PS 86-253
Weighing interaction strength in stream food webs: Importance of incorporating density data
Many streams in deforested regions of southern Brazil have silted channels with reduced structural heterogeneity and grassy banks. Historically, biodiversity in these streams was high, but now is declining. We experimentally investigated whether or not an increase in instream structural heterogeneity supporting more diverse basal resources is associated with a greater number of trophic interactions and changes in the distribution of interaction strengths. Three deforested streams (S1, S2, S3) were selected, and three treatments were done in within 5-m reaches of each stream: control (C, unaltered), wood (W, wood was added), and wood + leaf packs (WLP, with wood and leaf packs added). Treatments within each stream were located 20 m from each other, and each was sampled before the treatments were set up and resampled three months afterward. Primary producers, plankton, macroinvertebrates, and fish were collected, identified to the lowest taxonomic resolution possible (most to genus level) and all consumer taxa had their gut contents analyzed for estimation of trophic links. Interaction strengths were based on quantitative diet data (% composition) weighted by biomass and density of the consumers. Interaction strengths were then classified as weak, intermediate, or strong.
Results/Conclusions
A total of 73,065 specimens belonging to 154 consumer taxa were sampled and 4,266 interactions were identified based on dietary analysis. The number of trophic interactions was significantly higher after the experiment only for W and WLP reaches (Mann-Whitney test, PW=0.045; PWLP=0.045; PC=0.27), indicating that the experimental addition of basal resources promoted a greater food web complexity. At the end of the experiment, the number of strong interactions was significantly higher in the C and WLP reaches (PC and PWLP =0.045), mostly due to higher densities of larger consumers such as fish. The number of intermediate strength interactions was significantly higher in the WLP reaches (PWLP =0,045), where densities of small consumers (e.g., the caddisfly Smicridea, with densities from 27ind/m3 to 3,542ind/m3) were much higher. Despite these differences, the distribution of interaction strengths, dominated by intermediate-strength and fewer weak and strong interactions, was consistent through all 18 food webs. These findings contrast with those reported in the literature (i.e. many weak and few strong interactions). In our study, species density was a weighting factor (instead of considering only body-size), and therefore emphasizes the trophic importance of small but highly abundant organisms.