Niche Breadth and Dominance of Parasitic Insects Sharing the Same Host Species

A guild of six parasitic insects (Hymenoptera) which attack cocoons of the same sawfly populations was composed of four indigenous and two introduced species. The indigenous parasitoids included the ichneumonids, Pleolophus indistinctus (Prov.), Mastrus aciculatus (Prov.), Gelis urbanus (Brues) and Endasys subclavatus (Say), and the introduced species were the ichneumonid, Pleolophus basizonus (Grav.), and the eulophid, Dahlbominus fuscipennis (Zett.). Their distributions were sampled on five variable requirements, or resource sets. Litter moisture content and seasonal activity varied within plots, and host density, host species, and plant community varied between lots. Although the distributions of the indigenous species overlapped, in relative terms each species occupied one position in the niche space that was poorly exploited by all other guild members. Such an enclave permitted each of the first three species to develop a zone of dominance over the other parasitoids. The introduced species had no recognizable enclave. The most abundant guild member, P. basizononus, was dominant in the sites most favorable to litter—searching parasitoids. It had the broadest niche over the range of litter moisture content and length of seasonal activity, and it interacted more with all other species than any other guild member. It was a better competitor than the next most broadly adapted species, P. indistinctus. Competitive superiority is proposed as the driving force behind abundance, leading to dominance in favorable sites. The density—dependent interaction between individuals, which results in dispersal, appears to be responsible for the occupation of a broader niche.