The significance of the subterranean fauna in biogeographical reconstruction: examples from Cape Range peninsula, Western Australia

WA Museum Records and Supplements | Updated 9 years ago

Abstract: Cape Range peninsula contains a diverse troglobite (obligatory cave inhabitants) fauna, the only rich troglobite community known in Western Australia and in the semi-arid tropics. At least thirtyeight species of troglobite (or stygobiont) in this sparsely sampled area place it amongst the worlds faunistically diverse karst areas. The characteristics of the region and its subterranean fauna are broadly examined in this paper.

The area (with Barrow Island) has high generic endemism with c. 14 apparently endemic genera including amphipods, shrimps, snails, millipedes, schizomids, spiders, archaeognaths, thysanurans and fish.

In the context of cave biology the communities are not simple with up to seven troglobites occupying a single cave, together with many other speeies lacking such overt modification to cave life but seemingly out of place in a semi-arid climate. The affinities of the fauna are varied but a large element of the terrestrial fauna is derived from the humid tropics and is relietual from times when humid forest covered this region.

The cave systems in Cape Range are fossil and were clearly formed under more humid conditions, as evidenced by the cave fauna Stalagmite growth has been extraordinary slow, suggesting that the climate was not been substantially wetter than at present over the la-., 170,000 years. Hence, the climate required for cave formation and forest cover predates this. However, sympatric congenors both in the range and on the coastal plain suggest that climatic/eustatic fluctuations have led to fragmentation and rejoining of populations.

Genetic distance (D) between cave populations is used naively to establish a sequence of biological events in Cape Range in order to look for consistency both within the genetic data and with other evidence. A sequential chronology is presented from the Precambrian to the present as a series of working hypotheses, viz. D between altitudinally separated congeneric amphipods suggests a late Miocene/early Pliocene date for the uplift of the Cape Range anticline which supports palaeontological and geomorphological evidence. D between partly sympatric Stygiocaris species suggests speciation at 5 Ma, perhaps associated with the same event. Terrestrial fauna appears to have speciated later; paradoxosomatid millipedes speciated in the Pliocene (c. 3 Ma) and were separated into genetic provinces by c. 2 Ma which, by deduction, is when the wet forest was lost from the gorges. D for amphipods within the range suggests that the perched water tables in the range were isolated in the early Pleistocene (c. 1 Ma). D suggests that the isolation between caves of species populations occurred during the mid to late Pleistocene both in the range and on the coastal plain, presumably associated with Pleistocene climatic and eustatic fluctuations.

Hypotheses presented lead to the following predictions: elements of the Cape Range stygofauna should be found on the coast bordering the Pilbara Craton; elements of the Cape Range terrestrial freshwater fauna should occur along water courses that would have drained from the Pilbara Craton across the Cape Range Formation.

The conservation implications of troglobite biogeography are briefly examined.