smaller than themselves. The second is that larger predators tend to take larger
prey individuals on average, but this is chiefly because the prey maximum
increases with predator size. The prey minimum tends to increase more slowly
than the maximum; with the result that the prey-size range increases with
predator size (Woodward & Hildrew,2001, 2002b): the mechanisms behind
this are probably complex, but it is likely to be driven by a combination of
both predator gape limitation and the size distribution of the potential prey
assemblage. The consequence of this scaling is that the diets of individual
predators shift with changing predator body size: a phenomenon documented
by Hardy ( 1924 ) in one of the earliest published food webs, and termed meta-
phoetesis by Hutchinson (1959). Ontogenetic dietary shifts with growth are well
known but often overlooked in food-web studies, where each ‘node’ is effec-
tively averaged over all developmental stages (Woodward & Hildrew,2001;
Woodwardet al., 2005c), or in some cases represents only certain parts of the
developmental sequence. Given that organisms can move through three or
more orders of magnitude of body mass over the course of their development,
if the prey-size niche shifts in proportion then dietary changes can be profound,
and the consequences for interpretation of feeding interactions when indivi-
duals are aggregated to species can be considerable: indeed, ontogenetic differ-
ences within species can far exceed taxonomic differences among species
(Woodward & Hildrew, 2001 , 2002b).
Scaling to species
Although size constraints and predator–prey interactions operate at the level of
individuals, when examining consequences for community structure we are
usually interested in partitioning the interactions so as to make discrete associ-
ations between species, which are the entities in the system to which other
dynamic metrics are relevant (e.g. population growth rate). Recurrent size-
dependent patterns reported from species-averaged webs include ‘upper trian-
gularity’ and nested feeding niches within food-web matrices, such that (in
general) predators feed on prey that are smaller than themselves and diet
width expands with predator size (Fig.6.4). Analysis of a recently compiled
metadatabase of consumer resource body-size ratios (Broseet al., 2005) shows
a wide range of possible values for predatory invertebrates, although the aver-
age is close to two orders of magnitude in both marine (log-ratio 1.80.08SE)
and freshwater (log-ratio 1.90.06 SE) systems (Fig.6.5). Marine systems display
a wider range of ratios than is the case in freshwaters, presumably because the
greater volume of habitat space can support far larger consumers, in addition to
phylogenetic constraints: freshwater food webs are often dominated by insects,
whereas these arthropods are vanishingly rare in marine systems, where they
are replaced by other taxa, such as crustaceans, that can attain far larger body
sizes. Despite the average log-ratio being close to two, there are some prey
BODY SIZE AND PREDATORY INTERACTIONS IN FRESHWATERS 105