untitled

redistributed themselves in precisely this manner (Milinski 1979). Similar results have

been recorded in continuous food input experiments with numerous other species,

including mallard ducks (Harper 1982), cichlid fish (Godin and Keenleyside 1984),

and starlings (Inman 1990). Measurements in the field have been less supportive.

However, animals in preferred habitats generally obtain higher rates of food intake

than those relegated to poorer habitats (Sutherland 1996). Researchers frequently find

that individuals vary in the quantity of food that they acquire, with more dominant

or larger individuals securing more of the food delivered than lower-ranking indi-

viduals. This hierarchy suggests that although animals are capable of adjusting their

behavior in predictable ways to accommodate the presence of other competitors for

scarce resources, differences in dominance status tend to maintain differences in fitness

(Sutherland 1996).

One way to accommodate these effects is through a modified model known as the

ideal despotic distribution(Fretwell 1972). This model assumes that individuals choose

the best habitat possible on the basis of their dominance status. The most dominant

individuals choose first, followed by others in rank order of their dominance status.

Under these conditions, individuals of similar status might well choose to split their

time between two habitats offering similar levels of suitability, whereas high-ranking

individuals invariably choose the best habitat. More importantly, the ideal despotic

distribution predicts that there will be disparities in food intake, mortality risk, or

reproductive success among individuals. These differences dissolve when we focus

on individuals of similar rank.

The negative impact of other individuals on foraging success is sometimes termed

interferenceby ecologists. It can result from direct aggression, stealing of food from

other foragers, depletion of prey, or from uneaten prey scattering or hiding from other

foragers. If we presume that aggression is the main cause of interference, we can

predict how it will affect feeding rates. While searching for prey, individuals should

encounter other predators at random. If each encounter between predators resulted in

a wastage of wtime units, then the foraging rate, f(N, P), can be well approximated

by a modified Type II functional response (Beddington 1975; Ruxton et al. 1992):

f(N, P) =

This formula predicts that interference should increase with predator forager density

(Fig. 5.11). Similar logic can be used to develop an analogous model of interference

arising from food thievery, also known as kleptoparasitism(Holmgren 1995).

Numerous field experiments have demonstrated such an increase in interfer-

ence strength with forager abundance on organisms ranging from oystercatchers

(Haematopus ostralegus) (Goss-Custard and Durrell 1987) to caribou (Manseau 1996).

It is conventional to measure interference from plots of log intake versus log forager

abundance. Thus, Fig. 5.12 illustrates changes in intake of cockles by oystercatchers

in the Netherlands as a function of forager density (Sutherland 1996).

The ecological impact of interference can be profound (Beddington 1975;

DeAngelis et al. 1975), adding a strong density-dependent effect to consumer–

resource interactions that might otherwise be highly variable over time (see Chapters

12 and 19). Hence, interference can be a mechanism in the natural regulation of wildlife

aN

1 +ahN+awP

74 Chapter 5

5.6.2Interference

among foragers

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