Geographically distant regions typically have their own distinctive flora and fauna, attesting to chance differences in the establishment and diversification of particular taxa and the local interplay of environmental and biological forces that direct evolutionary change. As a consequence, when comparing ecological communities from different regions one cannot know in advance whether species from different taxonomic groups will display sufficient ecological similarity that assemblage structure will converge, or whether peculiarities of evolutionary history will result in differences. For example, our ability to appreciate the rich literature on the ecology of New Zealand streams is enhanced by the knowledge that many of its taxa are endemic, the mayfly Deleatidium is ubiquitous but Baetis is absent, shredders are rare, and one important group of shredders, caddisflies in the family Limnephilidae, also is absent (Winterbourn 1995).

Because ichthyologists have been collecting and describing the fishes of North America and Europe for several hundred years, the fish species and their distribution are well known (Hocutt and Wiley 1986). The European fish fauna is less diverse than North America, and regions within North America differ greatly. All of Canada and Alaska contain some 180 species of fish, considerably fewer than the rich Mississippi basin where most of the major adaptive radiations in North America have occurred. The Tennessee and Cumberland Rivers drainage realm alone includes some 250 species of fishes (Starnes and Etnier 1986). Species richness declines from east to west across the United States (Moyle and Cech 2006), due in part to major differences in extinction rates during the Pleistocene. As a consequence, the western United States contains only about one fourth as many fish species as does the east.

Comparison of species-area relationships for the fish faunas of Europe and North America provides further evidence that evolutionary and biogeographic history can have a profound influence on regional diversity (Figure 10.2). Postglacial recolonization was more restricted in Europe relative to North America because drainage divides in Europe tend to run from east to west and reestablishment from Iberia and the Adriatic was restricted by mountain ranges, therefore glaciated areas were recolonized largely from the Danube Basin. This likely limited both southward retreat and subsequent northward recolo-nization for the European fauna, whereas the North American fauna had a much larger area free from glaciation and easier routes for recolo-nization (Mahon 1984, Oberdorff et al. 1997). In both Europe and North America, the fish faunas of glaciated regions are species-poor in comparison with unglaciated regions, and contain species that are larger, more migratory, and give less parental care compared with the unglaciated regions of the Mississippi and Missouri basins (Moyle and Herbold 1987, Griffiths 2006).

Comparative studies of taxonomically related fish groups clearly show that history and bioge-ography can influence the taxonomic and ecological diversity of a region. The Nida River in south-central Poland and the Grand River in Ontario, Canada, are two river systems that exhibit similar gradients from the headwaters downstream and occupy similar climates. Thus, they might be expected to support about the same number of species, filling roughly similar ecological roles. There are in fact many

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