Effects of crowding

The most obvious omission, perhaps, from the predator-prey interactions we have modeled so far has been any acknowledgement that prey abundance may be limited by other prey, and predator moths and two natural enemies


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10 20 30 40 50 60 70 80 90

Log (number)

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10 20 30 40 50 60 70 80 90 Time (weeks)

Figure 10.5 Host generation-length cycles in the moth Plodia interpunctella (a) alone (black line) and (b) with a granulovirus (colored line). These dynamics may be compared with those in Figure 10.1c. In spite of a superficial similarity in pattern, analysis indicates that those in (a) are generated by intraspecific competition; those in (b) are simply modulated versions of those in (a) and are therefore not predator-prey cycles. However, those in Figure 10.1c are predator-prey cycles. (After Bjornstad et al., 2001.)

abundance by other predators. Prey are bound to be increasingly affected by intraspecific competition as their abundance increases; and predators, too, are likely to be limited at high densities by the availability of resting places, say, or safe refuges of their own, quite apart from their interaction with their most obvious resource, their prey.

More generally, predators have mutual interference been assumed in the models discussed thus far to consume prey at a rate that depends only on prey abundance (in Equation 10.2, for example, the consumption rate per predator is simply aN). In reality, consumption rate will also often depend on the abundance of the predators themselves. Most obviously, food shortage - the abundance of prey per predator - will commonly result in a reduction in the consumption rate per individual as predator density increases. However, even when food is not limited, the consumption rate can be reduced by a number of processes known collectively as mutual interference (Hassell, 1978). For example, many consumers interact behaviorally with other members of their population, leaving less time for feeding and therefore depressing the overall feeding rate. For instance, humming-birds actively and aggressively defend rich sources of nectar. Alternatively, an increase in consumer density may lead to an increased rate of emigration, or of consumers stealing food from one another (as do many gulls), or the prey themselves may respond to the presence of consumers and become less available for capture. All of these mechanisms give rise to a decline in predator consumption rate with predator density. Figure 10.6a, for example, shows significant reductions in consumption rate with abundance even at low densities of the crab Carcinus aestuarii foraging for the mussel Musculista senhousia; while Figure 10.6b shows that the kill rate of wolves, Canis lupus, preying on moose, Alces alces, in Isle Royale National Park, Michigan, USA, was lowest when there were most wolves.

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