Chapter Ten
INTRODUCTION
Very few studies have ever been conducted in Sumatran caves. Musper (1934) explored a number of caves in the hills around Gumai (near Lahat), and Bronson and Asmar (1976) describe a little of the biology of Tiangko Panjang cave in Sarko, Jambi (p. 53). Some brief studies of a few caves in northern Sumatra have been conducted by teams from CRES but the most detailed collecting work was undertaken by van der Meer Mohr (1936), also in North Sumatra. He describes the fauna of two caves south of Medan (also visited by a CRES team) and one near Balige, south of Lake Toba.
Most caves are found in limestone areas (fig. 8.10) but 'caves' were also created during the Japanese occupation when tunnels were constructed. These have many of the same features as natural caves and are not without interest. For example, the moss-nest swiftlet Aerodramus vanikorensis was found for the first time in Sumatra in a tunnel in the famous Ngarai Simanuk Gorge near Bukittinggi (Wells 1975). Even road tunnels, such as that between Kota Panjang and Bankinang, Riau, support populations of bats and swiftlets.
CAVE STRUCTURE
Rain water contains carbon dioxide from the atmosphere and is therefore slightly acid. This weak acid dissolves calcium carbonate (the main constituent of limestone) and forms channels which, in time, achieve the dimensions of caves, often with a stream running through them. Water dripping from the roof in the main chamber of the Quarry Cave, Lho'Nga, North Aceh, had a pH of 5.0. Dry caves exist where the stream flow has been diverted or where the limestone has been raised or tilted relative to the surrounding land. In Sumatra, most of the limestone emerges at intervals along the length of the Barisan Range and dates from the early and late Tertiary. The caves themselves can date from anytime between then and the present.
Figure 10.1. The formation of stalactites and stalagmites, a - water evaporates from drips, leaving deposits of calcium carbonate and impurities thereby forming a stalactite; b - water evaporates, also leaving a deposit but forming a squatter stalagmite; c - where water drips quickly no stalactite is formed; d -stalactite and stalagmite eventually join to form a single column; e - where the drips from a stalactite fall (or fell) into a river, no stalagmite is formed.
Two of the commonest features within a cave are stalactites and stalagmites which are columns of calcium carbonate with various impurities (which are the cause of the wide range of colours found). Their formation is described in figure 10.1. The surface of stalactites and stalagmites increases the surface area of a cave and therefore the living area available to its inhabitants.
The nature and depth of cave deposits and the manner in which they lie on one another can provide information on the past history of the cave and the surrounding area. Pollen grains may be found in organic and inorganic deposits and these too can help us to reconstruct the conditions of palaeo-environments.
THE CAVE AS A HABITAT
The salient features of a cave are its definable limits, its enclosed nature and the consequent reduction of light and comparative stability of climatic factors such as temperature, relative humidity and air flow (Bullock 1966). Variations in these features create a surprisingly wide range of effects which determine the type and number of animals which can inhabit a cave. Cave animals are divided into three ecological groups: (a) troglo-bites, obligate cave species unable to survive outside the cave environment; (b) troglophiles, facultative species that live and reproduce in caves but that are also found in similar dark, humid microhabitats outside the cave; and (c) trogloxenes, species that regularly inhabit caves for refuge but normally return to the outside environment to feed. Some other species wander into caves accidentally but cannot survive there (Howarth 1983).
The floor of most dry caves is composed largely of a layer of material formed from waste products and bodies of animals. A sample of 'soil' was taken from Kotabuluh cave, Tanah Karo, and the analysis (table 10.1) showed very low levels of carbon and nitrogen, but a high level of potassium and an extremely high level of phosphorus. Not surprisingly, local villagers are extracting this 'guano' to sell as fertiliser (see also the paper by Khobkhet [1980]).
Darkness
Exclusion of Photosynthesising Plants. Since light is essential for photosynthesis, it would be reasonable to expect that green plants are excluded from the dark parts of caves. This is essentially true for almost all caves but a form of 'lichen' consisting of a colony of gram-positive bacteria with a few blue-green algae between it and the substrate, has been reported from a cave wall in 'absolute' darkness in Nepal (Wilson 1977, 1981). The blue-green algae had high densities of thylakoids (the membranes on which photosynthesis occurs), and increased thylakoid density leads to greater photosynthetic efficiency. Interestingly, what appears to be a very similar 'lichen' was found at Kotabuluh cave by a CRES expedition. The most important effect of this virtually total exclusion of green plants is to make all cave-dwellers dependent on material brought in from the outside and to exclude all animals which feed directly on the above-ground parts of green plants. Plant roots can penetrate fissures above a cave and can commonly be seen attached to or hanging from the cave roof. Fungi and bacteria are not, of course, dependent on light and their role in caves is discussed below.
Figure 10. 2. 'Lichen' from Kotabuluh cave, Tanah Karo.
Vision. In near total darkness a cave-dweller becomes reliant on senses other than sight to detect food or enemies. This is not peculiar to cave animals - many nocturnal and cryptic animals depend almost exclusively on hearing, smell and touch (Bullock 1966). Clearly, however, diurnal animals can live in a cave provided their other senses are sufficiently acute, and sight may even be useful to animals ranging into the twilight areas, and also to those that range outside caves. For obligate cave-dwellers (troglobites) the presence or absence of eyes is immaterial, although the lack of evolutionary selection to maintain good sight permits deleterious variation to appear, resulting in the poor visual acuity characteristic of cave-dwellers in general and the total blindness of some (Holthuis 1979; Peck 1981; Roth 1980).
Figure 10.3. A Sumatran cave cricket showing its extremely long antennae.
Adaptation and Modification to Life in Darkness. Adaptations to life in darkness are not particularly associated with cave-dwelling. Troglobites, together with nocturnal animals and troglophiles, tend to have their non-visual senses well developed so that they are able to reproduce, eat and move around in the darkness. Some, for instance, have very long appendages, such as the legs of scutigerid centipedes and the antennae of cave crickets (fig. 10.3). Antennae can also function as chemoreceptors and may be sensitive to relative humidity (Howarth 1983). Other animals such as many bats and most swiftlets Aerodramus spp. (Medway 1962) have developed the faculty of echo-location. A sound is produced and the echoes which reflect back from solid objects are interpreted to give a 'picture' of the surroundings. The swiftlets click their tongues and produce a low-frequency sound (1.5 to 5.5 kHz) which is audible to humans and sounds like a wooden rattle. This enables them to detect large objects, allowing them to navigate, nest and breed within a totally dark cave, but is not sufficiently accurate to enable them to catch insects at night (Medway 1969a). It might be thought that the different activity periods of bats and swiftlets would represent temporal partitioning of a common food resource. In fact, swiftlets feed mainly on small wasp-like insects (Hymenoptera) (Hails and Amiruddin 1981; Medway 1962), whereas insectivorous bats concentrate on various moths and beetles (Gaisler 1979; Yalden and Morris 1975). Bats do interact ecologically with nocturnal birds and this is described elsewhere (Fenton and Fleming 1976).
Echo-location in bats has evolved independently at least twice, but in each case it is characterised by high-frequency sounds (20-130 kHz), above the threshold of human hearing, originating from the larynx or speech-box, and by the reception of echoes in complex and often large ears.
Mouse-eared bats (Verspertilionidae) use a predominantly frequency-modulating (FM) system; that is, the frequency of the sound they emit through their mouth varies and is given in very short pulses. When cruising in the open a pulse is emitted and some time is spent listening for echoes. When closing in on the prey, however, pulses are emitted rapidly so that the exact locations for the flying insect can be determined. A flying mouse-eared bat can detect objects less than 1 mm across.
Horseshoe bats (Rhinolophidae and Hipposideridae) mainly use a single rather than a variable frequency and each species uses a characteristic frequency. Instead of emitting the sound through their mouth, these bats keep the mouth shut and emit the sound through their nostrils which are positioned half a wavelength apart to give a stereo impression. The peculiar 'horseshoe' around the nostrils has the function of a megaphone, causing the sound to be emitted in a concentrated beam.
It used to be thought that bats using echo-location had no difficulty catching their insect prey, but it now appears that some moths can detect bats from 40 m away and before they have been detected. These moths have developed the ability to utter clicks which confuse the bat. Other moths have a variety of responses which makes it difficult for the bats to predict their behaviour. Some bats in their turn do not keep their echo-locating system 'switched-on' continuously so as to give little warning as possible to the moths, while others emit frequencies above the moth threshold of hearing (Fenton and Fullard 1981).
Whereas the bats mentioned above catch flying insects, false vampires (Megadermatidae) feed by picking insects off leaves, or lizards, frogs and small rodents off the ground. They have also been known to eat bats caught in mist-nets without themselves getting caught in the net (Medway 1967). To avoid swamping the echoes from their prey they 'whisper' their sounds which are FM like those of mouse-eared bats. Sometimes false vampires locate their prey solely by homing in on sounds made by the prey itself. This is similar to a frog-eating bat which has been studied in Panama and which can differentiate between the calls of edible and poisonous frogs and between the calls of small frogs and frogs too big to capture. Its efficiency at catching frogs has probably led to adaptations in the frogs' calls so that the males still call to attract females but in such a way as to reduce their chances of being caught (Tuttle and Ryan 1981).
Finally, the only fruit bat to echo-locate, the cave-dwelling rousette bat Rmisettus, uses a tongue-click like swiftlets (Fenton and Fullare 1981; Yalden and Tuttle 1975).
As well as the loss of eyes, many troglobite invertebrates have also lost cuticle pigments (van der Meer Mohr 1936) and wings. Some have developed a thinner cuticle, a larger, more slender body than their relatives outside the cave, adaptations to their feet allowing them to walk on wet surfaces, and also a lower metabolic rate (Barr 1968; Howarth 1983).
Diurnal Rhythms. Most animals have a clearly defined daily cycle of activity. Nocturnal species are active at night, diurnal species during the day, and crepuscular species around dawn and dusk. Such cycles are obviously associated with daylight and darkness and thus may not occur in a cave community. There are, however, certain events within a cave which may impose a daily rhythm on the inhabitants. The most important of these is the departure, and later return, of the bats. In their absence, food is not available for the predators and free-living ectoparasites in the roosts, and there is a halt to the rain of fresh faeces from the roof (Bullock 1966). Although data are lacking, it is probable therefore that a diurnal rhythm exists despite the absence of night and day.
Temperature, Humidity and Air Flow
The insulating role of the walls and roofs of caves effectively buffers the wide daily variations in temperature and humidity of the outside world. During the CRES expedition to the diy cave near Batu Katak, Langkat, it was found that the humidity in the cave did not fall below 97%, although the outside humidity at midday was about 75%. Similarly, the temperature in the cave varied between 24°-32°C. As would be expected the maxima and minima of humidity and temperature in the cave occurred some time after those outside. Conditions thus remain fairly stable from day to day, but there are still seasonal changes which can greatly alter conditions in caves. For example, during a rainy period the humidity and amount of free water within a cave tends to increase. Thus a chironomid midge which requires free water for breeding may only be able to reproduce when seasonal pools are full of water, while at other times the pools are dry and its numbers dwindle. Fluctuations also occur in other insect and bat numbers; some bats show distinct periodicity in breeding and mortality rates, but there is no actual evidence that this is caused by changes in the environment (McClure et al. 1967). These bat cycles affect other organisms: heavy mortality of bats permits a rapid increase in corpse-feeding organisms which would diminish later in the year due to lack of food.
Air movement is also buffered by the cave walls but still occurs as air is drawn out of the cave during the day when the air outside is warmer and lighter. This air movement follows a regular pattern but leaves pockets of stagnant air where spiders can weave delicate and complex webs, and preserves pockets of high humidity. Under such stable conditions the air disturbance caused by the approach of predators or prey may be discerned.
The constant high humidity in the deeper parts of caves appears to have led to many troglobite arthropods becoming morphologically similar to aquatic arthropods. In these deeper areas, the carbon dioxide levels may become relatively high if there is no inflow of air except from the cave mouth. Howarth (1983) has suggested that the lower metabolic rate of some troglobite arthropods may be a physiological response to this high carbon dioxide concentration. Many, if not most, of these troglobites can survive long periods of starvation, gorging themselves when food is available and storing a large amount of fat.
FOOD
As noted on page 315, all cave-dwellers are dependent for food on material brought into the cave from outside. Some animals feed on the plant roots attached to the cave roof, wood and other material washed in during floods (if the cave has a river running through it), or the organic matter percolating through from the surface. In Sumatra, however, the major providers of food are the bats and swiftlets which roost and breed in the cave but feed outside. Other cave shelterers such as porcupines may also contribute, but to a negligible extent in most caves.
Bats and swiftlets supply food in a number of ways:
• during their lives they produce faeces, collectively known as 'guano', which has nutritive value and is fed upon by various animals (coprophages) as well as providing a source of nourishment to fungi and bacteria;
• as live animals, they are hosts to many parasites, both internal and external, and provide food for predators;
• they moult hair and feathers and shed pieces of skin;
• they produce progeny which may be susceptible to different predators and parasites. For instance, eggs of swiftlets are attacked by a cave cricket Rhaphidophora oophaga which does not attack the birds themselves (Chopard 1929);
• when the bats and swiftlets die, their bodies form a source of food for various corpse-feeding organisms (necrophages).
Almost all animals provide food for others in these five ways, but within the cave ecosystem, in the absence of green plants, these are the only major sources of food.
The bats themselves roost on the roof and walls and form the basis of one community; their faeces and dead bodies fall to the cave floor and form the basis of another. Thus there is a distinct division of the animals into a roof community and a floor community (Bullock 1966).
Roof Community
The roof community includes all those animals which feed on live bats and swiftlets. These include only one major predator, the snake Elaphe taeniura. The snake waits, coiled in a crevice, until a bat alights nearby. Then, bracing itself with part of its body within the crevice, the snake lunges out and captures its victim, which is crushed in a quick coil and swallowed before the snake withdraws back into the crevice. The only other major predator on bats is the hawk Macheiramphus alcinus which roosts near large caves (Eccles et al. 1969).
There are many parasites, some internal, but mostly external. Some, such as the wingless nycteribiid flies (Marshall 1971), live almost their entire lives on bats, others, such as streblid flies and chigger mites (Trombiculidae), spend only part of their life cycle on bats, while some, such as the soft tick Ornithodoros, attach themselves to bats only when they are hungry (fig. 10.4). Most of these suck blood, but the large earwigs of the family Arixeniidae eat skin debris. These earwigs are quite large - equivalent to an insect 32 cm long on an average Sumatran man. A survey of parasites on bats in Thailand revealed no less than 116 species belonging to 13 families of insects and mites (Lekagul and McNeely 1977). It has been suggested that the great majority of insect species parasitic on tropical bats do in fact have only one species as a proper host (Marshall 1980). In caves containing nests of swiflets, the community is even more diverse, for these birds are also hosts to a variety of mites and insects.
The parasitic insects on bats show considerable convergence in many characteristics, although six families from four orders are represented. Many of them are flattened, some vertically and some horizontally, to ease their movement between the bat hairs. Most of the insects have tough but expandable 'skin' which allows for large meals of blood from the host. The skin often bears backward-pointing spines which lessen the chance of being dislodged by a scratching bat. They also have well-developed grasping claws. Wings have been lost in species belonging to all but one of the insect families concerned. The loss of wings and general lack of light have led, not surprisingly, to the loss or reduction of eyes in these parasitic insects (Marshall 1980).
Figure 10.4. Common parasites of bats: a) Ornithodoros - a soft tick; b) a nycteribiid fly; and c) streblid fly.
After Lekagul and McNeely 1977
Floor Community
On the cave floor coprophages and necrophages predominate. It is difficult to distinguish between these two; although a few forms are exclusively necrophagous, many of the coprophages will include dead bats or swiftlets in their diet. The majority of cave-dwelling bats are insect-eating and the faeces they produce are hard and dry and readily exploited by coprophages such as wood lice, cocoon-encased caterpillars of Tinea moths, flies and beetles. The faeces produced by the few species of fruit-eating bats that roost in caves, however, are amorphous and wet and are not generally utilised by coprophages. In this case, cockroaches ingest the faeces and the general coprophages feed in turn on the faeces of the cockroaches (Doyle 1969). Fungi and bacteria develop on the faeces and some coprophages exploit this food resource too. The cave crickets and small Psocoptera flies may feed on fungi (McClure et al. 1967).
The floor community includes many predators such as the scutigerid centipede, assassin bug Bagauda, and medium-sized spiders which feed on the coprophages and the small Tinea moths. Some of these predators and others such as large spiders, live on the walls and wait for wandering coprophages to come to them, or only venture to the ground when hungry. Small predators may enter the diet of the larger predators such as shrews Crocidura, the toad Bufo asper and the spider Liphistius. Liphistius is the world's most primitive spider as shown by its abdomen which, unlike that of other spiders, is still segmented. One species lives on dry stalactites, and builds large web-cases 4-5 cm long with a door about 2 cm across. These are loosely constructed, decorated with debris from the surrounding area, and the inside is lined with smooth white silk. The hinged flap of the door is held partly open by the spider crouching behind it. Radiating from the entrance in a semicircle are six to ten strands of silk, 12 to 15 cm long. If a small insect touches one of these the spider races out, catches the prey and returns (McClure et al. 1967). No species of Liphistius has yet been collected in Sumatran caves but the larger Liphistius sumatranus can be found on road banks near Bukittinggi (Bristowe 1976a, b; Whitten, pers. obs.).
Food Webs and Pyramids
The various relationships described may be drawn as a food web (fig. 10.5). As the understanding of a cave ecosystem grows, so its food web becomes more and more complex. For instance, for simplicity's sake, figure 10.5 omits the role of certain wasps parasitic on the tinaeid moths which themselves must be preyed upon by one of the predators. As food webs become more complicated (and it should be remembered that caves probably represent Sumatra's simplest ecosystem), there is good reason to produce generalised representations or models of food webs. In 1927, Charles Elton, one of the 'fathers' of ecology, noted that the animals at the base of a food chain are relatively abundant while those at the top end are relatively few in number, with a progressive decrease between the two extremes. This 'pyramid of numbers' is found in ecosystems all over the world and provides a useful means of comparing communities. To construct the pyramid, species are grouped together according to their food habits. Thus all the autotrophs (plants) are called 'primary producers', herbivores are called 'primary consumers', predators on herbivores are called 'secondary consumers', predators on secondary consumers are called 'tertiary consumers', etc. Generally, in a defined area, a large number of primary producers support a smaller number of primary consumers supporting an even smaller number of secondary consumers supporting one or two tertiary consumers (fig. 10.6a). Variations of the pyramid shape occur when, for instance, a single tree is considered (fig. 10.6b). An inverted pyramid can be formed if one considers a single animal, such as a bat, or a plant, which carries a large number of parasites, which are themselves parasitised by an even larger number of hyperpara-sites (fig. 10.6c). The information contained in a pyramid of numbers permits us to state the number of herbivores supported by a certain number of plants and so on. But for comparisons between ecosystems, a better approach is to use the weight of organisms (biomass) rather than numbers, and to show this as a 'pyramid of biomass' which usually has a similar shape to the pyramid of numbers (Phillipson 1966).
Figure 10.5. Simplified food web of a cave ecosystem.
Figure 10.6. Pyramids of numbers: a) with a large number of primary producers, b) with a single primary producer, c) the case of parasites and hyperparasites.
After Phillipson 1966
In Liang Pengerukan near Bohorok, Langkat, a team from CRES counted all the animals in 25 m2 of cave floor (or, in the case of wood lice, several 1 m2 samples within that area), and samples of these animals were later weighed. The total biomass of the 25 m2 was 63.38 g (2.5 g/m2), 90% of which was contributed by a single species of wood louse which also accounted for over 99% of the animals found. The heaviest animal, a cave cricket, was 900 times heavier than the lightest, the wood louse. A pyramid of numbers would overemphasise the wood lice, but a pyramid of biomass (fig. 10.7) demonstrates clearly the relationship between the 'amount' of primary and secondary consumers. Note the absence of producers. Tertiary consumers such as large spiders were seen in the vicinity but not in the sample area.
Figure 10.7. Pyramid of biomass using data from Liang Pengerukan.
DIFFERENCES WITHIN CAVES
Although a cave at first sight appears to be a fairly uniform habitat, this is not the case. One of the chief factors causing variation in the cave habitat is the distribution of bats, the producers of guano. Cave maps produced by CRES and by Doyle (1969) (fig. 10.8) show that the occupation of the roof by bats is very patchy. Physical factors also vary from place to place in the cave; thus, during a rainy period, standing water will accumulate in one part but not another. Similarly, some parts are subject to air movement. Others are not.
It is differences such as these that make different parts of the cave suitable for different species. Such factors probably explain the occurrence in Batu Caves, Selangor, Malaysia, of no less than 11 species of Psychodidae (McClure 1965), a family of moth-like flies (fig. 10.9). (Outside of caves the larvae of this family are important in sewage purification because they feed on microorganisms which would otherwise block the filter beds of the sewage plant.) The coexistence of this number of species in a cave requires that they each exploit a different set of limiting resources or, possibly, a similar set of limiting resources to different intensities. This is the principle of competitive exclusion, or Gause's theorem (named after the Russian scientist who first tried to make closely-related species coexist on the same nutritive medium in the laboratory). Many closely related species cannot live in the same place because their ecology and behaviour are too similar - that is, they compete for the same limiting resource to the exclusion of one or more of the species. This is illustrated by the exclusive distribution of three ecologically and behaviourally similar species of leaf monkey on Sumatra for which no area of overlap is known, and of the two species of freshwater crayfish described on page 155. The word 'limiting' is included in the statement of the competitive exclusion principle because only resources that limit population growth can provide the basis for competition. Non-limiting resources, like atmospheric oxygen, are superabundant compared to the needs of organisms, and their use by one organism does not preclude their use by another organism. So, although the 11 species of Psychodidae are all blood-suckers they must, somehow, be successfully dividing up the limiting resources between themselves.
Figure 10.8. Maps of caves to show distribution of bats: a) a plan of Gua Pondok, Malaysia; b) a plan of Liang Pengerukan; c) an elevation of Liang Pengerukan taken between points A and B.
After Doyle 7969
Figure 10.9. A psychodid fly.
DIFFERENCES BETWEEN CAVES
If different caves are considered, conditions vary still further - one may be well drained while another may have a river running through it; one may contain swiftlets, another may not. There are almost no data by which the fauna of different Sumatran caves can be compared, but some data were collected by CRES for the caves around Batu Katak. Although not exhaustive, the lists (tables 10.2 and 10.3) serve to show considerable differences.
A knowledge of comparative cave fauna richness is of great value because it provides a useful biological input to environmental impact assessments for limestone mines (such as at Lho'Nga and Indarung). In most cases it would be possible to avoid disturbing a particularly rich cave and to exploit another area. It should be remembered that the disappearance of a cave fauna can affect an area far wider than the cave itself. For instance, people would no longer be able to exploit (in a controlled, ecologically sustained manner) the extremely rich guano for fertiliser; the loss of insectivorous bats would mean an increase in local insect populations; and the loss of the cave fruit bat Eonycteris spelaea would result in fewer durians Durio zibethinus which they pollinate.
Durian trees usually have two flowering and fruiting periods each year, although the timing of these varies between localities. Durian flowers are large, feathery and white with copious nectar, and give off a heavy, sour and buttery odour. These features are typical of flowers which are pollinated by certain species of bats while they eat nectar and pollen. From research conducted in Peninsular Malaysia it appears that durians are pollinated almost exclusively by cave fruit bats (Soepadmo and Eow 1977) and thus we owe the production of durians to these bats and thence to the limestone caves in which they live.
When durian trees are in flower, cave fruit bats feed enthusiastically upon them, but they also feed on and assist the pollination of other food trees such as banana, mango, petai bean, jackfruit and rose-apple (Marshall 1983, 1984; Nur 1976; Start and Marshall 1975). They also feed on the flowers of a common tree of mangrove forests Sonneratia alba, and studies of pollen remains in the faeces of cave fruit bats in Batu Caves near Kuala Lumpur have revealed that these bats regularly fly to mangrove areas nearly 40 km away (Marshall 1983; Start and Marshall 1975).
Only two roosts of cave fruit bats have been recorded in North Sumatra, both of them discovered by teams from CRES. The first roost was found in one of the caves, Liang Rampah, near Penen south of Medan, and contained an estimated 1, 000-2, 000 individuals; none of the other five caves in the area were inhabited by this species. Interestingly, this bat does not seem to have been present when van der Meer Mohr (1936) visited the cave in 1935. A colony of similar size was found at Kotabuluh cave. Assuming that these bats can also fly at least 40 km to a food source at night, the areas they cover (± 1, 250 km2 per roost) include the famous durian-growing area around Sembahe/Sibolangit. However, only the Penen bats are likely to visit the other famous durian farm around Binjai (fig. 10.10). Although there may well be other roosts of this bat in the area, it is reasonable to assume that Penen cave is the only source of cave fruit bats for at least some of the area shown. If the value of the intact, undisturbed cave is to be estimated we would need to assess the value of durians for durian orchard owners, small-scale owners and collectors, roadside vendors, wholesalers, distributors and town vendors. Owners of mature durian orchards can earn considerable sums of money.
Some people may believe that cave fruit bats are 'only bats' and do not deserve protection, but our choice is clear: leave the bat roosts undisturbed and enjoy durians, or destroy the roosts and be content to have only the rarest taste of the fruit. There is almost no chance of successfully translocating bat colonies because, although we know that the cave fruit bat inhabits very few of the available caves, we do not know precisely what cave qualities it requires.
Data from CRES expeditions and from van der Meer Mohr 1936
Data from CRES expeditions and from van der Meer Mohr 1936
Figure 10.10. Areas within which cave fruit bats from Kotabuluh and Penen caves could pollinate durian flowers.
CAVES AS ISLANDS
For some cave animals, such as some species of cockroach and scutigerid centipede, the outside world is an acceptable living place, and they merely move into a cave when the opportunity arises. But many cave animals, such as species of the primitive spider Liphistius, the cave cricket and the assassin bug Bagauda, are more or less confined to caves. Their caves, then, represent habitat islands set among inhospitable habitat seas or, perhaps, barely hospitable habitat seas occupied by inhospitable animals.
As with other form of islands, dispersal of animals between them represents a serious problem. Parasitic animals on bats can be transferred between caves if they move off their host when it is visiting a food source and then attach themselves to a new bat from another cave when it in turn visits the food source (Start 1974), or when the host changes roosts. Some species of non-parasitic cave animals are quite widely distributed but they are poor competitors in the outside world and their means of dispersal is not known. It may be that generations ago they had a means of dispersal (such as strong flight) but this has been lost through a lack evolutionary pressure to select for it. Thus each cave or group of caves may contain a population isolated from others of the same species which is gradually shifting or has shifted away from the species type to evolve distinct subspecies or even species. Nowhere in the Sunda region has sufficient work been conducted to show if certain caves have endemic fauna, but this extremely likely.
Another respect in which caves are like islands, is in the relationship between cave size and number of species. This was studied in Greenbrier Valley (West Virginia, U.S.A) where a series of seven limestone exposures are set amid other rocks or divided by rivers. Thus cave-dwelling animals could move within a limestone block with relative ease but not between the blocks. The number of terrestrial species was significantly correlated with 'island' area, but there was no correlation between 'island' size and species number for the aquatic animals. These species, unlike the terrestrial ones, were often found near cave mouths - for example, in pools formed by drips from the top of the entrance - and thus had more chance of dispersal. In addition, there were probably subterranean aquatic connections between the caves, meaning that the caves do not form truly isolated islands for these species (Gorman 1979).