Community Ecology Processes, Models, and Applications

with terrestrial food webs, pelagic marine food

webs tend to be more strongly structured by body

size than by species, to contain more cannibalistic

and omnivorous species (Dunneet al. 2004) and to

contain more ‘loops’. This shifting ontogenetic

niche identity is well illustrated by stable nitrogen

isotope data for North Sea fishes, which show that

the trophic level of a species, averaged across onto-

genetic stages, was unrelated to its maximum size,

but was strongly related to body size at the individ-

ual level (Jenningset al. 2001; Fig. 8.2). Despite

such indeterminacy, however, both experiments

(Menge 1995) and observational data (Williams

and Martinez 2004; Thompsonet al. 2007) support

the reality of discrete trophic levels, at least at lower

positions in food webs. Moreover, maximum

trophic level in a system is clearly related to the

presence of species that can attain large size.

Cohenet al. (2003) showed for the pelagic food

web of Tuesday Lake, USA, that there are consistent

relationships among body size, numerical abun-

dance and trophic level of species, with primary

producers being both much smaller and more

abundant than predators. These relationships are

presumably also characteristic of pelagic marine

systems, which are similarly based on microscopic

algal producers. In the pelagic system, species rich-

ness also shows a pyramidal distribution with few

apex predators and many primary producer species

(Cohenet al. 2003; Petcheyet al. 2004). Predators, as

distinguished from parasites and pathogens, must

generally be larger than their prey (Broseet al.

2006), and simple allometric theory dictates that

the few larger, less abundant species high in the

food web also are slower growing than basal

species.

These considerations have several general impli-

cations for responses of marine communities to

human influence (Duffy 2002, 2003; Petcheyet al.

2004). First, the smaller populations and slow pop-

ulation growth rates of top predators should raise

their risk of extinction due to demographic and

C1

Top carnivore

Omnivores

Herbivores

H1

P1 P2 P3

Edible plants

1

2

3

P4 P5 P6

Inedible plants

‘Horizontal’ biodiversity

‘Vertical’

biodiversity H2

01 02

Figure 8.1Components of horizontal and vertical diversity in a schematic food web. Vertical diversity includes average
food chain length and degree of feeding from one (e.g. herbivores) compared with more than one (omnivores, and
cannibalistic top carnivore) trophic level. Horizontal diversity includes number of functional groups and degree of feeding
specialization (e.g. species H2) compared with generalism (e.g. H1, O1 and O2). Reproduced with permission from Duffy
et al. (2007).

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