Implications of species loss in freshwater fish assemblages

Freshwater systems are vulnerable to pollution and species loss often ensues. Are there additional implications for assemblage structure? Here we use Berger-Parker d, Simpson's 1/D and Simpson's E to measure the ecological diversity of pristine and perturbed freshwater fish assemblages in Trinidad, West Indies, and Oklahoma, USA. Although the impacted sites typically had fewer species than expected, they could not be distinguished from unperturbed ones of equivalent richness. Changes in the evenness of these assemblages are thus driven by changes in richness. One practical outcome is that diversity indices may not provide independent verification of the detrimental consequences of pollution. The similarity in structure of naturally and anthropogenically impoverished assemblages provides no grounds for complacency, however, since it ignores the evolutionary history of the species concerned. On the basis of our results we suggest that species provenance may be important in tests of ecological function. Moreover, these investigations should replicate natural patterns of evenness as well as richness.