Alien invasions from space observations: detecting feral goat impacts on Isla Isabela, Galapagos Islands with the AVHRR

Abstract

One of the greatest threats to native biodiversity is attributable to the invasion of habitats by alien species. In particular, feral animals can be some of the most aggressive and damaging introduced species in the natural environment, especially on isolated islands. Rapid growth of the feral goat population on Alcedo volcano, one of six volcanoes on the largest of the Galapagos Islands, Isabela, was first noted in 1991. Feral goat impacts on other islands in the archipelago have caused extensive degradation of native and endemic vegetation driving plant and animal species and subspecies to extinction or to the brink of extinction. Effective management of these feral herbivores demands knowledge of intensity and range of the areas that are being most heavily impacted. A time‐series analysis of AVHRR NDVI 10‐day maximum value composite datasets corresponding to the goat‐infested area and a control area where relatively few goats exist spanning the period of 1992–1996 were analysed according to local seasonality. Statistically significant reductions in NDVI values for vegetated areas in the study area relative to the control area were detected, which was tested against fieldwork surveys. This study suggest that analysis of 1 km² resolution AVHRR data may provide a cost‐effective means to evaluate the location and extent of vegetation impacts exerted by feral herbivores grazing in areas too remote for regular, comprehensive field observations.

Acknowledgements

The authors would like to express their gratitude to the Environmental Change Institute, University of Oxford, for use of facilities throughout the preparation of this paper, Howard Snell (former Science Director) and Marc Patry (former head of the Isabela goat eradication project) from the Charles Darwin Research Station, and in particular, Karl Campbell, in the Galapagos for the provision of useful suggestions and Dr Cleveland Hickman of Washington and Lee University.