Function Of Prey Size Research Articles

Experimental Tests of Optimal Habitat Use in Fish: The Role of Relative Habitat Profitability

Utilizing optimal foraging theory and laboratory estimates of foraging costs, we predict the choice of foods and use of habitats by fish in the field. These predictions are tested with the bluegill sunfish (Lepomis macrochirus) foraging in three habitats (open water, sediments, and vegetation) in a pond. Relations describing prey encounter rates in each habitat as a function of prey size, prey density, and fish size were derived from laboratory experiments. These relations permitted us to estimate prey encounter rates based on weekly prey samples in each habitat of the pond. We can then determined the optimal diet and profitability (net energy return) for each habitat through time. Predictions of optimal diet exhibited good qualitative correspondence to the actual diet of the fish in the open water and vegetation, although we consistently predicted a slightly narrower diet than the fish were choosing. The model correctly predicted the magnitude of the change in size selection on Daphnia pulex with fish size and with decline in prey density. Predictions of optimal diet in the sediments were considerably in error apparently due to a tendency for late—instar midges to burrow deep in the sediments, thereby becoming unavailable to the fish. In this case habitat profitabilities were computed simply on the basis of the actual observed diet. Predictions of optimal habitat use i.e., when the fish should switch habitats to maximize feeding rates, showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of our predictions. The actual habitat use pattern differs dramatically from a null model of random habitat use. We indicate how this approach may be useful in studying intra— and interspecific exploitative interactions.

Prey Vulnerability and Size Selection by Chaoborus Larvae

The planktonic larva of Chaoborus trivittatus (Diptera: Chaoboridae) feeds in an opportunistic manner on all sizes of its major prey, Daphnia pulicaria. Nevertheless, medium—sized prey constitute a greater portion of the diet than would be expected on the basis of their relative abundance in the environment alone. The differential vulnerability of food items is responsible for this apparent size—selection. Vulnerability is defined here as the product of the encounter rate between predator and prey and the capture success of the predator. The rate of encounter increases as a function of prey size, whereas the capture success of the predator decreases with prey size. This leads to a vulnerability curve that is concave to the prey size axis; hence, medium—size items should be overrepresented in the diet when the predator feeds indiscriminately. A model of foraging behavior, which incorporates this concept of differential vulnerability, is used to predict the optimal breadth of diet for Chaoborus feeding on a mixture of prey sizes. Because Chaoborus is an ambush predator, pursuit time is insignificant, and the profitability of prey size i(ei/ti) can be approximately by the joules assimilated from eating an individual of size i divided by the handling time. The energy content of Daphnia increases as approximately the third power of body length. Handling time, defined as the time taken to swallow an item, increases exponentially as a function of prey size. The profitability curve is concave to the prey—size axis, peaking in the middle of the prey size range. However, the foraging model predicts that the larva should include all sizes of Daphnia in its diet within the range of prey densities commonly found in nature. Furthermore, an analysis of the crop contents of C. trivittatus in Lake McDonald and in laboratory feeding experiments supports the hypothesis that larvae are opportunistic feeders on Daphnia; the size composition of the diet can be explained by the relative abundance of prey sizes in the environment weighted by their relative vulnerability. Since vulnerability may be correlated with profitability for many predator—prey combinations, it is possible that many apparent examples of optimal behavior can be explained simply on the basis of the differential vulnerability of prey.

SELECTIVE PREDATION BY TWO SWIFTS AND A SWALLOW IN CENTRAL AMERICA

SummarySamples of flying insects were made at three sites and during dry and wet seasons in Costa Rica and at two sites during the wet season in Panama. Compared with these were the diets of three aerially foraging birds: the Band‐rumped Swift Chaetura spinicauda and Rough‐winged Swallow Stelgidopteryx ruficollis in Costa Rica and the Short‐tailed Swift Chaetura brachyura in Panama.Among insects flying, Diptera accounted for between 52 and 75%, with smaller numbers of Hymenoptera (mean = 15%) and Coleoptera (10%). In bird stomach contents, however, Hymenoptera predominated (59–81%), Coleoptera were less common (mean = 18.5%) and Diptera least common (3.4%) of the three orders. The discrepancy between diets and samples of flying insects could not be accounted for solely on the basis of differential digestion. The composition of the swift and swallow diets was similar; there were no significant seasonal differences in the taxonomic composition of the diet of Chaetura spinicauda.Size distributions of beetle prey fitted log‐normal distributions in every case. Mean size of prey was significantly larger than that of insects available in all comparisons. Although there was no seasonal difference in the mean size of prey of Chaetura spinicauda, the variance in prey size, used as a measure of niche width, was significantly larger in the wet than in the dry season. Although insect densities may be higher, foraging time is reduced and the wet season is likely the worse season. This result therefore supports Emlen's hypothesis of narrower niches under optimal conditions and broader niches under suboptimal conditions.For increasing altitude above ground published data show a decrease in the density of insects flying, a decrease in the proportions of taxa favoured by the birds, and a significant decrease in the mean size of beetles. None of these trends can explain the differences between the birds' diets and the samples of flying insects.Preference of prey by these birds is concluded not to be a function of prey size alone, but to involve interaction among prey size, ease of capture, and local density. Because of different flight abilities, insect taxa will differ in ease of capture and, consequently, in mean size preferred by the predator, as well as proportions in the diet. This factor favours Hymenoptera and Coleoptera over Diptera as prey. Local concentrations of winged ants, bees and social wasps, and fig wasps make these prey differentially easy to exploit and explain the preference of Hymenoptera over Coleoptera. Large species of swifts are hypothesized to be the evolutionary consequence of the tendency of aerially foraging birds to exploit local concentrations of single prey species.