Importance of diversity in the functioning and structure of riverine communities

The natural diversity of biological communities provides many alternative pathways by which nutrients and energy may move through ecosystems. Many different species are capable of per-forming similar functional roles in nature. When disturbed or polluted, natural systems often become restored first through the activities of species that had not previous] y been common. Human activities often result in reduction or loss of diversity within ecosystems, particularly through the loss of rare species. Maintenance of diversity is vital because it provides protection against both natural and human-caused catastrophes. Diversity, in the broadest sense, charac-terizes and maintains the flexibility of bi-ological communities. Most papers written about diversity have stressed the impor-tance of genotypic diversity for the evolu-tion of species in changing environments. Variations of genotypes within species may be expressed as varieties and subspecies which thrive under differing transient en-vironmental conditions. For example, pro-tein electrophoresis has shown that there are genetic differences between individuals of species of insects living at the extremes of their range and those living where optimum growth conditions permit larger population development (Sweeney et al. 1987). Patrick (1949) and many others since have quantified species diversity in natural aquatic ecosystems and how various types of pollution alter this diversity. Concern about the importance of diversity of species in nature has-grown recently because our accelerated clearing of tropical forests has endangered many native species. Not only are some species destroyed directly but a “border effect ” extends for a great distance into the remaining forest and disrupts the ‘ I have chosen to write this article about diversit