01514nas a2200133   4500008004100000245005400041210005200095300001300147490000800160520109700168100001801265700001701283856008001300       2006                        eng d00aRedefining the community:a species-based approach0 aRedefining the communitya speciesbased approach  a464 -4720 v1123 aWe propose that effective community size can be defined on the basis of the web of indirect interactions experienced on average by each individual species. Indirect interaction chains are composed of links provided by direct interactions. We analyzed previously published data on 20 assemblages of species. Chain strengths were estimated by the weakest link and by the product of link strength. The average strength of the interaction chain decreased with increasing numbers of links with both models. Positive indirect interactions in chains with an even number of links offset negative direct interactions. We set the community size by the chain length where 95% of the indirect interactions are weaker than 10% of the mean of the absolute value of direct interaction strength. Using the multiplicative model, seven assemblages had a community size (web of interaction length) of three links, one of four links, and the remainder of communities were too small to set community size. The analysis suggests that effective communities of size are rarely investigated in ecological experiments.1 aDodds, W., K.1 aNelson, J.A.  uhttp://knz.lternet.edu/content/redefining-communitya-species-based-approach