Food-web topology varies with spatial scale in a patchy environment

The majority of food-web studies currently used to test ecological theory have integrated information over large spatial and temporal scales. We aimed to assess the degree to which food webs display patch-scale variation, and the consequences for emergent properties at the larger scale of the stream reach. Spatial heterogeneity in ecological conditions (habitat structure and food resources) and food-web structure were measured in three streams. All food webs were constructed using equivalent effort at a patch scale (0.06 m(2)) and a reach scale (30-m stream length). A mosaic of habitat structure and food resources was reflected in considerable variability in food-web structure among patches, but there was less variation within than among streams. The variability in food-web attributes among patches could not always be related to ecological conditions, but food resource availability affected connectedness of food webs and trophic structure (measured as functional feeding groups). Of particular note was the result that reach-summary food webs were consistently different from patch-specific food webs in each stream. Reach-scale food webs underestimated connectance but overestimated prey: predator ratios and the number of trophic links. Summary webs sometimes placed species together that, in fact, did not coexist in the field. Such within-site food-web heterogeneity needs to be taken into account in future multiple-site comparisons of food-web structure.