Effects of Invasive Goats (Capra hircus) on Mediterranean Island Communities

Abstract

Although islands exhibit great biodiversity, a high rate of endemism and simplified food webs make them highly susceptible to disturbances such as invasive species. Introduced feral goats (Capra hircus), a generalist herbivore, are among the most important invasive species on islands. Many endemic island plants have evolved without intense grazing pressure and have developed few to no defenses against herbivory. Concern about the effects of goats on island communities has led to increasing numbers of goat eradication programs. Unintended consequences may follow eradications because goat grazing can have complex, community-wide effects on island food webs. We evaluated the long-term effects of goat herbivory and removal in a system of 16 islands in the Aegean Sea (Greece) located within the globally important Mediterranean biodiversity hotspot. In this region, goats have always been an integral part of rural economies. The seasonal introduction of goats onto small Aegean islands is of special conservation concern because these islands are inhabited by particular plant communities that have evolved in the absence of herbivory. Our data suggest that goats change plant community assemblages: they significantly decreased the height, percent cover, and biomass of vegetation on an island. Additionally, goats significantly contribute to the desertification of islands by initiating a long-term erosion cycle that delays recovery even once goats have been removed. In contrast, arthropods, important primary consumers, do not appear to be affected by goat removal as any advantages obtained in absence of goats appear to benefit higher trophic levels. This study also reaffirms the role of seabirds in providing valuable N and P marine subsidies to terrestrial food webs of Mediterranean islets. These findings demonstrate that goats have serious, long-lasting effects on small island ecosystems and that additional remediation steps are needed following goat removal.