A TEST FOR A DIFFERENTIAL COLONIZATION AND COMPETITIVE ABILITY IN TWO GENERALIST PREDATORS

Abstract

Spatial subdivision of the landscape can reduce the intensity of interspecific interactions, whether they be predation or competition. Recent models examining the ways in which spatial pattern in the landscape will influence the outcome of species interactions assume a trade-off between colonization ability and competitive ability in the interacting species. We tested for differential colonization and competitive ability in two interacting species of wolf spider (Hogna helluo and Pardosa milvina) which co-occur in agricultural and early successional habitats in the eastern USA. We conducted our research in replicated experimental soybean plots in which we created islands of enhanced habitat by supplementing natural litter and enhancing weed growth. Our experimental manipulations were repeated once for each of the two spider species: (1) spiders added, (2) prey attractants (composted vegetable waste) added, (3) spiders and prey attractants added, and (4) one control, for a total of eight subplots. We replicated the experiment six times, three times each in conventionally tilled fields and conservation tilled fields. We censused these plots at the end of the summer and found that conspecific addition significantly elevated numbers of one species (H. helluo) but not the other (P. milvina). These findings were consistent with our predictions that H. helluo is limited in colonization ability when compared to P. milvina. The addition of prey attractants significantly elevated numbers of both species, but the effect was far stronger for the superior colonist (P. milvina). This indicates that P. milvina is able to track resource abundance (both habitat structure and prey), a trait correlated with colonization ability. We found no strong evidence for a negative effect of H. helluo addition on numbers of P. milvina, as would be predicted if H. helluo strongly competed with or significantly preyed upon P. milvina. Body condition of P. milvina was lowest in subplots with higher numbers of H. helluo, indicating that H. helluo may have influenced the foraging success of P. milvina. We also found a strong and unexpected effect of the context of the plots (background tillage regime) on numbers of P. milvina, with there being approximately twice the P. milvina in the subplots in conventionally tilled fields.