OUCC Proceedings 9 (1979)
The Ecological Relevance of Cave Communities
|OUCC Proceedings 9 Index|
by Simon Fowler
The curious faunas of caves of great interest for purely scientific reasons and this is justification enough for wanting to study biospeleology. However, with further study the unique environments and associated unique communities to be found in caves may yield some interesting ecological results. Two significant features of cave ecosystems which distinguish them from most other communities are as follows:
|1) Abiotic Factors:||In non-floodable passages the physical factors affecting any biological community (such as temperature, light(!), humidity etc.) are often incredibly stable and probably constant for relatively long periods of time.|
|2) Energy supply:||The net energy input into the systems is extremely low. It may depend entirely on organic matter from the surface, though the possibility of chemotrophic bacteria must not be overlooked.|
In the past twenty years it has almost become a biological axiom that community stability increases with community complexity (Elton 1958). Community stability is difficult to define but, for example, communities including the lemming are cyclically unstable probably for intrinsic reasons, whereas complex tropical communities are classically held to be very stable. Community complexity can be measured by some index which takes into account the number of species and species biomass etc. The actual evidence for this complexity/stability hypothesis is circumstantial, and recently the idea of environmental stability as the primary factor in determining community stability has become more popular (May 1975). Cave communities have a low species density (low species biomass and low species number) and a stable environment, so the two hypotheses predict opposing results: an unstable or stable community, respectively. The stability of cave communities is therefore potentially of crucial interest in this controversy.
The theory of island biogeography (MacArthur and Wilson 1967) is another controversial hypothesis which has been applied to cave communities (Culver 1970, 1971). Habitats can often be regarded as 'islands' in a 'sea' of unfavourable environment (e.g. genuine oceanic islands, forest remnants, nature reserves etc.). With only small population sizes (at least on small 'islands') there will be a reasonably high chance of random extinctions of species. This will reach an equilibrium with a rate of immigration and establishment of new species, or at least that is how the theory goes. The problem is that extinction rates, immigration rates etc. are very difficult to measure reliably. Many karst regions consist of islands of limestone, isolated by different rock types. Culver has suggested that the terrestrial hypogean faunas have been isolated in the islands since they were created in the Pleistocene. Thus the extinction rate has not been balanced by an immigration rate (since the latter is non-existent). The possibility of a permanently non-equilibrium island community adds another facet to the theory of island biogeography and the simple, sometimes undisturbed nature of cave communities may provide a chance of making a valid test of the hypothesis.
So, in two controversial areas of theoretical ecology that are of particular importance to the possible conservation of our own increasingly island habitat, biospeleology may be able to make a significant contribution.
|Culver, D.C. (1970)||Analysis of Simple Cave Communities I, Evolution 24, 463-473|
|Culver et al (1973)||Toward Predictive Cave Biogeography, Evolution 27, 689-695|
|Elton, C. (1958)||The Ecology of Invasions by Animals and Plants Chapman and Hall|
|MacArthur, E.H. and Wilson, E.D. (1967)||The Theory of Island Biogeography Princeton University Press|