Vegetation Recovery Near the Alcedo Volcano
Vegetation Recovery after Eradication of Introduced Feral Animals on Northern Isabela Island, Galápagos
In August 2006, the Ecuadorian Government announced to the national and international media and environmental communities the successful culmination of field activities associated with a near decade-long project, “Control of Invading Species in the Galápagos,” a multi-institutional project carried out in partnership with 12 national and international organizations, as well as the Galápagos National Park and the Charles Darwin Research Station. This project is the largest invasive species eradiation project ever undertaken anywhere in the world, and was financed primarily by the United Nations and the World Bank through the Global Environmental Facility. The goals were to eradicate invasive animals on Isabela and Santiago Islands and to begin the process of restoring the islands to their “native” conditions. Today, the Isabela Project team is confident that after along 8-years they have achieved the elimination of the feral goats from northern Isabela Island and neighboring Santiago Island. The goats were introduced to the Galápagos in centuries past as a food source for pirates, whalers, sealers, and early colonists. Goats were largely responsible for erosion and precipitous drops in wildlife populations of several charismatic species, including the giant tortoise, as a result of the damage caused to nesting sites, eating and trampling of eggs, and severe degradation of their habitat.
Methods
The strategy for this study was to locate study sites across a disturbance gradient from minimal-to-severe for historical-to-contemporary periods of animal invasion and eradiation on Isabela Island. Multi-date ASTER satellite data were processed to characterize greenness and disturbance classes. Field validation studies were conducted during the summer of 2007 to assess biomass conditions and ecological settings. GPS coordinates were collected and corrected related to re-vegetation, plant abundances, and ecosystem recovery. Preliminary spatial analyses masked out water and clouds from ASTER image pairs, one during the garúa seasons in 2001 and 2005, and one during the rainy seasons of 2003 and 2006. We reclassified the images to highlight vegetation and non-vegetation land cover types, using field photographs, GPS points, and notes to determine where vegetation was in each image. We then calculated global vegetation indices (NDVI) for each image and computed change detection images for seasonal pairs.
Garua Season (2001 - 2005) Rainy Season (2003 - 2006)
Results
Our analysis indicated that recovery of vegetation has begun on Isabela Island without the pressure of the goats, and some preliminary observations related to succession of vegetation and ecosystem restoration were suggested: (1) Overgrazing by the goats suppressed invasive plant populations and restricted their spatial distribution; (2) Elimination of the goats is correlated to the initial stages of an extensification process of invasive plant species that is now being observed; and (3) Feedbacks exist between land degradation caused by the goats, the trophic cascade effects produced by the eradiation, and the occurrences and spatial patterns of invasive plant species, with space and time lags.
