Abiotic Community Constraints in Extreme Environments: Epikarst Copepods as a Model System

The general hypothesis that the overall presence or absence of one or more species in an extreme habitat is determined by physico-chemical factors was investigated using epikarst copepod communities as a model system, an example of an extreme environment with specialized, often rare species. The relationship between the presence or absence of epikarst copepods from drips in six Slovenian caves and 12 physico-chemical factors (temperature, conductivity, pH, Ca2+, Na+, K+, Mg2+, NH4+, and Cl−, NO2−, NO3−, and SO42−) was explored. Statistical analyses included principal components analysis, logistic mixed models, stepwise logistic multivariate regression, classification trees, and random forests. Parametric statistical analyses demonstrated the overall importance of two variables—temperature and conductivity. The more flexible statistical approaches, namely categorical trees and random forests, indicate that temperature and concentrations of Ca2+ and Mg2+ were important. This may be because they are essential nutrients or, at least in the case of Ca2+, its importance in molting. The correlation of Cl− and NO3− with copepod abundance may be due to unmeasured variables that vary at the scale of individual cave, but in any case, the values have an anthropogenic component. This contrasts with factors important in individual species’ niche separation, which overlap with the community parameters only for NO3−.