Broad-scale Patterns of the Distribution of Body Sizes of Individuals in Ecological Communities

INTRODUCTION Body size is one of the most fundamental properties of an organism, affecting metabolic rate, lifespan, home range size, reproductive rate, and many other ecological attributes (e.g., Peters 1983, Calder 1984, Schmidt-Nielsen 1984). Ecologists have long been interested in using these regular patterns to gain insight into ecological systems (Peters 1983), and it is increasingly recognized that body size has important consequences for understanding many aspects of community structure and function (e.g., Siemann et al. 1996, Cohen et al. 2003, Brown et al. 2004). However, information on continental and global scale variation in body size at the level of entire communities is extremely limited and, given size’s key role in functional biology, quantifying and understanding broad scale patterns in body size represents a major challenge for ecology (Kaspari 2005). The few broad scale analyses of community size patterns that do exist typically focus on very simple characterizations of the distribution of body sizes within a community, most typically average body mass (Li 2002, Kaspari 2005). However, much richer information is available by looking at the entire distribution of body sizes of individuals within the community, otherwise known as the size-spectrum (Figure 1, Sheldon et al. 1972, Griffiths 1986, Kerr and Dickie 2001). Both the general and specific form of this size-spectrum can provide substantial information about the processes structuring communities by reflecting the influences of competition, predation and resource use (Griffiths 1986, Enquist and Niklas 2001, Kerr and Dickie 2001). In addition, understanding broad-scale patterns in the spectrum is essential for linking patterns relating individual properties and body size, to patterns in assemblages, communities, and ecosystems. This information is necessary because in order to link community-level behavior with that of the component individuals, relationships between properties of the individual and body size must be summed across all of the individuals in a community, which requires information on the distribution of individual body sizes (e.g., Peters 1983, Enquist et al. 2003, Savage 2004). In this project, I will take advantage of three continental-scale databases, to evaluate broad scale patterns in the distribution of body sizes of individuals. I will focus on terrestrial size-spectra, which have received very little attention, even at local scales (Kerr and Dickie 2001). I will evaluate patterns in the general and specific shape of these distributions as well as simpler, more traditional measures of the spectrum such as average mass. If we are to successfully link individuals and ecosystems, and understand the processes structuring local communities, it will be necessary to understand the patterns of, and forces structuring, the distribution of body sizes of individuals within and among assemblages, communities and ecosystems. Evaluation and quantification of broad scale patterns represents a first important step in this direction.