Ecology and biogeography of island parasitoid faunas

Abstract

Islands constitute natural laboratories for the study of evolutionary and ecological processes due to their discrete and isolated nature. Island biotas tend to be species–poor and disharmonic compared to the mainland; typically, interspecific competition is low, and entire groups of predators, parasitoids or pathogens are absent from their biotas, so the ecological space is often not fully saturated. Consequently, species from island assemblages often use a wider range of resources than their counterparts from the source mainland. Here, I investigate whether island parasitoid communities have proportionally more generalist species than their source mainland, and which factors determine island community structure. These questions were approached using data on the distribution of Ichneumonoidea (Hymenoptera) species worldwide and with data from a survey conducted in the Macaronesian region. Prior to the global analyses, I assessed whether islands and archipelagos follow the same species–area relationship, and identified which islands have comparable inventories. Globally, islands have proportionally more idiobionts (i.e. generalists) than continental areas. However, there is a latitudinal gradient in the level of generalism of island parasitoid faunas that correlates with some environmental factors and island characteristics; the species pool is the most important determinant of island community structure, together with temperature (for braconids) or biogeographical region (for ichneumonids). Host and parasitoid larvae collected in different islands of the Macaronesian region and adjacent mainland were assigned to Molecular Operational Taxonomic Units using a protocol based on host dissection and DNA barcoding. At this scale, mainland faunas have proportionally more koinobiont species and island communities have a greater proportion of idiobionts. Although overall parasitism rates were similar between islands and mainland, islands had higher idiobiont parasitism rates than expected by chance. In summary, results from this thesis indicate that indeed island parasitoid faunas are biased towards generalist species.