THE EFFECT OF SEASON AND ANIMAL SIZE ON THE CALORIC CONTENT OF DAPHNIA PULICARIA FORBES1,2

110

Estimations of invertebrate predation parameters usually rely on behavioral observations and on feeding rates in single, arbitrary prey densities. In this study, predation parameters were estimated directly from functional response data. This type of analysis relies on many prey densities and therefore should lead to more widely applicable parameter values.Functional response curves of third-and fourth-instar Chaoborus americanus preying on five sizeclasses of Daphnia pulex in the laboratory were examined. Data for each size-class were fitted to Rogers' random predator equation, a modification of Holling's type II functional response equation. The data fit the model well.The parameters a' (instantaneous attack rate) and T. (handling time) were calculated for each prey size from these curves. Attack rate reached a maximum, indicating highest vulnerability, at an intermediate size. Deduced handling times were greater than simple observation would suggest and were highly correlated with prey dry mass, suggesting digestion as the primary component.Results of mixed-size-class experiments were compared to predictions based on the parameters from single-size-class experiments. The ratio of predation-caused mortality rates for prey size-classes was consistent with the predictions, suggesting that no active behavioral selection was occurring. Feeding rates in mixed-size-class experiments were underestimated when experiments were run a month later than the single-size-class experiments during the 1st yr of this study. However, when single-and mixed-size-class experiments were run simultaneously during the 2nd yr, predictions were very close to observed feeding rates. This indicates the possibility that predator age and seasonal physiological changes affect functional response parameters.Attack rate increased with temperature, probably due to increased encounter rates as Daphnia swimming speed increased. Handling time decreased with increased temperature, probably due to increased digestive rate.The biological significance of Holling's parameters and those of other more behaviorally based models is discussed.

Section: Methods Of Analysismentioning

confidence: 97%

Functional Response of an Ambush Predator: Chaoborus americanus Predation on Daphnia pulex

Spitze¹

1985

110

“…In substituting this equation into the definition of patch payoffs note that FA = n and F 8 = Nn. To convert the number of items captured to energy obtained we used a value of 0.2193 J/item (Snow 1972) and an assimilation efficiency of 0.7 (Ware 1975). Because the value of p,(•) depends upon the number of fish in the patch, the solution for the predicted number of fish in Patch A is defined implicitly for the Capture Rate Matching IFD and the IFD With Costs II.…”

Section: Predicting Fish Distributionsmentioning

confidence: 99%

Ideal Free Distributions of Stream Fish: A Model and Test with Minnows, Rhinicthys Atratulus

Tyler¹,

Gilliam²

1995

Ideal Free theory has furthered our understanding of the processes determining the distribution of mobile foragers in a spatially heterogeneous (patchy) habitat. The Input Matching rule derived from Ideal Free theory has been used to predict forager distributions, but does not account for unshared environmentally induced costs that individuals may incur. Drift—feeding stream fish typically contend with such costs in the form of (1) an energetic costs of maintaining position while foraging in moving water, and (2) a decrease in the proportion of drift food items they can capture as current speed increases. These costs often differ between patches, and the cost of position maintenance is unaffected by changes in the number of fish foraging in a patch. Here, we developed an Ideal Free Distribution (IFD) model (IFD With Costs) to describe forager distributions in a habitat composed of patches in which foragers compete for food and incur an energetic cost when foraging that is unaffected by the number of competitors in the patch. We tested the ability of two versions of the IFD With Costs as well as two alternative IFD models to predict the distribution of minnows (Rinichtys atratulus) foraging in a laboratory flow—tank with two side—by—side patches. In all trials the patches differed in their supply of drift food, and in some trials the patches had different water velocities. When the patches' water velocities differed, the fish distributions (1) changed as the total supply of food in the two patches increased while the ratio of food supply in the patches remained constant, (2) did not match relative food availabilities in the patches, and (3) favored the slow water velocity patch. One version of the IFD With Costs predicted each of these results while none of the alternative models could account for these findings. The results support the hypothesis that fish quantitatively integrate energetic gains and costs in a manner similar to that described by the IFD With Costs when making patch selections.

“…This equation was obtained by linear regression (n = 63, r 2 = .98, P < .001) on data given by Snow (1972: Table 1, February 1970through January 1971. Some of the variance for large Daphnia is due to systematic seasonal changes in energy value (callg dry mass; Snow 1972: Table 2).…”

Section: Energetic Value Of Preymentioning

confidence: 99%

Prey Vulnerability and Size Selection by Chaoborus Larvae

Pastorok

1981

287

204

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.