9780521861724htl 1..2

summation of the feeding niches of the consumer individuals (Fig.6.6). Most

studies suggest that consumer species are, on average, about one to four orders

of magnitude larger than their prey, in terms of body mass (Woodwardet al.,

2005b) as in the Broseet al., ( 2005) database. However, because the size of the

protagonists is rarely measured for individual acts of predation, but taken from

the average size of individuals across the entire population, these reported

ratios are likely to be somewhat misleading. The effect of species averaging

can be seen in the Broadstone Stream food web where, if we calculate the

average consumer-resource ratio by dividing across the total number of feeding

links within the summary food web (i.e. all species pairs of predators and prey,

Pi

A B A B

A B A B

Pi

di dii diii div

Pii Piii Piv

(c)

(d)

(a)

(b)

predator

size

frequency

in diet

foodsize

niche

frequency

predator

size

frequency

in diet

predator

species size

food size

limits

prey species

sizes

distribution of

prey eaten

predator size

distrubution

food size

limits

prey size

distrubution

distribution of

prey eaten

foodsize

niche

frequency

Figure 6.6Scaling from the individual to species-averaged food webs. Panel (a) shows

resource use of a single predator (solid square symbol) feeding on one of a pair of prey

species (each having a distribution of individuals of different sizes). The predator exploits

different sizes of prey with differing effectiveness within the upper and lower size limits of

its feeding niche. Only a portion of the size spectrum of the prey species is exploited, with

upper- and lower-size refugia being evident. Curve B here represents non-exploited ‘prey’

species outside the predator’s size envelope of potential prey. (b) Resource use by four

different predator individuals of different life stages, reflecting ontogenetic shifts in diet

as a predator grows. (c) The distributions of body sizes of both consumers and resources,

across a predator population comprising of many individuals of different sizes of both

predator and prey (square symbols represent the average body size of each species).

(d) Trophic interactions depicted as single links between single nodes, which are the species

represented by their average size (square symbols), as in species-averaged food webs.

108 G. WOODWARD AND P. WARREN