Connectance in stream food webs

1. The properties of food webs are important both in theoretical ecology and environmental management, yet remain elusive. Here, we examined 12 new stream food webs of higher taxonomic resolution and completeness than any previously published data set and combined them with other 10 published stream webs. 2. Compared with most previously published food webs, these stream communities (containing between 22 and 212 species) had more feeding links per species, a higher fraction of intermediate species (0.78-0.88) and lower fractions of top (0.04-0.09) and 'basal' species (0.07-0.15). 3. The exponent of the log link-log species relationship (S-1.3) differed significantly (ANCOVA: P < 0.001) from the link-species 'law' (S-1) and the constant connectance hypothesis (S-2). Based on these results, the link scaling law and the constant connectance hypothesis must be rejected for food webs in stream systems. 4. Connectance was markedly lower than previously reported and decreased with web size. We attribute this to the body-size disparity of organisms at the top and the bottom of the web, flow disturbance and to the high physical heterogeneity and complexity of the sedimentary habitat. All these may reduce the fraction of possible feeding links that are realized.