Using Time and Energetic Measures of Cost in Estimating Prey Value for Fish Predators
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Date
1984
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Abstract
In the predator-prey interaction between redear sunfish (Lepomis mlcrolophus) and
benthic, freshwater snails (Physa, Helisoma, and Oxytrema), we document selective predation among genera and sizes of prey and use optimal foraging theory in an attempt to explain diet selection. In
experiments, sunfish strongly selected against Oxytrema and weakly discriminated between Physa and Helisoma, with Physa most often chosen; size selection within any genus did not occur. Among genera,
selection results were consistent with differences in shell strength and a time cost/benefit (C/B) construct
operationally defined as handling time divided by prey dry mass. Within any genus, neither shell strength (smallest snails had weakest shells) nor time C/B (largest snails had minimal C/B) provided
predictions consistent with results from selective-predation experiments. To explain this discrepancy,
we measured metabolic costs of handling and energy content of prey. Dividing net energy of prey (E, generated by subtracting the energetic cost of handling from prey energy content) by handling time (T) yielded values of E/T that were similar for Physa and Helisoma. Within either genus, E/T was
always highest for largest snails. Thus, this construct also provided predictions inconsistent with our
experimental results. Only an energetic CIB ratio, in which the energetic costs of handling were divided
by the energetic value of the prey, was consistent with selection of Physa over Helisoma and no size selectivity within either genus. Whether predators add prey to their diet based on this construct is
unknown at present. The lack of concurrence between theoretical predictions of how prey should be
ranked by predators (i.e., according to energy gained per unit time expended) and our experimental
results suggests these constructs should be re-evaluated. Unless we know the discriminatory ability of
our predators (in terms of how fine a difference in prey types they can assess) and the currency used
in making decisions, CIB or EIT constructs provide little insight into diet selection by predators.
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Keywords
foraging costs, Helisoma, Lepomis microlophus, optimal foraging, Oxytrema, redear sunfish, selective predation, diet selection, freshwater snails, fish predators
Citation
Stein, Roy A.; Goodman, Cheryl Gosse; Marschall, Elizabeth A. "Using Time and Energetic Measures of Cost in Estimating Prey Value for Fish Predators," Ecology, v. 65, no. 3, 1984, pp. 702-715.