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CHAPTER THIRTEEN

The consequences of body size in model

microbial ecosystems

OWEN L.PETCHEY

University of Sheffield

ZACHARY T.LONG

University of North Carolina at Chapel Hill

Virginia Institute of Marine Science

PETER J.MORIN

Rutgers University

Introduction
Patterns in the sizes of coexisting organisms have always intrigued ecologists
(Hutchinson, 1961 ). Some kinds of regularities are well known for some systems
(Sheldon, Prakash & Sutcliffe, 1972 ), and are less appreciated or rediscovered for
others (Enquist & Niklas, 2001 ;Cohen,Jonsson&Carpenter, 2003 ). One example
of this kind of pattern is the apparent constancy of total biomass within different
size fractions of organisms living in aquatic communities (Sheldon,et al. 1972;Cyr,
2000 ;Kerr&Dickie, 2001 ;Cohenet al., 2003; Sheldon, Sutcliffe & Paranjape, 1977 ;
Tilmanet al., 2001;Mulderet al., 2005). Essentially, over many orders of magnitude
of organism size, any particular size class holds about the same total biomass of
organisms per unit volume. The result is an inverse relationship between the log
of organism size and the log of organism abundance per unit volume, with a slope
of1. The apparent constancy of this relationship has even led some workers to
suggest, tongue in cheek, that it could be used to estimate the population size of
some organisms that have proven to be notoriously difficult to observe, once
assumptions about their averagesize were made (Sheldon & Kerr, 1972 , 1973).
Whether or not the elusive Loch Ness Monster (to which these calculations were
rather whimsically applied) actually exists, it appears that the total biomass of
organisms in some habitats is fixed by certain features of the habitat, most likely
the abundance of incoming energy and nutrients that drive productivity (Sheldon
et al., 1977; Cyr, 2000 ;Kerr&Dickie, 2001 ;Cohenet al., 2003;Mulderet al., 2005). A
recent flurry of research on the allometry of metabolism seeks to explain such
patterns mechanistically as a consequence of the ways that organisms capture and
transport energy and material (Brown, 2004 ). It is worth noting that none of these
patterns suggest that diversity, the number of different organisms that total
biomass is divided among, should necessarily influence the total standing stock
of biomass supported by a particular environment and its energetic regime.

Body Size: The Structure and Function of Aquatic Ecosystems, eds. Alan G. Hildrew, David G. Raffaelli and Ronni
Edmonds-Brown. Published by Cambridge University Press.#British Ecological Society 2007.