The structure of food webs along river networks

Do changes in the species composition of riverine fish assemblages along river networks lead to predictable changes in food-web structure? We assembled empirical "fish-centered" river food webs for three rivers located along a latitudinal gradient in the South Saskatchewan River Basin (SSRB) that differ in land-use impacts and geomorphology but flow through similar mountain, foothill, and prairie physiographic regions. We then calculated 17 food-web properties to determine whether the nine river food webs differed according to physiographic region or river sub-basin. There were no statistically significant differences in the 17 food-web properties calculated among the rivers. In contrast, fish species richness, connectance, the proportion of herbivores, and the proportion of cannibals changed longitudinally along the river network. Our results suggest that regional changes in river geomorphology and physicochemistry play an important role in determining longitudinal variation in food-web properties such as fish species richness and connectance. In contrast, the overall structure of river food webs may be relatively similar and insensitive to regional influences such as zoogeography. Further explorations of river and other food webs would greatly illuminate this suggestion.