zebra mussel (Dreissena), and why does excretion of N and P inDreissenascale so
disproportionately with body size (b¼1.38)? There are some fascinating insights
in this chapter into how body size could feed back onto ecosystem processes. For
instance, they note that the removal of large, migratory salmon has reduced
nutrient subsidies to many rivers and their catchments, and that the harvesting
of large fish will similarly feed back onto nutrient regeneration rates.
Joel Cohen(this volume) closed our symposium with a consideration of the
consequences, in both predator and parasite food chains, of the average body
mass of a consumer being related to the average mass of the species consumed
by a power law with an exponent less than 1. We were struck by Cohen’s finding
that ‘one kilogram of resource supports a predator of larger body mass in a
terrestrial community than in a (marine) coastal community’, though it is not at
all obvious why this should be so. This and other intriguing outcomes of his
analyses only serve to highlight the need for more data on body size in natural
systems.
Concluding remarks
We finish with some speculations. Body-size allometry, and its extension to the
metabolic theory of ecology, does pretty well at predicting some, but not all, of
the patterns and processes dealt with in this book, and hence provides our
chapter title. Whilst appealing to issues of scale may these days seem like the
resort of the scoundrel, it is obvious that patterns appear and disappear as
spatial and temporal resolution changes and analyses include more and more
heterogeneous groups of species at different trophic levels. Further, the appro-
priate measure of body size is difficult to specify a priori, and we are struck by
nature’s ingenuity in creating ways of cheating the scaling and limitations of
body size, allowing organisms to ‘punch above their weight’ in obtaining food,
and in the process making the ecologist’s task more difficult. Examples include
filterers that build large external feeding structures andCohen’s(this volume)
cases of social hunting that enables predators to feed on prey much larger than
themselves. Perhaps even the very large marine filter feeders are only possible
because they can exploit whole aggregations of prey, effectively a ‘superindi-
vidual’, with a combined body size much larger than the single krill
‘particle’. The greatest challenge to a size-based approach to food webs lies with
the enormous terrestrial primary producers, trees, the largest of all organisms
but which lie at the base of land-based food webs, though their biomass is
mainly metabolically inactive (Cousinset al., 2005 ).
Transient dynamics and temporal fluctuation are referred to in several
papers, and clearly point to the notion that patterns based on energetic rules
and allometry may be obscured at the scale at which it is possible to observe
them. It seems likely that the processes underlying metabolic theory provide an
envelope or a constraint outside which natural systems may not sustainably lie,
BODY SIZE: IMPORTANT, BUT NOT THE WHOLE STORY 333