Size-Abundance Relations in Communities

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Abstract
It is suggested that size-abundance relations in communities (community profiles) are frequently polymodal. I examine bird census data to test this possibility. The censuses and their analysis are subject to a number of biases, which are partially corrected for. At least 60% of the bird community profiles are shown to be polymodal, with the modes separated by 1.6-1.8 times. Organisms of particular sizes are much more likely than others to form abundance peaks. Community profiles for insects and planktonic particles also show evidence of polymodality. Not only do the abundance peaks contain more species than the troughs, but the mean abundance per species is also higher in the peaks. Some of the polymodality in bird communities is associated with the vegetation structure and with bird guild structure. There is evidence of geographic changes in the incidence of polymodality in bird and planktonic particle profiles. Polymodality does not arise from inroads into a uniform resource base caused by predation or competition. Because body size is determined by an array of life history pressures, one expects size differences between groups of organisms with different life-styles. African bovids, for example, are shown to fall into a number of more or less distinct size classes. In addition, it is suggested that adaptive suites are often discretely, and not continuously, distributed; the dichotomy between few large toxic seeds and many small edible ones produced by tropical legumes (Janzen 1969) is an example. Species stacking, whereby the development of a size-abundance peak depends on the existence of an abundance peak of smaller organisms as a food source, is suggested as another, possibly major, contributor to polymodality.