Body Size Patterns in Stream Communities: A Test of Holling’s Textural Discontinuity Hypothesis
Theoretical studies of the resilience of ecological systems to environmental change predict that the size distributions of species in ecosystems should have discontinuities that reflect similar discontinuities in ecosystem processes. Body size distributions should have many peaks and troughs (modes) for natural, undisturbed ecosystems, but that as disturbances increases, so the number of modes declines. If so, this prediction has implications for assessing the quality of real ecological systems and has potential for environmental monitoring.
This paper explores the relationship between water quality and body size patterns in stream communities in order to establish the potential of size based indicators for assessing environmental conditions as well as testing Holling’s (1992) proposition that lumpiness occurs in body size distributions across a broad range of spatial and temporal scales. Samples of the stream benthos were collected at different station in River Aire, Yorkshire, UK, which varies in water quality. All sites showed skewed distributions towards smaller size classes and most had two very obvious modes at medium and large size classes except for most polluted habitats. Analysis of the number of gaps using Holling’s (1992) BMDI, revealed wide variation in clean and intermediate water quality sites, though the most polluted site had the fewest gaps. However other disturbed sites had more gaps and for some clean site had fewer gaps. It is clear that size distributions in stream communities are lumpy in the sense that most sites showed more than one mode or many gaps but the number of gaps (discontinuities) is not correlated with disturbances, at least for freshwater quality.
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