Print PageThe importance of higher-order interactions, such as indirect effects and associational resistance (AR), in organizing ecological systems may not be known because of a lack of studies designed to account for such interactions and to measure the strength of their effects and the conditions under which such mechanisms act. In order to determine the importance of associational resistance in the Asphondylia–Borrichia–Iva system, we measured the gall densities on Borrichia frutescens and Iva frutescens monthly on six replicate islands. Patches of I. frutescens occurred on each island both immediately adjacent to and at some distance from B. frutescens, and densities were measured on both near and far Iva patches in an attempt to determine the effect of scale on associational resistance in this system. Additionally, galls were collected from Iva bushes from both types of patches on each island and cultured under lab conditions for the purpose of determining the species composition of the guild of parasitoids which attack Asphondylia borrichiae as a test of the mechanism proposed for the associational resistance, which is that the AR is mediated by natural enemies, especially Torymus umbilicatus.
Results/Conclusions
Although patterns of gall densities were consistent with that expected under the effect of AR (gall densities higher on I. frutescens patches distant from B. frutescens), the species composition of the guild community did not support the hypothesized mechanism of AR (parasitism rates generally and T. umbilicatus proportions specifically were higher on distant patches than on near patches). While AR is a real phenomenon, it may act on such a small scale that its role in community assembly is a minor one. Further, the evidence collected in this study does not directly support the mechanism proposed.
