It is worth stressing that foraging strategies will not always be strategies for simply maximizing feeding efficiency. On the contrary, natural selection will favor foragers that maximize their net benefits, and strategies will therefore often be modified by other, conflicting demands on the individuals concerned. In particular, the need to avoid predators will frequently affect an animal's foraging behavior.
This has been shown in work on foraging by nymphs of an aquatic insect predator, the backswimmer Notonecta hoffmanni (Sih, 1982). These animals pass through five nymphal instars (with I being the smallest and youngest, and V the oldest), and in the laboratory the first three instars are liable to be preyed upon by adults of the same species, such that the relative risk of predation from adults was:
These risks appear to modify the behavior of the nymphs, in that they tend (both in the laboratory and in the field) to avoid the coevolution: predator-prey arms races?
backswimmers forage suboptimally but avoid being preyed on ...
Open water Sediments Vegetation
100 80 60 40 20
rcto 40 20
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15 31 15 31 15 30 15 31 15 31 15 30 15 31 15 31 15 30 Jul Aug Sep Jul Aug Sep Jul Aug Sep
Figure 9.18 Seasonal patterns in (a) the predicted habitat profitabilities (net rate of energy gain) and (b) the actual percentage of the diet originating from each habitat, for three size classes of bluegill sunfish (Lepomis macrochirus). Piscivores were absent. (The 'vegetation' habitat is omitted from (b) for the sake of clarity - only 8-13% of the diet originated from this habitat for all size classes of fish.) There is good correspondence between the patterns in (a) and (b). (After Werner et al., 1983b.)
central areas of water bodies, where the concentration of adults is greatest. In fact, the relative degree of avoidance was the same as the relative risk of predation from adults:
Yet these central areas also contain the greatest concentration of prey items for the nymphs, and so, by avoiding predators, nymphs of instars I and II showed a reduction in feeding rate in the presence of adults (although those of instar III did not). The young nymphs displayed a less than maximal feeding rate as a result of their avoidance of predation, but an increased survivorship.
The modifying influence of predators on foraging behavior has also been studied by Werner et al. (1983b) working on bluegill sunfish. They estimated the net energy returns from foraging in three contrasting laboratory habitats - in open water, amongst water weeds and on bare sediment - and they examined how prey densities varied in comparable natural habitats in a lake through the seasons. They were then able to predict the time at which the sunfish should switch between different lake habitats so as to maximize their overall net energy returns. In the absence of predators, three sizes of sunfish behaved as predicted (Figure 9.18). But in a further field experiment, this time in the presence of predatory largemouth bass, the small sunfish restricted their foraging to the water weed habitat (Figure 9.19) (Werner et al., 1983a). Here, they were relatively safe from predation, although they could only achieve a markedly submaximal rate of energy intake. By contrast, the larger sunfish are more or less safe from predation by bass, and they continued to forage according to the optimal foraging predictions. In a similar vein, the nymphs of several species of algivorous mayflies largely restrict their feeding to the hours of darkness in streams that contain brown trout, reducing their overall feeding rates but also reducing the risk of predation (Townsend, 2003). In the case of mammals that feed at night, including mice, porcupines and hares, time spent feeding may be reduced in bright moonlight when predation risk is highest (Kie, 1999).
A foraging strategy is an integral part of an animal's overall pattern of behavior. The strategy is strongly influenced by the selective pressures favoring the maximization of feeding efficiency, but it may also be influenced by other, possibly conflicting demands. It is also worth pointing out one other thing. The places where animals occur, where they are maximally
... as do certain fish predation and the realized niche
abundant and where they choose to feed are all key components of their 'realized niches'. We saw in Chapter 8 that realized niches can be highly constrained by competitors. Here, we see that they can also be highly constrained by predators. This is also seen in the effects of predation by the barn owl (Tyto alba) on the foraging behavior of three heteromyid rodents, the Arizona pocket mouse (Perognathus amplus), Bailey's pocket mouse (P. baileyi) and Merriam's kangaroo rat (Dipodomys merriami) (Brown et al., 1988). In the presence of owls, all three species moved to microhabitats where they were less at risk from owl predation and where they reduced their foraging activity. However they did so to varying extents, such that the way in which the microhabitat was partitioned between them was quite different in the presence and absence of owls.
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