ecologists often present their data as a continuous size spectrum of individuals,
irrespective of taxonomic affinities, whereas terrestrial ecologists generally
view the species as the individual datum, and usually as a fixed entity – mean
adult body mass. The ways these two research communities present and
subsequently interpret their data affect their understanding of how aquatic
and terrestrial systems work (Raffaelli, Solan & Webb, 2005 ). To these specula-
tions we might add that aquatic systems show one feature for which there is no
obvious parallel in the terrestrial world: a viscous medium that greatly shapes
how organisms function and behave. This unique attribute of aquatic systems
has understandably been a major focus for aquatic ecology and has accordingly
produced a perspective that is quite different from that of terrestrial ecologists.
As the chapters in this book testify, body size has clearly remained a major
research focus for the ecological community, from ecophysiology to the eco-
system. In part this is because body size sets real mechanical limits on what
organisms can do (e.g. limits to the dimensions of prey that can be physically
ingested), and in part because body size is a super-parameter, which does well at
capturing a range of associated physiological and ecological traits, but is much
more easily measured (or found in books) than those traits themselves. A few of
the authors herein have set out explicitly to test such data against the predictions
of the metabolic theory of ecology (Brownet al., 2004), with varying degrees of
success, while others chose ostensibly different theoretical backgrounds against
which to consider patterns and dynamics related to body size.
Body size and metabolic theory
We consider first the few chapters that explicitly mention explorations of
metabolic theory, or at least consider metabolic theory against patterns in the
different fields of literature. With respect to life history,Atkinson and Hirst(this
volume) consider whether selection can alter the scaling exponent (b) relating
metabolic rate to body size, the core of metabolic theory. They ask to what
extent is b fixed at 3/4? They cite Glazier’s (2005) survey of 642 metabolic rates
during ontogeny, around half of which deviated from 3/4 and, interestingly,
with higher values (mean 0.95) in pelagic compared to benthic (mean 0.74)
species, perhaps due to the increased costs of buoyancy or avoiding predation
in pelagic species.Huryn and Benke(this volume) consider biomass turnover
and body size in stream benthic invertebrates, analyzing the relationships
between body size (M) and population density, biomass (B), production (P) and
P/B. They found an encouraging degree of fit with theory for three temperate
streams, with the relationship between P/B and body size (as mass) having a
scaling exponent varying between0.24 and0.27, bracketing the predicted
value of0.25.Huryn and Benke(this volume) consider this precise fit ‘extra-
ordinary when considering the large ecological differences between the
streams’, forested or grassland, with or without fish, etc. The ‘snag’ community
BODY SIZE: IMPORTANT, BUT NOT THE WHOLE STORY 327