partitioning can be seen clearly within the Broadstone Stream food web, where
diet width increases with predator size when only macrofaunal prey are consid-
ered, but the opposite is true when links to both the macrofaunal and meiofau-
nal prey assemblage are included (Woodwardet al., 2005c).
In addition to ontogenetic and seasonal effects, population size structures can
also be altered through extrinsic factors. For instance, strong size-selective
predation, as occurs in commercial fisheries (e.g. Policansky & Magnuson,
1998 ), can extirpate prey species and change community structure (Brooks,
1968 ; Brooks & Dodson, 1965 ), but where predation falls on some parts of a
population only, and is not so strong, the effect may be to alter the size struc-
ture, which could then have consequences for the feeding links from the
affected population to others, and change the size structure of the wider food
web (e.g. Blumenshineet al., 2000; Jennings, this volume).Macroecological patterns: consequences of body-size
shifts in response to environmental gradients
Although body size affects feeding interactions at the individual level, the
effects of such constraints also ramify through higher levels of organization,
to the community and ecosystem (Persson & De Roos, this volume). The implicit
suggestion is that, if individual interactions are determined largely by body size,
then alterations in the distribution of body sizes within communities are also
likely to have powerful effects at macroecological scales. Linked to this is the
question of how size distributions change with species richness, and what the
likely consequences are for food-web dynamics and ecosystem functioning
across environmental gradients. The number of species and the distribution
of body sizes within a food web are determined by a combination of biotic and
environmental factors, and the relative importance of the two varies over differ-
ent spatial scales. For instance, at biogeographical scales, species richness
decreases and body mass increases with latitude (Ricklefs,2004), whereas at
finer spatial scales, anthropogenic stressors often play a key structuring role in
many aquatic communities (Gilleret al., 2004; Petcheyet al., 2004).
Predictable shifts in the taxonomic composition of a community in response
to environmental gradients are well known, but much less attention has been
paid to how the size spectrum changes across abiotic gradients, with the excep-
tion of some marine systems, where abundance-biomass curves have been used
for some time to infer environmental stress (e.g. Warwick & Clarke,1991;
Warwick, this volume). Stressor-induced species loss is highly non-random
with respect to body size: large, rare species, and particularly those at the higher
trophic levels, are typically the first to be lost, and this can alter the size
spectrum and distribution of interaction strengths within the food web
(Petcheyet al., 2004; Woodwardet al., 2005b). The disproportionate loss of
large species is likely to have particularly strong effects on the ‘functioning’ of112 G. WOODWARD AND P. WARREN