Arthropod Community Responses to Manipulation of a Bitrophic Predator Guild

Abstract

We used replicated field enclosures to manipulate population densities of two species of bitrophic predators in a terrestrial old—field community: a mantid (Tenodera sinensis), and a wolf spider (Lycosa rabida). The treatments consisted of adding mantids alone (8 individuals/enclosure), lycosids alone (10 individuals/enclosure), and lycosids and mantids together (8 + 10 individuals). A control consisted of enclosures to which no predators were added. The impact of these predators on numbers and biomass of other arthropods in the community was examined on several levels: overall community, different size (body length) categories, and major taxa. We asked whether the impact of these predators in combination could be predicted from their separate effects. Mantids depressed total numbers (10—15%) and biomass (50%), in the arthropod community over the course of 10 d. This effect was confined to the largest size categories in the community. Lycosids had no measurable effect at this level of resolution. Mantids depressed abundance of acridids in both mantid and mantid/lycosid enclosures, but again lycosids had no impact. Lycosids alone enhanced abundance of gryllids, but not in the presence of mantids. Both mantids and lycosids depressed numbers of small spiders (also members of this guild), but this effect was not additive. Interactions among members of bitrophic generalist predator guilds may contribute to the commonness of nonadditive and higher order effects in manipulative experiments. Depending upon the level of resolution, it may be impractical to predict the impact of the whole guild from summing the individual effects of single predator species on terrestrial arthropod communities.