Bioconservation and Tasmanian cave fauna
At first sight, the caves of Tasmania and other parts of cool temperate Australia do not appear to offer a great deal of biological diversity. Although they may act as traps for vertebrate animals, and so provide valuable fossil and sub-fossil bone material, no vertebrates live in Tasmanian caves; bats do not use them with any frequency and cave fish are unknown, in contrast to the caves of the tropics and sub-tropics, where bats, swiftlets and oilbirds often use caves as roosts in huge numbers, providing a supply of food through their droppings which in turn supports a rich fauna of scavengers and their predators. Probably because our caves are cool (rarely exceeding 10°C) and wet, Tasmanian bats and birds do not roost in them, eliminating the possibility of this whole food chain.
Does this mean that these southern caves are biologically uninteresting and that their fauna is impoverished and uninteresting? On the contrary, recent work in Tasmania has shown that the diversity of our caves is on a par with that of much warmer caves in Australia, and that they provide habitat for a number of interesting and important species. For many years, and still for many people, the Tasmanian cave fauna was apparently restricted to the conspicuous species: glow worms, the vast congregations of cave crickets often seen near cave entrances, and one of their predators, the slightly-eerie Tasmanian cave spider. Animals smaller than these tended to go unseen, perhaps because cavers were concentrating more on travelling through caves, or on the large scale of cavern and decoration, rather than what was under their noses. But collections by those who have the eye for small things, such as Albert Goede, Arthur Clarke and Stefan Eberhard, have shown that there is much, much more.
SURVEY OF CAVE BIODIVERSITY
A grant from the Australian Heritage Commission's National Estate Grants program in 1988-9 allowed us to collate material and information on Tasmanian cave invertebrates, and to send Stefan Eberhard down caves all over the state (Eberhard et al, 1991).
During the course of our investigation (October 1988 to December 1989), 39 caves in 17 karst areas were sampled intensively for invertebrate animals. Occasional collections made by one of us (SME) since 1980 were included in the database and they included collections from approximately 55 caves in 7 karst areas. We also incorporated collections made in the Western Tasmania World Heritage Area under the Directed Wildlife Research Programme of the Department of Parks, Wildlife and Heritage; these include 38 caves in 13 karst areas (Eberhard 1987; 1988; 1989). In addition, biological data were obtained from the literature, or from other scientists, to give data from a total of 133 caves in 30 karst areas. This represents approximately 14% of the known caves and about 56% of the cavernous karst areas in the State. Two dolerite boulder caves from a pseudokarst area at Mt Arthur were also sampled.
From these collections, many species were identified, but it is necessary to distinguish between some grades of cave species, since not all can be described as true cave-dwellers. Any animal living in a cave can be defined as a cavernicole. The traditional system for the ecological classification of cavernicoles consists of three major categories (Vandel 1965). Troglobites are obligatory cavernicoles, unable to survive except in caves or similar hypogean habitats. Troglobites are often distinguished by morphological specialisations called troglomorphisms, which may include loss or rudimentation of eyes and pigment, and attenuation of the body, appendages, or sensory hairs (Holsinger & Culver 1988). Troglophiles are facultative cavernicoles. They may live permanently in caves, and successfully complete their life cycles there, but they can also do this in suitable epigean or endogean habitats. Trogloxenes are habitually found in caves, but they cannot complete their whole life cycle there and must return periodically to the surface or entrance zone for food.
The survey showed that Tasmanian caves have the richest faunal assemblages in temperate Australia, with many more cave-adapted species than the literature had suggested. Although the taxonomic resolution in some groups is low, because so little collecting has been done, more than 150 spp. of animals, representing some 130 families, were identified. Species in at least 34 genera can be classified as troglobites. Material supplied to taxonomists has already yielded published descriptions of 3 new genera and 10 new species, and many more taxa remain to be described.
These cave animals spanned almost the entire range of terrestrial and freshwater animals groups, including flatworms, annelid worms, nematodes, arachnids, crustaceans, millipedes, insects and molluscs. Some of the significant groups are discussed briefly below.
Harvestmen (Class Arachnida, Order Opiliones) Tasmania has a rich fauna of both epigean and cavernicolous harvestmen (G. Hunt pers. comm.). The dominant cavernicolous group includes both troglophiles and troglobites belonging to the suborder Laniatores, family Triaenonychidae. Troglobitic forms are present in Hickmanoxyomma, Mestonia, Nuncioides, Notonuncia, Glyptobunus, Lomanella, and a new genus close to Lomanella (J.L. Hickman & G. Hunt pers. comm.). Leionuncia includes a possibly troglobitic form.
Spiders (Class Arachnida, Order Aranea)
Some 19 families of spiders have been recorded in Tasmanian caves. Of these, eight contain strongly troglobitic species (Amaurobiidae, Anapidae, Austrochilidae, Micropholcommatidae, Mysmenidae, Synotaxidae, Textricellidae and Theridiidae). Although it is a troglophile, rather than a troglobite, the best known and perhaps the most significant spider is the Tasmanian Cave Spider, Hickmania troglodytes. It belongs to a superfamily, the Austrochiloidea, which is among the most primitive of the araneomorph spiders, and so it is of considerable systematic interest.
Shrimps, woodlice and yabbies (Class Crustacea) The crustacean fauna of Tasmania is of world significance, and its cave representatives are no exception. Aquatic species are found in most caves, and terrestrial woodlice (along with millipedes) are the terrestrial species found deepest in Tasmanian caves. The mountain shrimps, Anaspides spp., are phylogenetically important as ancient representatives of the early stock from which the advanced crustaceans arose. Many cave populations are known in cave streams and lakes, including some, such as that at Wolfe's Hole, which are likely to be described as new species. Other smaller relatives, species of Eucrenonaspides, Micraspides and Koonunga, are also widespread, but found in drip pools, seepages and similar habitats out of the main flow of water.
Aquatic amphipods in the genera Antipodeus and Austrogammarus are common and widespread. They include several highly troglomorphic, but undescribed, species. Other cave-dwelling amphipods in the genera Giniphargus and a genus similar to the West Australian Hurleya. These animals are from very ancient, pre-Gondwana stock.
Surprisingly, in view of the diverse cave crayfish fauna in the east and south of the USA, Australian caves do not appear to have been colonised by crayfish extensively. The normally surface-dwelling giant crayfish, Astacopsis gouldi, has been recorded from several caves, most notably the tourist cave at Gunns Plains, but apart from anecdotal reports, only one specimen of a crayfish showing troglomorphic characteristics is known; this was a juvenile Parastacoides sp.
Beetles (Order Coleoptera)
Tasmanian caves have a distinctive and highly interesting cave beetle fauna. Troglobites are represented in the family Carabidae (tribes Trechini and Zolini). In Tasmania, troglobitic trechines are represented in the genera Tasmanotrechus and Goedetrechus; they are generally very rare in any given cave, and many collections are represented by single specimens only. They are confined mostly to the deep cave zone, where they are found under stones, in or near flood litter on riparian siltbanks, on moist substrates near water, or near seepages.
Zolines are generally more abundant than trechines, not as highly troglomorphic, and they are not generally restricted to riparian type habitats. They may be found on siltbanks, walls, roof and floor and may be found quite close to entrances, including the twilight zone.
Aquatic snails (Phylum Mollusca: Class Gastropoda, Family Hydrobiidae)
Tasmanian caves support a rich fauna of freshwater hydrobiid gastropods, a highly speciose group which typically have limited dispersal powers and restricted distributions (Ponder et al 1988). All the species collected from caves so far are new and include both epigean forms and fully stygobiontic forms. There are four principal cavernicolous genera: Beddomeia, Phrantela, Fluvidona and "Fluviopupa". Some, or all, of these genera may be found together at the same site, but there is a high degree of site specificity in the collections. An unusual Beddomeia sp. is known from a drip pool at Cracroft. Closely related to Beddomeia is the highly cave adapted Pseudotricula eberhardi which is found only at Precipitous Bluff (Ponder, 1992); it occurs abundantly in stream passages there. Also from Precipitous Bluff is a single specimen of another unusual hydrobiid, possibly representing a new genus and species with affinities to Phrantela (W. Ponder pers. comm.). Six species of cavernicolous hydrobiid are known from Precipitous Bluff.
DIVERSITY AND SIGNIFICANCE OF THE TASMANIAN CAVE FAUNA
How do the recorded levels of species diversity compare between different caves and karst areas within Tasmania, and also with mainland Australia? Table 1 shows that Tasmanian caves appear to support a substantially higher biodiversity than mainland temperate caves. However, comparisons of biodiversity such as this are rather crude, because there will inevitably be differences in sampling intensity between different areas and the fauna of many karst areas is still incompletely known. The comparison is also complicated by biogeographical and historical factors, and the area effect, i.e. in a geographical region where climatic conditions are historically similar, areas with extensive, continuous exposures of cavernous limestone will harbour more diverse troglobite faunas than areas with limited, discontinuous exposures of limestone (Holsinger & Culver 1988).
The age and diversity of cave fauna appears to be related to the geology and geomorphic history of karst systems. Cave faunas show a tendency towards greatest diversity in larger and higher relief carbonate outcrops. Tasmania is the stronghold of the richest cave faunal assemblages in temperate zone Australia. Situated at a climatic, and geographic extreme of range, and with many undisturbed sites, this region will provide highly important evidence in elucidating the evolutionary history of the Australian cave biota (Kiernan & Eberhard, 1993).
|Tasmanian Karst Areas|
|Precipitous Bluff||15||32||Eberhard et al, 1991|
|Ida Bay||5||31||Richards & Ollier, 1976|
|Ida Bay||11||30||Eberhard, 1990a|
|Mt Anne||5||12||Eberhard et al, 1991|
|Mt Ronald Cross||3||25||Eberhard et al, 1991|
|Bubs Hill||6||55||Clarke, 1989|
|Bubs Hill (BH203)||?||55||Houshold & Clarke, 1988|
|Kubla/Genghis Khan (MC1/39)||11||71||Eberhard, 1990b|
|Nullarbor Plain||6+||?||Richards, 1971c|
|Victoria (5 karst areas)||6||?||Davey & White, 1986|
|Wombeyan Caves, NSW||2 (+5*)||?||Smith, 1982|
|Bungonia Caves, NSW||2*||?||Wellings, 1977|
|Yanchep, WA||?||25||Jasinska & Knott, 1991|
|Cape Range, WA||15||?||Humphreys, 1989|
|Bayliss Cave, N Qld||24||?||Howarth, 1988|
|*: second level troglophiles, sensu Hamilton-Smith, 1967|
INTENSIVE STUDIES: LITTLE TRIMMER CAVE
The fauna of Little Trimmer Cave (Mole Creek, Tasmania) has been monitored at regular intervals since the beginning of 1991, in an ongoing project. The slow rate of cave biological processes means that long-term studies are essential to understand the ecology of cave organisms, and this approach is yielding much important information.
Little Trimmer contains a rich invertebrate fauna due, at least in part, to the wide variety of substrates and, consequently, habitat types within the cave. The Little Trimmer Program was devised for the regular monitoring of the major species and habitat types within the cave.
Despite sheltered and relatively stable environmental conditions, the compositions, numbers, and location of cave cricket (Micropathus cavernicola) and troglophilic spider (Hickmania troglodytes) populations vary according to seasonally-influenced changes in air flow and humidity. Most notably, juvenile forms of both species migrate deeper into the cave, where conditions are more constant and favourable, over the warmer months. In the case of the juvenile spiders, this migration appears to be due to competitive/predatory displacement and/or food requirements. The emergence of H. troglodytes young from egg cases is also tied with the cooler months, and occurs shortly after the return of juveniles from the deeper cave zones. The stream dwelling amphipods (Antipodeus sp.) show similar seasonal tendencies, with a mainly late-winter to spring mating period and protective burrowing behaviour prior to the stream drying up over the summer months. This burrowing allows the amphipods to survive seasonal dry spells, and correlates closely with increasing calcium concentrations in the remaining water. For these reasons, the life cycle of the species would appear to be almost directly dependent upon the yearly changes in stream flow. In contrast, the (as yet un-named) amaurobiid spiders inhabit deeper parts of the cave and they are not as prone to external seasonal influence.
Despite its phylogenetic and ecological importance, little has previously been recorded about the ecology and behaviour of Hickmania troglodytes. Our study has already shown that the mobility of individuals varies according to age/size, sex, and reproductive status. As well as the observations on the emergence of young, and population structure and stability mentioned before, feeding, courting and mating, and egg-sac producing behaviours have been recorded. With the exception of feeding, these tend to be long and delicate procedures, very prone to disruption. Courting appears to be very involved, and dangerous for the male! He reduces the hazard by gripping the female in a manner analogous to a wrestling hold during mating, a maneuver made possible by a combination of the length and number of his legs, and a curious curved structure towards the end of the second leg pair, which neatly fits around the base of her fangs and keeps them prised apart. These curved structures may also be used to strum the web of the female on the initial approach, a signal identifying him as not being prey. Despite all these precautions, the hapless male frequently becomes a post-encounter snack!
The female takes a great deal of care with egg-sac construction, and unlike the simple, seemingly random egg mass contained in the sacs of most other spiders, this contains a highly ordered and rigid thimble like structure, which hangs clear of the sides and, like a thermos, may serve to buffer the eggs from external temperature change, mechanical buffeting, infection, and so on. Incubation times are very long, between eight to ten months, a period that is very much in excess of the usual spider incubation period of approximately six weeks. Despite this, emerging young appear to be less well developed than those of more advanced spiders, and the silk of the egg-sac itself is exceptionally resistant to fungal and bacterial degradation, even though it may be well exposed to such agents (e.g. in the damp cave atmosphere, or when the surface is camouflaged with decaying wood).
Apocryphal stories have claimed these spiders to have a life-span of up to forty years, a figure that at the outset of the project seemed extreme. While forty may still be a little excessive, rates of growth, feeding, frequency of moulting, and other factors suggest that this spider may live for as many as 10 to 20 years.
Such a long incubation period and general longevity highlights the difficulties inherent in cave studies, where processes are very slow. Little Trimmer Cave presents an ideal opportunity to obtain long term ecological data regarding the life histories and population dynamics of several cave dwelling species.
GENERAL SIGNIFICANCE OF CAVE FAUNAS
The very oddity of cave animals with their loss of eyes and pigment, and exaggerated morphology has been an overwhelming source of interest for zoologists who have sought to explain such things as the selective pressures which bring about the loss of features. But the ecology of cave communities is also of great significance. Culver (1982) proposes five reasons why caves provide useful opportunities for testing models and concepts currently used in ecology. 1) Selective pressures are simple and easy to identify, such as scarcity of food and absence of light. 2) Conditions are relatively stable so that populations are likely to be near equilibrium. 3) The number of species present is small, so that the number of possible pairwise interactions between species is small. 4) There are many caves so that the number of replicated systems is large, and caves provide reasonably similar sets of conditions in different parts of the world at the same latitude. 5) Because cave communities are simple, they more nearly approximate the simple theoretical systems on which ecological hypotheses have been developed.
Against these advantages must be set two factors: first, the scarcity of cave animals, both locally in terms of population size within a single cave, and in the restricted distributions of species; and second, the slow speed at which the animals develop and grow. These combine to make designing rigorous experiments difficult.
Because caves are entirely dependent for their fundamental energy sources on outside events, they are very vulnerable to what goes on outside, often at some distance from the cave. Since water is usually the vehicle which brings food into temperate caves, their fauna is at risk from land use practices which affect the quality of water entering them.
More direct effects are also important and caves may be affected by quarrying, rubbish dumping, the in-filling of entrances or the clearance of vegetation from around entrances. The latter is particularly significant for caves supporting large cricket populations. The crickets are dependent on undisturbed surrounding vegetation for food, and their droppings provide important food for troglobites. Thus, although they are not obligate cave dwellers, reduction in the numbers of crickets may have a detrimental affect on the true cave fauna.
Visitors are a significant potential source of disturbance, and some cave habitats are very vulnerable to them. Indirect disturbance alone (lights, movement) affects cave spiders, which may desert their egg sacs. This probably accounts for the rarity of cave spiders in tourist caves. The seepage pools which support the small syncarid Eucrenonaspides oinotheke can be destroyed by a single careless footstep, as can the silt banks which support many of the terrestrial invertebrates deep in caves. Even visitation for scientific purposes should be carefully regulated in view of the delicacy of the habitats.
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