IS THERE AN INFLUENCE OF HISTORICAL EVENTS ON CONTEMPORARY FISH SPECIES RICHNESS IN RIVERS - COMPARISONS BETWEEN WESTERN-EUROPE AND NORTH-AMERICA

Freshwater fish species richness on 132 West European and North Americ an rivers is analysed using eleven variables related to contemporary e cology (nine) and history (two). This is done in order to examine the relative and joint effects of both historical and ongoing processes on the contemporary richness of these two regional fish faunas. Relation ships are quantified by simple and stepwise multiple regression proced ures. Species-area curves are presented for the fish faunas within bot h continents. We show that ecological factors statistically explain mo st of the variation in freshwater fish species richness for both conti nents. Effects of historical factors are shown to be statistically sig nificant, but add only a little to the variance already explained by e cological factors. Our analyses further indicate that rivers (which fl ow directly into the ocean) support fewer species of fish than do simi larly sized tributaries. The immigration-extinction hypothesis appears to provide a plausible explanation for this observed pattern. The fac t that in our final model, a continental effect is still highly signif icant, leads us not to exclude the possibility of some other historica l influences in generating different overall species richness levels o n the two continents.