Mapping terrestrial anthropogenic degradation on the inhabited islands of the Galapagos Archipelago

The Galapagos Archipelago is renowned for its high endemism but little effort has been made to quantify the human disturbance that compromises the islands' ecological integrity. We provide a quantitative assessment of anthropogenic degradation, which we define as areas either transformed by direct human activity or heavily invaded by four of the most prevalent alien plant species (Psidium guajava, Rubus niveus, Cinchona pubescens and Syzygium jambos). We assessed how the amount of degraded area varied among the six major vegetation zones (bare ground, littoral, arid, transition, humid and very humid) across five inhabited or formerly inhabited islands. Overall, we found that 37,833 ha (5.5%) of the Archipelago have been completely degraded. The islands that have suffered the greatest human impact (13,000-14,000 ha each) are Santa Cruz (the most populous) and Isabela (the largest). When vegetation type is considered the humid and very humid vegetation zones have been most affected by humans (29 and 45%, respectively). On San Cristobal and Santa Cruz 100 and 76%, respectively, of the very humid zone and 94 and 88%, respectively, of the humid zone have been transformed. These results are underestimations as mapping of the anthropogenic change in some vegetation zones (e.g. on Floreana) is poor, and the analysis did not take into account the effects of introduced animals. Nevertheless, this research points to an urgent need to prioritize restoration efforts in humid and very humid vegetation zones and to improve spatial mapping across the Archipelago to obtain a better understanding of the impacts of humans.