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Cross-scale structure and resilience in animal communities.
Allen, Craig*,1, Sendzimir, Jan 2, Peterson, Garry3, 1 South Carolina Cooperative Fish and Wildlife Research Unit, Clemson, South Carolina2 International Institute for Applied Systems Analysis, Laxenburg, Austria3 McGill University, Montreal, Quebec
ABSTRACT- Species interact with scale-dependent sets of ecological structures and processes that determine their functional opportunities. The cross-scale resilience model proposes that animal communities are compartmentalized by scale. It suggests that strong interactions within a range of scale lead to a diversity of functions within a scale, because members of the same functional group are likely to strongly interact and tend to coexist only by operating at different scales, reducing competitive interactions. It further suggests that ecological resilience is generated by diverse overlapping functions within a scale and by apparent functional redundancy at different scales, thereby reinforcing function across scales. To test those propositions we, 1) determined if the body mass distributions from a large number of diverse ecosystems were discontinuously distributed, 2) determined if body mass distributions were over-dispersed at the community level and at the within-aggregation ('scaled ') level, and 3) determined if functional groups were distributed evenly across scales. We utilized 33 terrestrial vertebrate data sets from 22 ecosystems. We tested for discontinuities in animal body size distributions with null models and cluster analysis. Spacing between species (segment length along the body mass axis) within each body mass aggregation was calculated and the evenness of spacing was quantified by determining the variance of segment lengths within aggregations. Observed variances were compared with simulations, with low variances indicating more even spacing. We utilized feeding guilds as surrogates for functional groups, and determined the variance in the distribution of guilds among body mass distributions, and compared that with the output of simulations. Observed segment length variances tended to be small relative to the variance distributions generated by simulation. There was also low variance in the distribution of feeding guilds. Our results suggest that body mass distributions are discontinuous, and support the predictions of the cross-scale resilience model.
Key words: discontinuities, ecosystems, cross-scale, community assembly