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Foraging behaviour of the intertidal beetle Thinopinus pictus (Staphylinidae) Richards, Laura Jean


Optimal foraging models generally assume that predators are capable of making appropriate foraging decisions and that these decisions affect fitness. I tested these assumptions in a study of the intertidal beetle Thinopinus pictus Leconte (Staphlyinidae). Adult beetles live on sand beaches in temporary burrows from which they emerge at night to prey on amphipods Orchestoidea californiana (Brandt). I also present some data for isopods Alloniscus perconvexus Dana, a less important prey species. I measured amphipod activity patterns by pitfall trapping, and beetle activity patterns by direct counts of the number of beetles active on the beach in 1-h searches. In general, there was a good correspondence between beetle and amphipod temporal and spatial activity patterns. However, by manipulating the spatial distribution of prey, I showed that beetles arrived at foraging sites independently of prey availability. Prey capture rate was low, with a mean of 75 min between captures, so that beetles were not always successful in obtaining food during a night. Food deprivation for up to 4-d intervals did not affect beetle survival or oviposition rates in laboratory experiments. I constructed models of amphipod size selection by beetles, using the size distributions of amphipods measured on the beach, and the results of laboratory experiments on capture success, reaction distance and feeding rates. Capture success decreased and the probability that an amphipod was detected increased with increasing amphipod size. Beetles observed during beach searches selected larger sizes of amphipods than predicted from availability and vulnerability of different sizes. To apply an optimal foraging model, I estimated the profitability of different sizes of amphipods from the number of amphipods of a given size required to satiate a beetle in the laboratory. Profitability was highest for large amphipods and lowest for small amphipods and isopods. However, amphipod abundance on the beach was always below the threshold at which specialization on larger sizes was predicted to occur. Male beetles were active longer than female beetles during the night, and fewer male beetles were observed feeding. Male beetles tended to be found higher on the beach and to include more isopods in their diet than female beetles. In laboratory experiments I showed that amphipods were highly preferred over isopods by both sexes of beetles. Male and female beetles were approximately the same size and consumed equal numbers of prey items. I conclude that male foraging behaviour was altered by search for mates. I present an optimal diet model for two prey types, based on the expected foraging time required for a predator to reach satiation. Predictions differ in some cases from a model based on maximization of the rate of energy intake. Foraging time may be minimized by a predator which begins as a specialist and then expands its diet to include lower value prey when it is near satiation. Laboratory experiments on Thinopinus give weak support for these predictions, but I present alternative interpretations of the results. I suggest that most invertebrate predators which forage on active prey are limited in their ability to assess variations. in prey abundance. Future studies should emphasize how patchiness in prey availability affects foraging behaviour.

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