What do food choice experiments measure? Differences in prey behavior and traits constrain consumption for a generalist predator

Background/Question/Methods

Choice experiments have been a cornerstone in the study of predator-prey interactions.  In experimental arenas, animals preferentially consuming one type of prey over another likely indicate differences in nutritional, chemical, or physical properties of the prey simultaneously offered.  Relatively less is known about how prey behavior may explain, or interact with other traits to explain, predator consumption.

Results/Conclusions

In the South African intertidal, the flatworm Planocera gilchristi is a generalist predator of snails and other animals.  When three abundant snails were offered in pair-wise combinations, the flatworm significantly consumed the littorinid snail Afrolittorina knysnaensis over the snail Oxystele tigrina, but not over the congeneric O. variegata, which was consumed in similar amounts.  In turn, O. variegata was significantly preferred over its congener O.  tigrina. When flatworms were confined to feeding on a single snail species, similar patterns emerged with O.  tigrina being consumed at significantly lower rates than A. knysnaensis, and O. variegata consumption falling between those two.  Snails in the genus Oxystele show a number of defensive behaviors including rapid withdrawal into the shell, accelerated locomotion, release and drop from vertical surfaces, and vigorous twisting of the shell to release the grip of predators.  When such behaviors were constrained by tethering, consumption patterns changed considerably. With both species of Oxystele being consumed significantly more than A. knysnaensis, and no differences in consumption between the two congeners when offered simultaneously.  In terms of profitability, A. knysnaensis contained significantly less edible matter, and flatworms experienced a significantly higher handling time consuming this prey than either of the other two.  In fact, when a range of sizes for each of the three prey was offered to flatworms, predators switched to smaller-sized A. knysnaensis and consumed significantly larger individuals of either Oxystele species.  Results show the need to consider predator preference along with the single and interactive effects of prey traits and behaviors explaining consumer-prey interactions.