The distribution of body size in a stream community: one system, many patterns

Summary 1 We investigated abundance–size spectra and body size–density allometry using an exceptionally detailed data set from a stony stream. The body size distribution of the whole metazoan community was expressed in terms of both density and biomass over a 14-month survey, and the relationships of body size with population density and taxon richness were also evaluated. 2 The distribution of density among size classes varied considerably between sampling occasions, although that of biomass was rather more consistent. This suggests that the body size distribution is dynamic and does not always fall into any single pattern or category, as has been assumed hitherto. 3 Analysis of the modes in the density–body size distribution in individual sample-units revealed small-scale spatial variation, with little consistency in the number or position of modes in any one month. 4 This stream community did not conform to the −0·75 (or indeed to any other significant regression) relationship between body size and population density, although such patterns are very sensitive to methodological details. 5 In data aggregated over the whole study, most metazoan taxa fell into the size range < 10–100 µg. The annual average density– and biomass–body size spectra appeared tri- and bimodal, respectively (on log–log scales), with minor troughs in the density spectrum at about 100 and 0·1 µg body mass. This could indicate, at this scale of analysis, discontinuities in the way in which benthic species exploit their environment. 6 Spatial and temporal heterogeneity in size spectra, and the lack of clear density–body size allometry, could be due to biotic or physicochemical fluctuations and environmental change (including seasonal and longer-term changes in acidity) that cause rapid species turnover and changes in density and distribution, particularly among the meiofauna. Small and rare species may thus be in the process of colonization or decline and the pattern would indicate a lack of equilibrium, particularly among organisms in the smallest size classes, at this temporal scale of the study.

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