Searching for the signal of competition in plant-mediated interactions among coexisting gall insects on broad-leaved paperbark

Abstract

The central prediction of competition theory is that a reciprocal struggle for resources should manifest itself among coexisting species under conditions of increasing functional similarity (e.g. similar feeding niche), density and spatio-temporal overlap. We investigated whether the signal from such a struggle was evident in a community of phytophagous gall insects on Melaleuca quinquenervia meeting these conditions. Specifically, we examined plant-mediated interactions among three species galling vegetative tips with varying degrees of temporal overlap. The abundance of early gallers (Sphaerococcus ferrugineus Froggatt and Fergusonina turneri Taylor) did not influence resource availability for the late galler (Lophodiplosis indendata Gagné), suggesting the absence of temporally separated exploitative competition. However, the abundance of S. ferrugineus was positively correlated with the abundance of L. indentata, which suggests facilitation or similar responses to the prevailing conditions. Examination of the reciprocal impacts of the late galler on early gallers paradoxically revealed that though galling by L. indentata may be reducing the resource availability for early gallers, the abundance of L. indentata was positively influenced by the abundance of early gallers (an effect that is stronger for S. ferrugineus than F. turneri) suggesting facilitation. The strong influence of site on the interactions among the different cecidogenic species indicated that any role for competition/facilitation may be spatially constrained. We explore potential processes that may be driving the patterns that we detected and also the implications for the use of these species as biological control agents for M. quinquenervia.