Behavior that leads to aggregated distributions

locating a patch food is patchily distributed

For all consumers, food is distributed patchily. The patches may be natural and discrete physical objects: a bush

There are various types of behavior underlying the aggregative responses of consumers, but they fall into two broad categories: those involved with the location of profitable patches, and the responses of consumers once within a patch. The first category includes all examples in which consumers perceive, at a distance, the existence of heterogeneity in the distribution of their prey.

Within the second category -responses of consumers within patches area-restricted search - there are two main aspects of behavior. The first is a change in the consumer's pattern of searching after encountering items of food. In particular, there is often a slowing down of movement and an increased rate of turning immediately following the intake of food, both of which lead to the consumer remaining in the vicinity of its last food item ('area-restricted search'). Alternatively, or in addition, consumers may simply abandon unprofitable patches more rapidly than they abandon profitable ones. Both types of behavior were evident when the carnivorous, net-spinning larva of the caddis-fly Plectrocnemia conspersa feeds on chironomid (midge) larvae in a laboratory stream. Caddis in their nets were provided with one prey item at the beginning of the experiment and then fed daily rations of

Figure 9.20 Aggregative responses: (a) coccinellid larvae (Coccinella septempunctata) spend more time on leaves with high densities of their aphid prey (Brevicoryne brassicae) (after Hassell & May, 1974); (b) redshank (Tringa totanus) aggregate in patches with higher densities of their amphipod prey (Corophium volutator) (after Goss-Custard, 1970); (c) direct density dependence when the parasitoid Delia radicum attacks Trybliographa rapae; and (d) direct density dependence when the parasitoid Aspidiotiphagus citrinus attacks Fiorinia externa. (e) But direct density dependence is not always the case: inverse density dependence when the parasitoid Ooencyrtus kuwanai attacks Lymantria dispar. ((c-e) after Pacala & Hassall, 1991.)

Figure 9.20 Aggregative responses: (a) coccinellid larvae (Coccinella septempunctata) spend more time on leaves with high densities of their aphid prey (Brevicoryne brassicae) (after Hassell & May, 1974); (b) redshank (Tringa totanus) aggregate in patches with higher densities of their amphipod prey (Corophium volutator) (after Goss-Custard, 1970); (c) direct density dependence when the parasitoid Delia radicum attacks Trybliographa rapae; and (d) direct density dependence when the parasitoid Aspidiotiphagus citrinus attacks Fiorinia externa. (e) But direct density dependence is not always the case: inverse density dependence when the parasitoid Ooencyrtus kuwanai attacks Lymantria dispar. ((c-e) after Pacala & Hassall, 1991.)

zero, one or three prey. The tendency to abandon the net was lowest at the higher feeding rates (Townsend & Hildrew, 1980). Plectrocnemia's behavior in relation to prey patches also has an element of area-restricted search: the likelihood that it will spin a net in the first place depends on whether it happens to encounter a food item (which it can consume even without a net) (Figure 9.21a). Overall, therefore, a net is more likely to be constructed, and less likely to be abandoned, in a rich patch. These two behaviors account for a directly density-dependent aggregative response in the natural stream environment observed for much of the year (Figure 9.21b).

The difference in the rates of abandonment of patches of high and low profitability can be achieved in a number of ways, but two are especially easy to envisage. A consumer might leave a patch when its feeding rate drops below a threshold level, or a consumer might have a giving-up time - it might abandon a patch whenever a particular time interval passes without the successful capture of food. Whichever mechanism is used, or indeed if the consumer simply uses area-restricted search, the consequences will be the same:

individuals will spend longer in more profitable patches, and these patches will therefore generally contain more consumers.

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