Chapter Twelve
INTRODUCTION
This chapter does not attempt an exhaustive review of all the different agricultural ecosystems found on Sumatra, nor an analysis of ecological constraints to tropical agriculture (for which see Janzen 1983d). Nor does this chapter discuss the fascinating home garden ecosystems which cover 20% of the available arable land in some areas of Indonesia, particularly Central Java (Penny and Ginting 1980; Sastrapradja 1979; Widagda 1981). Elsewhere, where Javanese have been settled for 20 years or so, home gardens can also be found but they do not yet cover a significant area of the arable land of Sumatra.
GREEN REVOLUTION
Some decades ago widespread famine was seen as the inevitable fate of the increasing population of Asia. The famines have not yet come, rice fields are still productive, and the real price of rice has halved. The first high-yielding variety (HW) of rice from the International Rice Research Institute (IRRI) was distributed in 1966 and to this can be traced the turn-around in fortunes that was dubbed the Green Revolution. The first variety distributed widely was a cross between an Indonesian variety and a dwarf Chinese species, and it had shorter and stronger stems that were able to support the heavier seed heads produced with applications of nitrogen fertilizer and 'protected' by pesticides.
The Green Revolution brought few if any benefits to upland populations, largely because its fundamental features of improved seed and larger quantities of fertilizers cannot be applied to upland areas because of the greater range of topographies, soil types, and a lack of dependable water supplies.
The overall success of the rice intensification programmes known as BIMAS (or 'mass guidance') during the 1970s can be counted as one of the most impressive achievements of the post-1965 government, although the first programmes began in 1963. It involved major investments in new or rehabilitated irrigation systems, fertilizer plants, transport and storage networks, and the establishment of effective research facilities, extension services, and administrative bureaucracies, all with trained staff, as well as a rural banking network, and local cooperatives. The crowning achievement has been a remarkable increase in rice production, increasing from about 11 million to 30 million tons. As a result, Indonesia has moved from being the world's largest rice purchaser to the status (albeit precarious) of being self-sufficient. Even so, the programme has not been without its problems.
For example, after the HW were introduced it was recognized that valuable local varieties were being lost and so the International Rice Research Institute in Los Baños, Philippines, set up a gene bank. The government's encouragement for the growing of the relatively few high-yielding varieties (by 1975 half of the sawah were planted to just four varieties) led to increasing genetic uniformity which, predictably, opened the crop to disease and insect pests. This vulnerability was aggravated by increasing uses of pesticides and fertilisers, closer spacing of plants, and double- or triple-cropping each year without pause. The first signs of trouble appeared in 1974 with attacks by the brown planthopper Nilaparuata lugens which damages the rice directly by sucking sap from the phloem of the leaf sheaths, and by introducing the 'grassy stunt' and 'ragged stunt' viruses. The viruses are a serious problem only when the pest populations are high, but the planthopper numbers proved difficult to control because planthoppers have:
• only about four weeks between generations;
• females which lay 100-300 eggs in the two-week laying period;
• males which can mate on the day after emergence and live for nearly a month;
• eggs which are lodged deep in the leaf sheath out of the reach of pesticides;
• tolerance to great crowding (up to 6, 000 hoppers per rice hill);
• tremendous mobility, being able to fly for 10 hours;
• a complex life cycle that produces winged adults only every other generation; and
• voracious appetites which can turn a healthy green plant to a withered brown shadow in just two days. This condition is known as 'hopperburn'.
Four new IRRI varieties with the BPH-1 gene for resistance to the planthopper were introduced in 1975, just one year after the first outbreak, but these became susceptible after just 4-5 cropping seasons. This happened in part because the 'new' varieties were quite similar genetically to their predecessors, and because of the enormous reproductive potential of the female planthopper. In the first four years of infestation, an estimated three million tons of rice (worth some US$500 million) was lost as a result of the planthopper. One of the newer varieties, IR36, remains resistant.
Meanwhile, Indonesia's own rice breeding programme had produced some excellent varieties such as Cisadane, with good taste, even higher yields than IR36, and which fared well in wetter conditions. These served to increase the number of varieties planted, and helped Indonesia to achieve rice self-sufficiency and surplus by 1984, although this was founded upon a genetic base even more uniform than that of the 1970s. In 1985 the plant-hoppers were again causing serious damage to the rice crops, having evolved the ability to overcome the genetic resistance of almost all the varieties, and these increases in damage were generally preceded by increases in pesticide use. A new IRRI variety, IR64, appeared to be resistant to the insects and was very rapidly adopted throughout, causing Indonesia's rice production to be pivoted on a yet more narrow base, supported by still heavier use of government-promoted, highly-subsidized pesticides.
It was at this time that ecologists' views began to be heard. They had been intrigued that an insect which had been of such minor importance should so quickly become of such significance that the Cabinet itself was holding meetings to discuss its control. It was explained that, under 'natural' conditions, over 100 predators, parasites and diseases kept the planthopper numbers under control, neutralizing its tremendous reproductive potential, and that the broad-spectrum insecticides that had been used against the pests were even more damaging against the predatory spiders, beetles, dragonflies, and other insects. It was also explained that sublethal doses of insecticides caused the development of resistance and resurgence of the planthoppers, and that this was probably due to the stimulation of egg production, and to the destruction of natural enemies.
And the planthopper is just one of over 400 species of insects, mites and ticks which have developed resistance to one or more pesticides. The resistance spreads through populations because some individuals survive pesticide applications because of behavioural, biochemical or physiological adaptations, and their genes are therefore passed on to the next generation. Repeated applications thus lead to the entire population becoming resistant.
INTEGRATED PEST MANAGEMENT
In late 1986 President Soeharto took a bold step and instituted a number of landmark ecological measures, the most radical of which was the ban on the use of 57 varieties of organophosphate chemicals on rice. These had been implicated in the resurgence or explosions of pest numbers and the unintentioned demise of their predators. Never before had any country adopted ecological solutions to the problems of their major crop in such a sweeping manner. The Presidential Instruction placed emphasis on promoting the use of a hormone which prevents the larvae from developing into adults, and a restricted range of conventional pesticides were permitted only when severe pest outbreaks occurred. These had been supposed to affect only the pest insects attacking the plants, but it is now known that other insects, including parasitoids, drink from the plants, and that the chemicals drip from the leaf tips into the water surface where many important insect predators live.
The Presidential Decree was effectively the start of Indonesia's world-leading position in IPM, replacing regular calendar spraying with a variety of biological and cultivation controls, and spraying only when defined levels of infestation were exceeded. In addition, within two years of the decree, Indonesia had removed all the pesticide subsidies. The benefits of the decree have been highly visible and this demonstrates how development which works with the natural ecosystem rather than against it can achieve dramatic results:
• yields have continued to increase and have become more stable;
• farmers are saving money previously spent on pesticides;
• the Government is saving $150 million annually on subsidies;
• the Government has made direct savings of $1 billion;
• no serious outbreaks of brown planthopper have occurred;
• water quality has almost certainly improved;
• over 300, 000 farmers have been trained for one season, and 2, 000 extension agents trained intensively for 15 months; and
• trained farmers pass on their knowledge and experience.
Thus the evidence supports the idea that insecticides have had a major destabilizing effect on overall yields, causing losses of millions of tons of rice, wasting billions of dollars in hard currency, and degrading the health and well-being of farmers and their environment. All this for the lack of understanding of how a rice field functions. The importance of ecology has perhaps never been so clear.
The problems of pesticide and fertilizer misuse will not be solved, however, until even more farmers have been trained and become confident to manage their fields, and until the banned pesticides stop being used on secondary crops on the same land where rice is grown, thereby eliminating useful predators and parasites. The continued availability of these pesticides also means that clandestine spraying on rice is very easy. It is clearly urgent to extend IPM to other crops.
NON-INSECT PESTS
Rats and birds are ubiquitous pests which also cause considerable damage. Soerjani (1980) considers rats to be the most important group of pests hindering agricultural production in Southeast Asia. Rats are known to occur in almost all agricultural crops and to damage many stored products (BIOTROP 1980; COPR 1978; Estioko 1980; Myllymaki 1979; Soekarna et al. 1980). They present the additional hazard of being a health risk to humans; the organisms causing scrub typhus, meningo-encephalitis, plague and leptospirosis are all known to be carried by parasites of agricultural or urban rats (Lim et al. 1980; Sustriayu 1980).
The number of bird species inhabiting rice fields, plantations and other areas of permanent agriculture is quite considerable. For example, 72 species were observed in these areas in Aceh by Chasen and Hoogerwerf (1941). This represents about half the number of bird species observed in primary and mature secondary lowland forest, but in fact only two species, or 1% of the combined species list, were seen in both areas. Seventy-five species were seen in clearings, pastures and young secondary growth, but when compared with those seen in agricultural areas only 23 (19%) were common to both lists. This serves to demonstrate how few of the forest birds can survive in disturbed areas. The birds of agricultural areas tend to have large geographic distributions and many can become pests. Among the more serious pests are: all of which are members of the sparrow family (Ploceidae).
|
White-rumped munia |
Lonchura striata |
|
White-headed munia |
Lonchura maja |
|
Baya weaver |
Ploceus philippensis |
|
Red avadavat |
Amandava amandava |
|
Pin-tailed parrotfinch |
Erythrura prasina |
Bird control poses considerable problems since birds generally range far wider and into more habitats than insect pests. Poisoning does not seem to be a viable control method because effective poisons are usually toxic to other animals, including man. Alternative methods, such as destroying communal roosts of weaver birds, can have temporary, local effects. In his account of bird pests, Adisoemarto (1980) repeatedly states that more ecological information is required for effective control programs.
A locally problematic group of pests on a wide range of fruit trees is fruit bats. Village-level control programs are rarely sufficiently selective and do not confront the problem that whereas some bats cause economic losses to fruit harvests (Adisoemarto 1980), others are essential to the fruit harvest in their role as pollinators (Khobkhet 1980) (p. 329). The flying fox Pteropus vampyrus is easy to shoot because of its size, but although it does eat fruit, observations reveal that it takes only fruit of a ripeness beyond what could be harvested and transported to markets.
RICE-FIELD ECOLOGY
Anyone who cares to sit by a rice field for a short while will see that the rice field is quite a complex ecosystem. The water surface is disturbed by the movements of mosquito larvae, the dragonflies hawk just above the rice and particularly around the bunds, and other creeping, crawling animals can be seen among the rice stems themselves. When it is considered that rice is the world's most important food crop, that it grows in probably the most complex ecosystem of any crop, it is remarkable that only now is the plant being studied from an ecological perspective.
The conventional wisdom is that the pests' natural enemies (predators and parasitoids) attack either herbivores or other natural enemies. In this simplistic system, changes in the size of the enemies' populations would necessarily follow behind those of the pests. There are in fact two additional pathways of energy flow, both of which are independent of the herbivores. In the first, predators and parasitoids feed on organisms such as the larvae of beetles, flies, and springtails, and bacteria, which decompose organic material or detritus in the soil, most of which is derived from weeds, rice straw, and algae. In the second, the bacteria are fed upon by zooplankton which also feed on phytoplankton. The large zooplankton are fed upon by filter-feeding organisms such as chironomid midges and mosquitoes which themselves become prey. In fields where insecticides are not used, the populations of filter-feeders and detritivores build up early in the season, causing the populations of natural enemies to increase accordingly. These in turn act as a buffer against damaging increases in pest populations. If pesticides are used early in the season, then the population of natural enemies is low, pest populations are released from natural controls, and the phenomenon of 'pesticide resurgence' occurs (W. Settle, pers. comm.). Linkages between these systems can be extended to form a simplified rice-field food web (fig. 12.1).
As a result of the Indonesian IPM programme, hundreds of thousands of farmers have learned that spiders are among the most voracious predators on rice hoppers and other small rice pests. Of the various species, the wolf spiders Lycosa pseudoannulata are the best performers, and are easily recognizable, having a fork-shaped mark on their back. They are highly mobile animals and colonize rice fields early in the development of the crop and can control the prey before damaging levels are reached. The female lives for three to four months and can lay 200-400 eggs during that time. After the spiderlings hatch, as many as 80 of them can be seen riding on her back. Wolf spiders do not spin webs but hunt their prey directly among the base of the plants, jumping over the water when disturbed. The spiderlings eat planthoppers and leafhopper nymphs, and the adults also eat stem borer moths. Each individual spider will eat 5-15 prey each day. IPM farmers know these statistics, not because they have been told them by an extension agent, but because they have all made these observations by housing planthoppers in cages - with and without spiders. The foundation of the Indonesian IPM programme is that farmers learn every important concept by doing experiments and making observations themselves.
Figure 12.1. Simplified food web of a rice field.
After Whitten et al. 1997
Wolf spiders are just one of a wide range of spiders and insects, including beetles, damselflies, earwigs, pond skaters, ripple bugs, ants and wasps, that prey directly on the pests of rice. Of the approximately 650 species of arthropods collected in rice fields by IPM researchers, some 65% are predators and parasites, 20% are detritivores and filter-feeding species, and 15% are herbivores (W. Settle, pers. comm.). In addition to all these there are also numerous pathogenic fungi and viruses.
Certain birds probably play a role in controlling insect numbers, and the more complex (layered) the vegetation is around the crops, the more predatory birds can be supported. Also, even simple measures, like placing bamboo poles around the fields to act as perches and lookout posts, enable birds such as drongos, treeswifts, wood swallows and falconets to hunt more efficiently and thereby to help the farmers. Exactly how beneficial they are has yet to be assessed. The same applies to two of the most conspicuous birds in some rice-field areas: the Javan pond heron Ardeola speciosa and cattle egret Bubulcus ibis. The herons forage in rice fields mainly just after they have been ploughed or planted and eat mainly dragonfly and waterbeetle larvae, molecrickets and spiders. The egrets are generally seen in wet rice fields while they are being ploughed, and on dry harvested rice fields, where they feed mainly on grasshoppers, crickets and spiders. The egret is generally considered to be beneficial since their main prey damage crops and compete with cattle (Kalshoven 1981). These rice pests include the short-horned grasshoppers Oxya japonica and Stenocatantops splendens, and the locust Locusta migratoria. The long-horned grasshoppers Conocephalus are also eaten but this situation is complicated by the fact that some members of the genus are beneficial by the fact of their predilection for the eggs of the pestilential stinkbug Leptocorisa oratorius. They also eat spiders but it is not known how this affects pest populations. The mole cricket Gryllotalpa sp. is a pest because it lives among the rice roots, feeding on the lower stem and loosening the soil which causes the plants to wilt (Kalshoven 1981).
Rice-field frogs are also beneficial biological control agents. Some animals eaten by the frogs are predators themselves, but the vast majority are potential pests. Rice also suffers serious damage from rats and introduced golden snails, and these are discussed elsewhere.
The Ecology of Rice-Field Rats
Rats cause the loss of least 12 million tons of rice each year worldwide, and it is possible that rats and birds combined may have more serious impacts on rice crops than all other types of pests (COPR 1976). In the Sunda Region rats are probably responsible for a 5%-6% overall loss of rice yield, but losses in individual fields can be far greater.
Rice fields in Sumatra are inhabited to different extents by at least eight species of rodent. The most serious pest among them is the rice-field rat Rattus argentiventer, which damages rice crops throughout Southeast Asia, although the wood rat R. tiomanicus and Whitehead's rat Maxomys white-headi are locally the dominant species (Adisoemarto 1980; Lim 1974; Soekarna et al. 1980). Two species of mice, the ryukyu mouse Mus caroli and the fawn-cauled mouse Mus ceruicolor, have been introduced into the northern half of Sumatra where they are confined to rice fields (Marshall 1977). It is possible that the large bandicoot Bandicota indica, which can measure 50 cm (head to tail) and weigh half a kilogram, has increased its range through introduction by man, because its flesh is commonly eaten. Two further species, the little rat Rattus exulans and the house rat Rattus rattus diardi, are generally found in peripheral areas but will enter rice fields to feed. These species are usually only found close to man's buildings or farms.
Analyses of stomach contents of these rodents has shown that they all eat insects and snails, and that the amount of plant material they ingest depends on the type of habitat and varies between species (Lim 1974). Rats eat rice at all stages of its development and although the theft of ripening rice grains is a direct loss from the harvest, the shredding of growing stems to eat the tender growing shoot does more absolute damage. A single, ripe seed head may represent a rat's daily food requirement, but the same rat may eat 100 growing shoots in a day before it is satisfied.
The reproduction of these rats clearly follows an r-strategy. The gestation period of the rice-field rat, for example, is 21 days, a litter generally numbers seven, and about eight litters are produced per female per year. Females could theoretically have more litters but reproduction tends to occur only when an abundant food supply is available, that is, towards the end of the rice-growing cycle. Male rats are capable of breeding at two months of age and female rats at 11/2 months (Lam 1983). The mean life span of the rat is only about 4-7 months (Harrison 1956), but even by seven months a single female could have raised about 20 young.
The very high intrinsic rate of growth among rats is the major reason why attempts to exterminate them using traps or poisons are effective only locally or for short periods. If any rats are left, their litter size is likely to be larger than normal (up to 11) because competition for food is less and so the population will increase quickly. If, as rarely happens, all the rats in an area are killed, the unoccupied rice fields represent a wonderful opportunity for rats from other areas to colonise. If one thinks of an animal's niche as its 'profession', then when rats have been exterminated from a field, hundreds of 'jobs' become vacant. It should be remembered that rats do not produce large numbers of offspring for the good of the species or because that many are needed to fill all vacant 'jobs'. They are produced because rats are locked into their r-strategy and each female produces as many fast-growing, healthy young as she can; in this way her hereditary line is not smothered by others. Many of the rats die young (as is shown by the life-span figures above) but this is a consequence of the r-strategy. If one mother rat produced just a few offspring to whom she devoted her time in fetching food and defending them, the available 'jobs' would be filled by the larger number of other rats from other mothers.
Thus, control of rats in the long term has to be centred on land management. We can take an ecological lesson from the dipterocarps, the family of relatively common trees in lowland forest (p. 221). These trees avoid undue loss of their seeds by all fruiting together with long intervals between fruiting periods. This prevents any particular seed predators maintaining a population of size that would pose a serious problem to the dipterocarps. Rats can live for only a few days on a diet of just rice stems (COPR 1976) and this may be the only food at certain times, particularly if insect populations are also controlled in some way. Thus, if rice is planted and therefore harvested at the same time over large areas, and if scrub and other neighbouring habitats - where rats could find alternative foods when rice grain was not available - were removed or utilised, rat populations could control be kept within bounds. These are thus two of the main methods of rat control encouraged by the Department of Agriculture, the others being the digging out of rat holes and poisoning (Soekarna et al. 1980).
The niches of the different rodent species in rice fields clearly overlap - they share a major food item and many of them make their burrows in the banks between the fields. Each species must compete in some way with the others and this must be important in determining the relative abundances of the species and therefore the amount and type of rice damage. Very little is known about the actual ecology of these animals, and detailed studies of their movements (Harrison 1958; Taylor 1978), food (Lim 1974) and inter-specific relations could be of considerable economic benefit.
PLANTATIONS
Introduction
It is sometimes said, without a great deal of evidence, that plantations (monocultures of trees) mimic the functions of natural forest ecosystem. This analogy contains a certain amount of truth but should not be taken too literally. Mature plantations certainly protect the soil, water and a few of the indigenous biota more than, say, rice fields, but they cannot approach the efficiency of mature natural forest. There is tremendous soil loss, for example, when forest or old plantations are laid bare prior to planting. Unfortunately, the myth that plantations are as good as forest in many aspects is perpetuated and is sometimes used as an excuse for extensive forest clearance.
The two main plantation crops in Sumatra, rubber Hevea brasiliensis and oil palm Elaeis guineensis, have rather different ecological characteristics.
Rubber is intolerant of swampy conditions and, in the past, poorly drained areas within a plantation were left as forest or secondary growth from which various forms of wildlife could make sorties into the plantations. Conversely, under natural conditions, oil palm is a tree of riverine forest, but those now in cultivation are tolerant of drier conditions. Almost no residual areas of forest are left in an oil palm plantation because virtually all the land can be utilised.
The branches of rubber form a stable and long-lasting substrate on which nests of squirrels and birds can be built. Large birds-nest ferns Asplenium nidus and other epiphytic ferns are common on old trees. The leaves of oil palm, however, continually change their position - from the vertical young spear to the dangling dead leaf. Only a few animals, such as weaver birds, are able to make nests on these leaves. The dead leaf bases remaining on the tree are colonised by various epiphytic ferns which, because of the generally very moist air, are extremely abundant (fig. 12.2). A useful guide to epiphytic ferns has been written by Piggott (1979).
Figure 12.2. Three species of epiphytic fern commonly found in plantations.
Harvesting the rubber latex usually disturbs nothing above head height, whereas the cutting of the infructescence away from the crown of the oil palm disturbs the only place where animals could set up a permanent base. The only animals that seem to be tolerant of this more or less weekly disturbance are squirrels (Duckett 1982) and rats, and the control of the latter is discussed below.
Rubber has one major fruiting season each year. The fruit are hard and have limited appeal to wildlife. Oil palm produces oil-rich, brightly coloured fruit which are obviously far more attractive as a food source (Duckett 1976).
The plant species covering the ground between the oil palm or rubber trees also influence the general abundance of animals (Ahmad 1980). For example, ferns and long grass between short oil palms provide an excellent environment for rats. Poorly kept smallholder estates where rubber trees are densely planted and many other plants are allowed to grow have a more diverse biota but do not make economic sense.
Large Pests
The major reason that plantations sustain a very low diversity of wildlife is the highly restricted floral diversity. A walk of 500 m through a well-maintained estate is unlikely to reveal more than 15 plant species (including ferns). This leads to an impoverished fauna of pollinators, leaf-eaters, parasites on other insects, etc. The impression gained in certain areas is that monkeys and other animals live in plantations. These have almost always come from an area of neighbouring forest or an isolated forest block (Bennett and Caldecott 1981), or are immature animals seeking unoccupied forest to avoid personal extinction. Visits by large animals to plantations, particularly young oil palm plantations, can cause severe economic loss. The main offenders are elephants and forest pigs. In these cases, which are reported almost every month and are a continual threat in certain places, it is pointless to blame the animals. The situation is almost always caused by careless land-use planning. If a favoured elephant food (such as sugar cane or young oil palm) is planted next to elephant habitat or across a traditional elephant pathway (Groeneveldt 1936, 1938; van Heurn 1937), of course the elephants will eat the food, and there are few effective ways of dissuading elephants from their avowed intention.
The answer to the elephant (bear/pig/ tiger) problem lies in the formation of buffer zones around forested natural ecosystems. These zones aim to:
• contain plant species which are low-grade food items for potential agricultural pests so that the animals are not attracted to the area, and this in turn will make their predators search elsewhere for food; and
• provide local inhabitants with products they had traditionally taken from the forest (timber, rotan, medicine, firewood), or provide a cash income to enable the people to buy these products.
Buffer zones and the merits of different potential crops are discussed by MacKinnon (1981).
Rats and Their Predators
As described on page 368, one of the consequences of forested land being converted to other forms of land use is that some species of birds have increased in number or have entered and colonised new areas. One such is the barn owl Tyto alba, one of the world's most widely distributed birds, which has spread from Java into southern Sumatra (Holmes 1977) and has thence spread quickly into northern Sumatra.
In other parts of its range, the barn owl eats a range of small animals but in plantations it appears to feed almost entirely on rats, particularly the wood rat. The diet of owls is easy to determine because they regurgitate pellets containing the fur and bones of their prey about 8 to 12 hours after a meal. These pellets may be collected from beneath the roosts. Of 2, 839 pellets examined in Peninsular Malaysia, 90% contained remains of rats and the rest were composed of insect, shrews and frogs (Lenton 1980). More than half (55%) of the rats were rice-field rats, a third (32%) were wood rats and the remainder (13%) were little rats. Whether this reflects the actual relative abundance of the rats is unknown. The rats have different activity patterns and some may successfully avoid owls in this way. For example, house rats and rice-field rats have finished most of their activity by midnight, but wood rats maintain moderate activity all night long. House rats start becoming active before dusk, rice-field rats at about dusk and wood rats after dusk (Ahmad 1980).
The owls reproduce rapidly; the clutch size averages 6.6 eggs per nest and two or three broods can be raised per year. At this level of production an adult pair accounts for about 1, 300 rats per year in the feeding of themselves and their young (Lenton 1983).
Rats in oil palm plantations live at considerable densities - an estimate for the wood rat is 250/ha (Wood 1969) and so barn owls can be an extremely important control agent and some plantations are breeding owls in captivity and then releasing the young at a nest box site. Nest sites for barn owls in oil palm plantations are limited and so nest boxes (0.5 x 0.5 x 1 m) on the top of telegraph poles have been provided. There is a limit to how close adult pairs can live to each other and 20 ha has been suggested as the minimum area required. This area could contain 5, 000 rats so that the owls could be responsible for the removal of about a quarter of the standing crop of rats per year. (It should be remembered that this is less than a quarter of all rats that live in an area during any period because rats are continually reproducing and dying.) In a study of the effects of owl predation on small mammal populations in England, it was found that 20%-30% of the standing crop of mice was taken every two months (Southern and Lowe 1982).
Snakes also appear to be significant predators on rats in oil palm plantations and for this reason plantation managers make little effort to control them. The stomach contents of five snake species from an oil palm plantation in Peninsular Malaysia were examined. Rats predominated, but other vertebrates included frogs, lizards and birds. The results are shown in table 12.1. Five live individuals of each of the five species were kept in cages and for one year five wood rats were placed in each cage at the start of each week. On the seventh day any remaining rats were removed prior to a new set being provided.
To calculate how many rats a snake of each species would devour in nature, where the whole range of prey would be available, the figures have to be adjusted. Thus the experimental feeding rate is multiplied by the percentage of dead snakes examined which had rats in their stomachs (table 12.2). So, if an average rat-eating snake eats 1.5 rats per week, it would consume 78 rats per year. The density at which these snakes live is not known but 2/ha would not be unreasonable. With a standing crop of 250 rats per ha, it can be seen that the snakes do take a considerable proportion (Lim 1976).
Pest Control by Predators
Caution is needed before one concludes that a predator, particularly a vertebrate predator, is actually controlling the number of its prey (Erlinge et al. 1984). Tigers and other large forest predators certainly do not control the numbers of deer, pigs, mouse deer or other common prey species. Detailed studies of large predators have dispelled the myths that these are master killers from which a prey animal, once detected, has no chance of escape. Instead, these large predators tend to take the young, the old and the sick. The remainder can successfully outrun, outwit or intimidate the predators such that a relatively small percentage of hunting attempts end with a kill.
After Lim 1976
Many insect predators, however, certainly do control the numbers of their prey species. This was clearly demonstrated in the 1960s and early 1970s when DDT and other long-lasting poisons were used to combat pests such as bagworms (a common pest on perennial crops) in oil palm plantations. In many cases the pests escaped the spraying because of behavioural characteristics. In the case of bagworms, the worst of the pests, for example, the larvae, pupae and adult females are protected by the bags they construct around themselves (fig. 12.3). In contrast, the 20 or so species of wasps that laid eggs in the young bagworms, and the larvae of which slowly ate the bagworm from the inside, had no such protection and were killed by the chemicals. The result was an increase in the bagworm population. A cessation of spraying resulted in an increase in predators and a decrease in bagworms (Conway 1972, 1982). Thus the predator wasps were truly controlling the bagworm population.
The major distinction between tigers and parasitic wasps in the way they predate is that only the wasp will certainly kill its prey every time it attacks. In addition, the tiger's prey can try to evade the tiger by running away but the wasp's prey cannot. Even with this efficiency, the wasps are highly unlikely to reduce bagworms to extinction because the more they predate upon the bagworms the harder it is for the wasps to find the remaining individuals or populations. If prey are hard to find, the population of wasps will decrease, which therefore reduces the pressure on the bag-worms, thereby allowing them to increase, and so on. Thus, after the wasps have reduced the bagworm population to a certain level, it is the numbers of prey that control the numbers of predators. The relationship between owls and rats, however, is similar to that between tigers and deer.
After Lim 1976
Figure 12.3. Bagworms (Psychidae) are serious potential pests on a range of perennial crops. The caterpillars make bags of silk which they cover with twigs, etc. (species differ in the material used). When moving, only the head, thorax and legs are exposed but if disturbed all these are pulled inside the bag which is then closed. The caterpillars pupate inside the bag and the male emerges as a typical grey/brown moth. The female emerges as a degenerate, wingless adult, capable of little more than building her own bag and producing eggs. The male copulates with her through the open end of her bag and the newly hatched caterpillars leave the parental bag and start making their own bags, often using material from the parental bag. The adult female dies inside her bag.
In the section on rats and their predators (p. 395) it was stated that pairs of barn owls are unlikely to live in areas of less than 20 ha, even if the prey density increases. So while territory size for animals is related to the abundance of available resources, there are strict upper and lower limits. Detailed studies of owls and their prey in temperate and arctic regions, where the size of rodent populations exhibit considerable natural fluctuations, have shown that the resident population of owls remains more or less the same, but the number of young birds raised varies considerably depending on the availability of prey (Pitelka et al. 1955; Sastrapradja 1979). This is a reflection of the striving to maximise reproductive success. Insufficient studies have been conducted in plantations and other relatively constant ecosystems in the Sunda Region to know whether similar natural fluctuations occur in their rodent populations (Flemming 1975). Fluctuations are created in plantations, however, by the rat poisoning programs which usually severely reduce the populations. The number of eggs laid by the owls, and the number of young raised, would be expected to reflect these population changes.
Owls are not going to defeat the rat problem but they may be seen as a means by which the period between rat poisoning programs can be lengthened because owls can slow the rats' population growth.