The biogeography of host-parasite dynamics is an area that has received little attention in studies of island ecology. While a few studies have shed insight on patterns of parasitism in insular host populations, more empirical evidence is needed to ascertain how isolation impacts parasites. Biogeography generally theorizes that the physical size of islands and the duration of each island’s isolation can be driving geographic factors controlling species interactions and populations dynamics. To test this, we assessed the effect of island structure and population isolation on the endemic insular lizard Podarcis erhardii and its native hemogregarine parasite (Apicomplexa: Adeleorina) in the Cyclades (Aegean Sea). We analyzed the relationships of prevalence and parasitemia of hemogregarine infection with several factors concerning the island (size, time of isolation, spatial isolation, population density) and host (body size) levels using regression and structural equation models, respectively. Regressions indicate that islands with greater host density and islands which have been isolated for shorter timespans tend to have higher hemogregarine prevalences; structural equation models suggest a similar pattern for parasitemia. We hypothesize this may be driven by insular density compensation. Hosts on islands that are more temporally and spatially isolated also tend to have higher prevalence and parasitemia of hemogregarines. Our results indicate that island area, island isolation, and host population density are likely to be significant drivers of changes in host-parasite interactions in fragmented populations.

Island taxa are frequently susceptible to introduced predators. This susceptibility is thought to be caused by the loss of key ancestral antipredator physiological adaptations during long periods of evolution under reduced predation pressures. Here, we test the hypothesis that island species have reduced locomotor abilities. While locomotor abilities are critical for escaping predation, little is known on how the presence of different types of native predators influences these abilities by maintaining selective pressure. To fill this gap, we documented sprint speed in the Aegean wall lizard (Podarcis erhardii) from the Aegean islands (Greece) with varying levels of predation pressure. We show that lizards from islands where mammalian predators were present sprinted fastest. Lizards sprinted at an intermediate speed where predators other than mammals were present, and lizards sprinted slowest on islands where no predators were present. Longer periods of evolutionary isolation in island environments were also associated with diminished sprinting speeds. These results indicate that lizards from the lowest-predation islands are the most vulnerable and preventing the introduction of invasive predators should be prioritized for these island systems.