Print PagePopulations of island vertebrates are generally small and thus inherently susceptible to extinction. Successful management of such populations depends on understanding the factors that determine them. We investigated these questions by surveying 30+ landbridge islands in the NE Mediterranean Sea that were inhabited by the endemic Aegean Wall lizard (Podarcis erhardii). Over the course of 6 field seasons we collected data on lizard population densities and several relevant ecological factors. These included presence of predators, presence of nesting seabird populations, as well as intensity and nature of goat grazing.
Results/Conclusions
The study islands ranged greatly in size (0.001-400km2) and were covered with xerophytic vegetation dominated by evergreen Juniperus phoenicea and Pistacea lentiscus. Islands with areas less than 1km2 were free of snake predators. At the same time, many of these smaller islands were also home to substantial seabird colonies. All of the larger islands (>6km2) were grazed year-around while goats were released on a seasonal basis onto a subset of the smaller islands. Goat release had particularly strong effects on the vegetative cover of smaller islands altering both vegetation structure, as well as plant community composition. Lizard population densities on the study islands varied over more than one order of magnitude. Presence of Long-nosed vipers (Vipera ammodytes) had the strongest depressing effect on lizard populations. However, on those islands that were too small to support snakes, lizard populations were positively correlated with density of seabirds and negatively correlated with intensity of goat grazing. Goats impact lizard populations in multiple ways: they reduce availability of thermal refugia, decreased available biomass and introduced disease-carrying invasive ticks into the lizard populations. These results suggest that reptile populations on islands are shaped by a hierarchical interplay of predation, grazing, marine subsidies, and parasitism.
