clear patterns of macroinvertebrate body size are ‘sometimes apparent but
rarely account for a large proportion of variation along the (environmental)
dimension of interest’. They attribute this result to the complexity of multiple
stressors in natural systems and to the inclusion of mixtures of organisms with
different ‘solutions’ (including body size) to particular environmental condi-
tions. We also wonder whether in such systems, population and community
dynamics will often be transient and thus the ‘snapshot’ patterns often weak.
Disturbance is one obvious cause of fluctuations, though others are feasible, and
we return to this notion below.
Jennings and Reynolds (this volume) relate body size to the pressures of
commercial fishing and to fisheries management (see also Atkinson & Hirst,
this volume). The effects of fishing on the population size structure of exploited
species have been stark and, apart from the well-known phenomenon of ‘fish-
ing down the food chain’ in overexploited systems, Jennings and Reynolds (this
volume) have managed to use theory and data to predict that the abundance of
large fish (16–66 kg body weight) in the North Sea may now be a remarkable
99.2% lower than before exploitation. Jennings and Reynolds’ paper is wide
ranging, and also deals with community-level phenomena such as trophic
structure and biomass size spectra, and they point out the practical applications
of food-web theory in the adoption of ecosystem and community-level manage-
ment of fisheries and the oceans. In terms of trophic structure, they highlight
the absence of clear trophic steps (trophic levels) in most aquatic (as opposed to
terrestrial) ecosystems, and the prevalence of life-cycle omnivory and size-based
predation, all arguments for a size-based analysis of the ecosystem impacts of
exploitation.
Two other chapters focus on body size in relation to species interactions,
food-webs and food web theory. Woodward and Warren (this volume) provide
much evidence for the importance of body size in determining feeding links,
food-web structure and life-cycle omnivory. Communities of freshwater benthic
invertebrates evidently provide some of the clearest evidence of all of the
primacy of body size in natural food webs, thus chiming with Jennings and
Reynolds’ (this volume) conclusions. They analyze explicitly the limits, both
upper and lower, to the size disparity observed between predator and prey – i.e.
both upper and lower size refugia exist – and show the effect of basing estimates
of size disparity on individuals and on species averages. If species averages are
taken (i.e. the average size of the predator and the average size of the prey) it
turns out that the predator is about ten times greater than the prey, whereas if
individual links are considered (i.e. the size of the particular predator and its
actual prey) the mean size disparity is about 100.Jones and Jeppesen(this
volume) consider further the role of body size in relation to trophic structure,
and in their case to the occurrence of alternative regimes and in particular of
trophic cascades, many examples of which seem to be ‘wet’ (Strong,1992). In the330 A. G. HILDREWETAL.