The Hypogean Fauna in Serbia: From Surface to Soil to Caves

Bozidar Curcic, Ivica Radovic


    In several areas of evolutionary ecology the study of caves can play an increasingly important role. There is, it seems to us, an important question that has received less attention. Are complex systems, such as the tropics or organisms with complex life histories, the appropriate situations in which to test the models? In retrospect, the answer is no. This is not because the results when applied to complex situations, predictions of the model itself are qualitatively different from predictions in simple situations. For example, a two-species model of competition, using the much-maligned Lotka-Volterra equations of competition, predicts in the case of a stable equilibrium that the abundance of the competitors will be negatively correlated through time and that selection will tend to result in character displacement. But by adding even one more species using the same Lotka-Volterra competition equations, abundance can be either positively or negatively correlated, and either character convergence or displacement can occur, according to the theory. As a consequence, many tests of ecological models were misapplied because they used two-species models for multiple-species systems. Too often, these hypotheses were poorly tested or not really tested at all. But the demonstration that the hypothesis is false is not surprising, because not only the null hypothesis but the hypothesis itself was often ill formed.
    A more appropriate place to test many ecological models is in relatively simple situations like caves. To reiterate, cave communities are simple, allowing more detailed examination of interactions; there are many replicate communities and natural occurrences of species additions and removals; and at least some assumptions of the models, such as near-equilibrium conditions, are more likely to be satisfied in caves than in more complex systems. But there have been weaknesses in the use of caves as model systems. The first is the virtual absence of experimental manipulations that would provide stronger tests of the theory. The second is the absence of long-term studies on fluctuations in age structure, population size or species composition. These kinds of data are critical for the examination of underlying assumptions of equilibrium and steady state. The third weakness, which has been all too evident throughout this study, is the lack of sufficient data to make a strong test of a hypothesis. Data on area effect and frequency of various species combinations in caves will never be as extensive as data on, for example, birds on islands, because the species pool is much smaller. However the data base can be increased, and when it is it should be “cleaner”, because in many cases transient species will be identifiable.
    Suppose we take an optimistic look at the future and assume that the major ecological processes and patterns of cave faunae can be described and predicted by realistic, general models. Is there any reason to believe that is also a step toward explaining more complex communities? We really don’t know. Perhaps highly diverse communities such as those in the tropics have a completely different mode of organization, involving more mutualisms, higher-order interactions, and the like. But it seems equally, or even more, likely, that complex communities consist of relatively small subsets of strongly interacting species. Cave communities may then serve as a paradigm for a strongly interacting subset of species.
    Experimental and theoretical studies concerned with the population genetics - based on the analyses of changes in the frequency of individual genes and, eventually, polygenic systems - though more sophisticated, do not always provide a thorough insight into the adaptation processes occurring in natural populations (Kosswig C., 1948, 1955, 1965). Unfortunately the complex genetic system analyzed so far, such as evolution and maintenance of sexual process as well as gene recombination and the mode of establishing of non-random associations between genes, have not been fully experimentally verified. It seems highly probable that adequate experimental studies regarding cave species (due to their specific population structure and the degree of polymorphism) may greatly contribute to the elucidation of fundamental problems in respect to the genetical aspects of their speciation in underground habitats.
    However, work on cave organisms may be important in reforging the connection between population genetics and adaptation (Kosswig C., Battalgil F., 1943). Since the use of gel electrophoresis to detect genetic variation, modern population genetics has become increasingly unconcerned with the phenotype above the level of enzymes. In the meantime, the study of morphological and physiological adaptation has, with some notable exceptions, fallen into decline because it no longer appears “modern”. Both fields would benefit from a closer connection. The work of Poulson (1963) on physiological and morphological adaptation in cave fish, that of Christiansen on behavioral and morphological convergence in cave Collembola (1961), and the work of Wilkens on the genetics and morphology of regressive structures in cave fish (1971) provide an excellent starting place for considering the genetic basis of morphological and physiological changes. A genetic analysis, using either the classical techniques of quantitative genetics or the modern techniques of gel electrophoresis, could begin to address problems of how much genetic change accompanies morphological variation. It is an unsatisfactory state of affairs when adaptation and genetic variation are treated as wholly separate - as they are in this study and in the work of nearly all cave biologists.
    Anyone concerned about the protection and preservation of cave environments has doubtless cringed at some of our suggestions for species addition experiments and genetic analysis. Most cave biologists have been guilty of overcollecting at one time or another, and we all need to develop a greater sense of our effects on cave faunae. In our attempt to develop a stronger data base, we have caused a severe decline in the populations of several cave arachnids and diplopods. This problem is not easily resolved, but cave biologists should ask themselves if they really need the specimens they are about to collect, and whether any real advance in scientific knowledge will result from the collection.