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The Effect of Seasonal Variation on the Community Structure and Food-Web Attributes of Two Streams: Implications for Food-Web Science
Ross M. Thompson and Colin R. Townsend
Vol. 87, No. 1 (Oct., 1999), pp. 75-88
Stable URL: http://www.jstor.org/stable/3546998
Page Count: 14
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Benthic communities from two grassland streams in New Zealand were contrasted to quantify seasonal variation in food-web and community attributes. Connectivity food webs were used to describe trophic links and energy flows. Community attributes were used to describe the distribution of species and biomass across taxa and functional feeding groups. The food webs produced are amongst the largest published - reflecting detailed taxonomy across fish, algae and macro-invertebrates. All food-web attributes - species richness, number of links, connectance, mean chain length, average number of links down and prey:predator ratio - showed significant variation across seasons. Community structure showed less seasonality, and low variation overall. Variation in food supply was important in structuring the community, e.g. primary production was correlated with the vertical dimensions (mean chain length) of the food webs and seemed to determine the proportion of species and biomass that were composed of browsers. Organic matter availability (as seston or standing crop) seemed to be important in dictating the proportion of species and biomass that were composed of collector-gatherers and filter feeders. Cluster analysis reveals that the two streams differed in fundamental ways. Dempsters Stream was characterised by a structurally complex, species-rich community, with a high proportion of predators and a predominance of browsers. This appears to be based on high algal productivity. Sutton Stream was characterised by a simpler food web with a greater contribution of collector-gatherers and filter feeders, reflecting higher availability and standing crop of organic matter. When clustered by food-web attributes, species richness was important in dictating which communities clustered together, identifying the species-rich spring and particularly summer food webs of Dempsters Stream as distinct from the other communities. On the other hand, when clustered according to community attributes, the spring and summer Dempsters communities grouped with the autumn and summer Sutton communities, whereas the autumn Dempsters community was most distinctive, due to a high species richness of Plecoptera and a high predator biomass. A comparison of seasonal and whole year summary food webs showed that web size, the number of links, and the average number of links down were over-estimated in the summary webs. Other attributes of the summary food webs, such as connectance, mean chain lengths and prey:predator ratios approximated the average of those from the seasonal food webs. The summary webs clustered separately from all the other food webs due to their higher species richness. Thus a food web gathered in a single season does not accurately reflect the food-web structure during any other season, or of the whole year.
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