Body Size: The metabolic theory of ecology and the role of body size in marine and freshwater ecosystems

Introduction Body size is the single most important axis of biodiversity. Organisms range in body size over about 22 orders of magnitude, from tiny bacteria such as Mycoplasma weighing 10 13 g to giant Sequoia trees weighing 10 g. Such size variation is a pervasive feature of aquatic ecosystems, where the size spectrum spans at least 20 orders of magnitude, from the smallest free-living bacteria at about 10 12 g to the great whales at about 10 g (e.g., Sheldon et al., 1972; Kerr & Dickie, 2001). Nearly all characteristics of organisms, from their structure and function at molecular, cellular and whole-organism levels to ecological and evolutionary dynamics, are correlated with body size (e.g., Peters, 1983; McMahon & Bonner, 1983; Calder, 1984; Schmidt-Nielsen, 1984). These relationships are almost always well described by allometric equations, power functions of the form: Y 1⁄4 Y0M (1:1)