Patterns of diversity, density, and biomass of ectothermic vertebrates in ten small streams along a north American river continuum

We sampled density, diversity, and biomass of fish and amphibians to test t he hypothesis that vertebrate diversity in low- (second-and third-) order, low-elevation streams flowing into large, higher-order streams or rivers is greater than that in low-order headwater streams flowing into streams of t he same order or one order higher. Also tested was the hypothesis that vert ebrate biomass and density among streams were related to elevation, stream gradient, and drainage basin area. In ten second-order and third-order stre ams (2.2 to 5.0 m wetted width in summer) in the Pacific Northwest, USA. ve rtebrate diversity increased with decreasing elevation. Elevation and strea m gradient accounted for 86% of the variation among streams in vertebrate d iversity. In contrast, elevation, stream gradient or drainage basin area we re not significantly related to either vertebrate density or biomass. The o bserved pattern of progressively greater vertebrate diversity downstream fr om the headwaters was consistent with the river continuum concept, but the concept must be modified slightly to account for the diversity in low-eleva tion, low-order streams being higher than in low-order headwater streams. F rom a management perspective, since similarly-sized low-order streams do no t have the same species assemblages, it is necessary to protect small strea ms through a range of elevations if the full complement of species is to be protected.