneensis macarangae, occupies Macaranga in Peninsular Malaysia (Whitmore 1967) and it is likely that the same species is found on Sumatra.
The ants bring young scale insects (generally a species of Coccus [Khoo 1974]) into the twigs where the scale insects suck sap from the tree tissue. The scale insects excrete sugary fluid (honey-dew) on which the ants feed. Macaranga does not only provide the ants with a safe place to keep their 'cattle' but it also forms small, white, oily food bodies on the stipules (small leaf-like extension at the base of the petiole) which are utilised by the ants (Corner 1952).
While it is clear how the ants benefit from the Macaranga, no thorough investigation has been made of the other side of the relationship. It is presumed, however, that the ants protect the tree from caterpillar and other pest damage. It would be easy enough to test this hypothesis by comparing insect damage on two sets of trees of the same species in which one set had the ants systematically killed and the other set was left alone as a control.
UNCOMMON LOWLAND FORESTS
Heath Forest/Padang Vegetation
Disturbance of heath forest results in impoverished vegetation. Repeated disturbance results in vegetation which, because of the extremely low mineral status of the soil, appears unable to develop back into heath forest (Mitchell 1963) (p. 257). Disturbance to padang vegetation, which may possibly be a natural ecosystem in its own right (albeit impoverished), results in even poorer padang, easily burned and increasingly slow to recover. It has been suggested that the large sandy area of impoverished vegetation near Gunung Tua (Labuhan Batu/South Tapanuli) used to be heath forest (van der Voort 1939).
The only study on the effects of disturbance on heath forest in Indonesia was conducted by Riswan (1981) in East Kalimantan. He clear-felled two 0.5 ha plots in primary heath forest, burned the vegetation in one and removed all the surface vegetation from the other without burning. Transects 100 x 1 m were established and divided into 100 1 x 1 m quadrats and the regrowth was monitored. After 18 weeks all the quadrats in the unburnt plot were occupied by some living vegetation. There were only two seedlings (two species) but 29 species had sprouted from the base of the felled trunks. In contrast, 13 of the quadrats in the burnt plot remained bare after the same time period. In the other 87 quadrats there were five seedlings of one species but only 19 species were sprouting from felled tree bases. In a nearby 'mature' disturbed heath forest there were only eight tree species present compared with 27 species in an adjacent primary heath forest. Riswan (1981) joined virtually all other authors who have experience of heath forest in strongly recommending that:
1) All heath forests should be retained in their natural state and classified as conservation areas; and that
2) Heath forests should be used for education, research and recreation.
Reclamation of the bare, sandy, glaring-white tin mine tailings, which are in areas probably once covered with heath forest, presents a considerable challenge to a plant ecologist. Soepraptohardjo and Barus (1974) write, "Because of the low potential of tin tailings, they are generally forgotten about, becoming mounds of white in an otherwise green landscape. It is the responsibility of P.T. Timah (the major tin-mining company) to seek ways of rehabilitating the tailings." Similar views were expressed in the environmental impact assessment of the P.T. Koba Tin operation (LAPI-ITB 1980). Reclamation experiments with various foreign shrubs and trees have been started (Siagian and Harahap 1981) but tree species from heath forest may represent an important element in any such reclamation. Brunig (1973) estimated that a total of 849 species of tree from 428 genera lived in the heath forests of Sarawak and Brunei. The area of heath forest on Bangka and Belitung Islands is much less, so a correspondingly lower number of species would be expected. However, it is these trees, which have adapted to atrociously poor soils, that should be used in trials aimed at reclamation. Van Steenis (pers. comm.) has suggested Ploiarium, Rhodamia and Rhodomyrtus as possible trial genera. There is little hope of recreating heath forest (Mitchell 1963) but the use of local species is likely to meet with much more success than the use of foreign species adapted to quite different conditions. Enormously successful reclamation of kaolin soils in southwest England has been achieved through soil conditioning and the careful choice of appropriate indigenous plants (Allaby 1983).
Considering the size and influence of P.T. Timah on Bangka and Belitung Islands, it is recommended that they make strenuous efforts to protect heath forest in and around their concession areas, and to begin a program of identifying tree species suitable for use in reclamation projects.
Ironwood Forest
There appears to be no documentation on the effects of disturbance on ironwood forest. One of the characteristics of ironwood trees is that after being felled, large numbers of suckers grow freely from the base of the trunk. It is not clear how many, if any, of these survive to become mature trees. These suckers grow vigorously even in full light, such as at the side of logging roads. This, plus the initial predominance of ironwood (seeds and saplings) in the forest, probably means that ironwood would regenerate quite successfully in the long term. For notes on germination requirements for ironwood seed see Koopman and Verhoef (1938) and page 265.
Forest on Limestone
Limestone hills are generally drier than hills consisting of other rock, and fires, either natural or deliberate, are the chief causes of the destruction of forest on limestone. The thin soil is highly organic and can itself burn in the fires and so regeneration may be very slow. Bare rock is colonised first by bryophytes and ferns and then eventually clumps of shrubs and small trees grow where litter has accumulated (Anderson 1965). Forest on limestone has few commercially important tree species but is destroyed by mining.
MOUNTAINS
During the ascent of Mt. Kemiri by a CRES team it was repeatedly remarked that signs of prior expeditions were extremely obvious, particularly in the upper montane and subalpine zones. A ditch and a soil pit dug by a geomorphological expedition two years before still had sharp edges and the piles of earth at their sides were still quite loose. There can have been hardly any heavy rain to smooth the edges or to wash the piles of earth back into the ditch. Campsites used by army expeditions in the 1940's were still largely clear of vegetation. Van Steenis (1938) remarked on the flattened empty biscuit tins at the summit left by the expedition which set up the triangulation pillar six years before his 1937 expedition. In 1982 these tins were still there although they were slightly rusty. Lastly, in 1982 the burnt slopes were essentially unchanged in character from the photographs at the end of his 1938 paper.
Most people in Sumatra live in climates where vegetation can grow vertically as much as 8 m in three years. It is important, therefore, that those people who climb Sumatra's mountains for recreation or study should remember just how long-lasting the effects of disturbance can be on mountains. The campsite at about 3, 200 m on Mt. Kerinci was, at the time that it was visited by a CRES team, littered with tins, paper and plastic wrappings and there were signs of fires which had got out of control. An untended fire on Mt. Sinabung destroyed most of the upper zone vegetation there in 1981.
The only known detailed studies of regeneration of tropical vegetation at high altitudes were both conducted in Costa Rica. The regeneration of subalpine vegetation studied by Janzen (1973b) followed a fire three years previously and he describes fire ecologically as a generalist herbivore. The two shrubs he studied in detail were a Vaccinium and a Hypericum, both of which have species occurring on Sumatran mountains. The fire killed the above-ground parts of the shrubs but suckers had shooted from the base of the plants. On average, the suckers had grown less than 50 cm in the three years. The soil surface exposed by the fire did not, as one would expect in lowland areas, become covered with fast-growing pioneer species. Instead, much of the bare soil remained unvegetated. In other places liverworts and mosses had started to grow. Very few seedlings of the surrounding plants were found.
Three years after the fire, dead stems of plants were still standing and even those that had fallen to the ground showed no signs of decay. This extremely slow rate of decomposition is caused partly by the low temperatures but also by the absence of (or extreme lack of) most of the lowland decomposers such as ants, termites, and earthworms. This in turn may be due to the extremely moist condition of the soil, branches, and logs which never warm up to any great extent (Janzen 1973b).
Regeneration of Costa Rican oak forest, which is similar to that found in Sumatra from 1, 500-2, 000 m, was also found to be extremely slow (Ewel 1980). Repeated cutting would deplete the seed stocks irrevocably and it was concluded that montane forests are "truly the tropics' most fragile ecosystems." Disturbance of oak/laurel forest in Peninsular Malaysia leads to a vegetation dominated by tree ferns and the scrambling fern Gleichenia (Wyatt-Smith 1963).
The Sumatran pine Pinus merkusii is an indigenous tree occasionally found at about 1, 000-1, 600 m in lower montane forest in northern and western Sumatra. Trunks of large specimens can reach a metre in diameter, considerably larger than most of the trees of the same species seen in plantations, such as around Lake Toba, the first of which were planted in 1927 (van Alphen de Veer 1953). Young pines in primary forest are generally found growing as pioneers on, for example, landslides. Large areas (possibly 150, 000 ha) of unplanted pine forests occur in parts of central Aceh and these are sometimes thought to be natural. In fact, they have been caused by man. Felling and repeated burning of lower montane forest has increased the area of suitable habitat for the pioneer pines. Once over 3 m tall the trees are quite resistant to fires (Whitmore 1984), unlike most of the broad-leaf trees with which they had once shared the land. These pine forests are maintained by the cattle owners who burn the undergrowth to encourage the growth of tender young grass for cattle to graze.
Figure 11.15. Limestone quarrying for roads, lime and cement can have major impacts on the range-restricted species living in caves.
CAVES
The study of the effects of disturbance on caves is an open book. At one extreme, it is clear that opening up a cave by mining the limestone and allowing the sunlight in will utterly destroy the specialised cave communities. There is no such thing as regeneration of cave life. Instead, a succession of plants from the limestone flora (p. 273) will colonise the rocks where light newly penetrates.
The resilience of the cave fauna to disturbance is unknown. The extraction of insectivorous-bat guano should not be complete, to avoid endangering the cave floor community. This sort of exploitation would obviously be better undertaken after an ecological survey and assessment of the possible impacts of different collection techniques. Similarly, the harvesting of swiftlet nests for food should be conducted so as to cause as little disturbance as possible to the birds. A nest harvesting plan has been worked out for Niah cave in Sarawak where some 4, 000, 000 swiftlets nest (Medway 1957).
It is interesting that Ngalau Indah near Payakumbuh, the only Sumatran cave where tourists are catered for, still seems to have sizeable populations of bats and swiftlets despite the almost daily disturbance. It is not known, however, if some less-resilient species have abandoned the cave.
Bats are sensitive to disturbance and when caves are visited during daylight hours, strong lights and loud noises should be avoided in the darker chambers. Catching bats requires skill and practice and should not be attempted except with good reason. Scientific collecting of bats should be conducted in moderation and other studies should cause as little disturbance as possible. The consequences of bats abandoning caves are described on pages 329 and 334.
Limestone mining can cause disturbance in a variety of ways, even if it is not immediately adjacent to a cave. For example, explosions used to break rock free can cause shock waves that break stalactites and stalagmites or cause thin cave roofs to cave in (Sani 1976; Sardar 1980). Even so, there are still quite a number of bat species roosting in the Quarry cave at Lho'Nga but, again, it is not known what species were present before quarrying began.