Selective predation in a polyphagous invertebrate predator, Pardosa vancouveri (Arachnida, Araneae)

Abstract

Selective predation (i.e., non-random feeding) was studied in a laboratory system that allowed individual lycosid spiders, Pardosa vancouveri. to feed on prey populations consisting of one or two kinds of insects. Various combinations of male and female fruit flies (DrosoPhila melanogaster) ‘small’ and 'large' beetle larvae (Tenebrio solitor) , and nymphal milkweed bugs (Oncopeltus fasciatus) were used as prey. Spiders of both sexes and two age classes were used as predators. In 14 experiments, the spiders shoved strong selection in six, moderate to weak selection in five, and very weak or no selection in three. Generally, fruit flies were selected by the spiders over mealworms or milkweed bugs, and 'small1 mealworms over 'large’ ones. Very weak or no selection occurred between male and female flies and between beetle larvae and milkweed bugs. Subadult and adult spiders always showed similar selective tendencies. Prior feeding experiences did not alter feeding selections, but changes in the physical complexity of the environment did. Thus, by adding tree leaves to the cages, the number of flies eaten by the spiders decreased significantly. Potential benefits to the spiders which were measured included percent maturing, percent surviving, rate of weight gain, size gain, and production of false egg-sacs, in prey combinations that produced moderate to strong selection, 7 out of 28 benefit measurements statistically favored the spiders. Prey combinations that produced weak selection tendencies gave no benefit measurements (N » 26) that statistically favored the spiders. Spiders fed the less-selected prey exhibited no benefits that were superior to those fed the more-selected prey. Spiders fed the more-selected prey did just as well as spiders fed both prey. It is hypothesized that this particular selective predation process involves prey sampling, memory of prey attributes, and selection of prey that possess certain attributes. Selected prey seem to confer certain benefits to the predator. As most benefits were associated with increased biomass within time limits, £. vancouveri tends to be more an energy maximizer than a time minimizer.