Structure and function of urban stream ecosystems

河流群落结构中最有趣的规律之一是，尽管不同分类群和生命阶段的丰度存在巨大的时空变化，但其大小分布却具有显著的可预测性。我们的研究经验表明，河流生物的大小分布与物理栖息地特征相关，尽管这种模式的实际机制尚未得到测试。我和我的学生提议测试两种可以解释观察到的模式的假设机制。第一种假设认为，由于河床的分形特性，可用空间限制了不同大小生物的密度。第二个假设是，随着尺寸增大，营养水平的变化可以解释丰度随着尺寸增大而普遍下降的原因。这两种假设将在自然条件下进行验证，方法是利用近距离数字摄影测量技术在毫米尺度上描述栖息地配置，估计不同大小生物的可用空间，并利用氮稳定同位素估计不同大小类别生物的平均营养水平。

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.