. LIFE HISTORY OMNIVORY ALSO REDUCES STABILITY, BUT MUCH LESSINTE...

1987). Life history omnivory also reduces stability, but much less

interactions are weak. (The most perfectly compartmentalized

than single life stage omnivory does. Intriguingly, omnivory is not

destabilizing in donor-control models, and omnivores are common

community possesses only linear food chains.) Do food webs

tend to be compartmentalized?

in decomposer food webs (Walter, 1987; Usio & Townsend, 2001;

Woodward & Hildrew, 2002), to which donor-control dynamics

Not surprisingly, in studies where habitat divisions are major

and unequivocal, there is a clear tendency for compartments to

can be applied.

map onto habitats. For instance, Figure 20.18 shows the results

In fact, an increasing number of studies indicate that omnivory

of a classic study describing the major interactions within and

is not uncommon at all, and that earlier indications of its rarity

between three interconnected habitats on Bear Island in the

were an artefact of the webs being only poorly described (Polis

Arctic Ocean (Summerhayes & Elton, 1923). There is a signific-

& Strong, 1996; Winemiller, 1996). For example, Sprules and

antly smaller number of interactions between habitats than

Bowerman (1988) found omnivory to be common in plankton

would be expected by chance (Pimm & Lawton, 1980).

food webs in North American glacial lakes, having identified all

On the other hand, when habitat divisions are subtler, the

their zooplankton to species level and produced webs that were

much more reliable as a result (Figure 20.17). Polis (1991) found

evidence for compartments is typically poor, and there are even

greater difficulties in providing a clear demonstration of com-

similar results in his detailed study of a desert sand community.

What is more, later modeling studies have undermined the whole

partments (or the lack of them) within habitats. Early analyses,

certainly, suggested that food webs within habitats are only as

suggestion that omnivory is inherently destabilizing. Dunne

compartmentalized as would be expected by chance alone (Pimm

et al.’s (2002) simulation study detected no relationship between

& Lawton, 1980; Pimm, 1982). More recently, though, promising

the level of omnivory and the stability of webs to species removal,

methodological advances have been made that seem capable of

while other models indicate that omnivory may in fact stabilize

identifying compartments within larger webs, especially when the

food webs (McCann & Hastings, 1997). It is sobering to note that

Figure 20.18 The major interactions

17

27 18

within and between three interconnected

habitats on Bear Island in the Arctic Ocean.