An established body of research has demonstrated that parasites intimately affect the ecology and evolution of their host populations. Host-parasite investigations are notoriously difficult to conduct because of the complexity of factors influencing this relationship. Research in island ecosystems has been particularly productive because of the relative simplicity and tractability of such systems. Island populations are considered to be particularly susceptible to introduced disease; however the ecology and distribution of native diseases in island systems are underrepresented in current literature. We investigated the host-parasite interactions in island populations of the endemic lizard Podarcis erhardii (Reptilia: Lacertidae) which are infected by hemogregarines (Apicomplexa: Adeleorina). Research was conducted across 23 islands in the Cyclades archipelago (Aegean Sea, Greece). Sites were located on land-bridge islands all of which were separated from the neighboring mainland by rising sea levels during the last 18,000 years. Because lizards in this system are poor overwater dispersers, island separation regimes also reflect the degree of isolation of the respective lizard populations. Study islands varied in age (time of isolation), area (km2), vegetation structure, and host population density. The dynamics of host-parasite interactions were assessed in terms of 1) how infection and intensity of infection (parasitemia) affects the host’s condition and immune function; 2) how island characteristics affect the distribution and intensity of hemogregarine infection. We study these questions using a combination of field and laboratory work.
This research indicates that host condition is negatively correlated with hemogregarine infection and parasitemia, and that immune function is negatively affected by parasite presence and parasitemia. In addition, both infection prevalence and parasitemia are clearly shaped by island traits. We found that 1) host condition is negatively correlated with infection and greater parasitemia, and immune function is negatively with infection and parasitemia; 2) infection and parasitemia are lower in populations on islands which are larger, have been isolated for a longer period, have less vegetation cover, and harbor more dense host populations. This study provides insight to the impacts of island biogeography and spatial structure on host-parasite interactions.