THE NORTH-AMERICAN LATITUDINAL GRADIENT IN SPECIES RICHNESS AND GEOGRAPHICAL RANGE OF FRESH-WATER CRAYFISH AND AMPHIPODS

Species richness undergoes a progressive decline with increasing latitude from 30-degrees to 60-degrees-N for crayfish and from 35-degrees to 70-degrees-N for freshwater amphipods. Rapoport's rule, which predicts a correlation between latitude and geographical range of distribution, is realized for both these crustaceans. The latitudinal gradients are determined by the mean annual temperature range, surface geology, temporal emergence from glaciation, and possibly strength of competitive interactions. Empirical differences in the magnitude of Rapoport's rule among taxa (mollusks, fish, mammals, reptiles and amphibians, crayfish, amphipods) reflect dispersion capabilities and ecophysiological properties.