but probably less than one third. This compilation indicates that the greatest invertebrate diversity is located in a few groups, including several minute, interstitial phyla (Nematoda, Rotatoria, Annelida, and Platyhelminthes) and the highly diverse Diptera, especially the midge family Chironomidae. In general, it is the smallest taxa that are most diverse (Palmer et al. 1997). The Broadstone Stream is relatively depauperate because of high natural acidity; however, its count of 131 invertebrate species likewise reflects high representation by small taxa that are often overlooked (Schmid-Araya et al. 2002).

Studies of the number of diatom species that colonize glass slides suspended in the current also suggest that local species richness is proportional to regional species richness (Patrick 1975). In two species-rich streams of the eastern United States, between 79 and 129 diatom species colonized glass slides; in two species-poor streams on Dominica, West Indies, the range was 46-61 species. Comparisons of species-rich and species-poor streams in the United States gave a like result: 160 species were collected on slides in a stream where the species pool was approximately 250; fewer than 30 were found in a stream with roughly 100 total species.

Fish inventories can be complete, particularly in temperate regions where local diversity frequently is of the order of 10-100 species and taxonomic uncertainty is minimal. As many as 50-100 species can be collected from a stream reach, although reports in the range of 20-50 species are more common (Horwitz 1978). Despite the scarcity of complete inventories of invertebrates and algae, there can be little doubt that, at least in temperate streams, the number of taxa in both groups exceeds the number of fish species by an order of magnitude.

In summary, there are numerous factors that contribute to some areas being relatively rich in species while other areas are less so. A first level of explanation must take into account regional diversity, which is influenced by climate history, topography, and geography; intensity of the sampling effort at both within and between-habitat levels; and the variability of the physical environment and the habitat. There is also good reason to believe that interactions between species, which in turn probably vary in their intensity depending upon environmental conditions, play a major role in determining local species richness. This provides the link between the topics of species diversity and community structure, and we turn now to the latter.

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