Ecosystem Size and Functional Group Relative Abundance Drive Stream Community Body Size Structure

Temperate streams are among the most well-studied ecosystems globally, yet there remains much uncertainty regarding the processes that regulate food web structure. Here we explore the relative contribution of food web composition and watershed factors on inter-stream variation in size spectra slopes (λ). Size spectra describe the decrease in abundance with increasing body size of individuals within a community (N∼M λ). We estimated λ for invertebrate, fish, and combined food webs to test effects of functional groups and watershed variables. The mean λ for combined and invertebrate food webs aligned well with theoretic predictions (mean = −2.04 and −2.02 respectively, range =−2.58 to −1.51). In contrast, the mean fish slope was shallower and more variable among sites (mean = −1.85, range = −2.71 to −1.33). Drainage area, a proxy for ecosystem size, was positively related to λ in all three data groupings. The fish-only λs were also positively related to agricultural land cover and negatively associated with the relative abundance of predators and omnivores. These results demonstrate the potential for drainage area to increase the relative abundance of large consumers and highlight the role of functional group dominance. Such insights are difficult to obtain without a size spectrum approach.