3 FOOD WEB STRUCTURE, PRODUCTIVITY ANDACT AS ‘ECOLOGICAL ENGINEERS’...

20.3 Food web structure, productivity and

act as ‘ecological engineers’ (see Section 13.1). Beavers, for

stability

example, in cutting down a tree and building a dam, create a

habitat on which hundreds of species rely. Keystone mutualists

(Mills et al., 1993) may also exert influence out of proportion

Any ecological community can be characterized by its structure

to their abundance: examples include a pollinating insect on

(number of species, interaction strength within the food web,

which an ecologically dominant plant relies, or a nitrogen-fixing

average length of food chains, etc.), by certain quantities (espe-

cially biomass and the rate of production of biomass, which

bacterium supporting a legume and hence the whole structure

we can summarize as ‘productivity’) and by its temporal stability

of a plant community and the animals reliant on it. Certainly,

keystone species are limited neither to top predators nor con-

(Worm & Duffy, 2003). In the remainder of this chapter, we

examine some of the interrelationships between these three.

sumers mediating coexistence amongst their prey. For example,

Much of the very considerable recent interest in this area has

munity depends on the environment in which it exists, as well

been generated by the understandable concern to know what might

as on the densities and characteristics of the component species.

A community that is stable only within a narrow range of envir-

be the consequences of the inexorable decline in biodiversity

onmental conditions, or for only a very limited range of species’

(a key aspect of structure) for the stability and productivity of

characteristics, is said to be dynamically fragile. Conversely, one that

biological communities.

We will be particularly concerned with the effects of food

is stable within a wide range of conditions and characteristics is

said to be dynamically robust.

web structure (food web complexity in this section; food chain

Lastly, it remains for us to specify the aspect of the com-

length and a number of other measures in Section 20.4) on the

munity on which we will focus. Ecologists have often taken a

stability of the structure itself and the stability of community pro-

ductivity. It should be emphasized at the outset, however, that

demographic approach. They have concentrated on the structure

of a community. However, it is also possible to focus on the

progress in our understanding of food webs depends critically on

the quality of data that are gathered from natural communities.

stability of ecosystem processes, especially productivity.

Recently, several authors have called this into doubt, particularly

for earlier studies, pointing out that organisms have often been