The Strait of Gibraltar poses an effective barrier to host-specialised but not to host-generalised lineages of avian Haemosporidia.

One of the major concerns with ongoing environmental global change is the ability of parasites to shift their distribution (both geographically and across hosts) and to increase in virulence. To understand the structure, diversity and connectivity of parasite communities across the Mediterranean Sea, we used avian haemosporidian communities associated with forest birds of northwestern Africa and northwestern Iberia as a model system. We characterised host specificity of lineages and tested whether host generalists are more likely to cross the biogeographic barrier imposed by the Strait of Gibraltar than host specialists. We sampled 321 birds of 43 species in northwestern Africa and 735 birds of 49 species in northwestern Iberia. Using a PCR-based approach to amplify Plasmodium, Haemoproteus and Leucocytozoon parasites, we retrieved 969 sequences representing 200 unique cytochrome-b lineages. Haemosporidians infected a significantly higher proportion of birds in northwestern Africa (78.5%) than in northwestern Iberia (50.5%). Relative diversity of different haemosporidian genera did not differ between our study areas, but Plasmodium was overrepresented among individual infections in northwestern Iberia. Haemoproteus and Leucocytozoon lineages were predominantly host-specialised and Plasmodium lineages were host-generalised. The number of regions occupied by lineages was significantly associated with their host specificity and abundance. These data are consistent with the positive abundance-occupancy relationship and patterns of host specificity among different haemosporidian genera observed in other studies.