Structure and function of urban stream ecosystems

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One of the most intriguing regularity in the structure of stream communities is that, although there is tremendous spatial and temporal variability in the abundance of various taxa and life stages, the size distribution is remarkably predictable. Our research has shown empirically that size distributions of stream organisms are correlated to physical habitat characteristics although the actual mechanisms underlying the patterns have not been tested. My students and I propose to test two hypothesized mechanisms that could account for the observed patterns. The first hypothesis states that available space, because of the fractal nature of stream beds, constrains densities of organisms of different size. The second hypothesis is that changes in trophic level with increasing size accounts for the general decline in abundance with increasing size. These two hypotheses will be tested in natural conditions by estimating available space for organisms of different sizes using close range digital photogrammetry techniques to describe habitat configuration at the mm scale, and by the use of nitrogen stable isotopes to estimate the average trophic level of organisms in different size classes.