macroecological scales are the result of species interactions at the population
scale which, in turn, are driven by the activities of individuals.
Here we consider some of the consequences of using body size to understand
the organization of trophic links as we scale from the individual to the com-
munity, with a focus on size constraints on feeding and the body-size distribu-
tions of organisms in food webs. We draw mainly on examples from freshwater
systems, especially benthic ones, where most of our own work has been based,
and two data sets in particular: the invertebrate food webs from an acidic pond
(Skipwith Pond, Warren,1989) and an acidic woodland stream (Broadstone
Stream, Woodward & Hildrew,2002b). We must also make an important caveat
at this point: we are restricting ourselves to ‘traditional’ predatory relation-
ships, so parasitic interactions will not be considered in this chapter. There
are two reasons for this: first, very little is known of parasites in freshwater food
webs, where they appear to be far rarer than in marine and terrestrial systems,
and second, parasites often show inverse (or no) clear relationships with body
size of their ‘prey’ (e.g. Huxham, Beaney & Raffaelli,1996; Memmott, Martinez &
Cohen,2000).Feeding and size: processes at the individual level
Organisms are subject to natural selection for strategies and morphological,
behavioural and physiological adaptations that maximize foraging efficiency,
subject to other constraints (Schoener, 1971 ; Pyke, Pulliam & Charnov,1977;
Stephens & Krebs,1986 ). Such adaptations can include body size itself (Schoener,
1969 ; Lundberg & Persson,1993), but also the many specialized feeding adapta-
tions that aquatic organisms exhibit (Humphries, this volume). The trade-offs
inherent in maximizing foraging efficiency inevitably restrict the range of re-
source types that are exploited effectively (Pykeet al., 1977 ;Pyke,1984 ;Stephens
&Krebs,1986 ), and size plays a key role here because it is a component of both
the costs (e.g. handling time) and benefits (e.g. prey energy content) of foraging.
Size constraints on foraging can arise at each of the individual stages of the
predation sequence, from the initial encounter to the rate of digestion of
ingested prey (Thompson,1975; Perssonet al., 1998; Woodward & Hildrew,
2002c). They can also arise at different points within each stage: for instance,
attack rate has several subcomponents that commonly scale with body size,
including the reactive distance of the predator (e.g. Breck & Gitter, 1983 ), the
speed of movement of both predator and prey (e.g. Fry & Cox, 1970 ; Breck &
Glass, 1973 ), and the probability of capture (e.g. Smyly,1980; Pastorok, 1981 ).
Handling time also comprises elements that are often strongly size dependent:
these include, in order of temporal sequence, pursuit and subduing time, inges-
tion time, and digestion time (Woodward & Hildrew,2002c). Thus, while the
energetic reward of prey to a predator is likely to increase with prey mass, the
costs of consuming it are also likely to be size dependent, but in more complex100 G. WOODWARD AND P. WARREN