Community structure of the parasitic Philornis fly on Trinidad

Host specificity can be used to determine how likely parasites are to persist following host extinction, or to become established following their introduction into new areas. The phylogenetic structure of communities is context dependent. We examined the historical associations of 30 bird species and 10 Philornis parasitic fly species occurring on the Island of Trinidad by comparing their phylogenies. Do Philornis species on Trinidad feed on closely related hosts more often than expected by chance? We first reconciled the host and the parasite trees. Then, we examined the relationship of the parasite community similarity of host birds and the phylogenetic distance between the hosts. We calculated faunal overlap for all 30 host species with known phylogenetic relationships based on DNA sequence divergence. Overlap was measured by the Sorensen coefficient (So). We estimated phylogenetic distance between birds from branch length based on the number of substitutions per site from maximum likelihood analysis. We used linear regression to analyze the direction of this relationship. Four of the Philornis species were monophagous (either effectively specialized or due to limited sampling). The six remaining Philornis species were not restricted by their host phylogenetic relatedness, feeding on a variety of birds in distant genera. Parasite community similarity, defined as the fraction of the total parasite species on two host species that are shared between the hosts, was negatively correlated with phylogenetic distance. The regression of community similarity against phylogenetic distance was significant (ANOVA, F_1,5 = 7.83, P = 0.03). Declining community similarity with increasing phylogenetic distance between hosts indicates that Philornis on Trinidad tend to feed on closely related birds more often than on distantly related birds. The interaction between host ecology and host phylogeny may play a role in determining which bird species are parasitized by Philornis and the extent and frequency of host switching.