Feedbacks between Invasive Plants and Invasive Animals in the Galápagos Islands: Challenges to Ecosystem Restoration

Will the Eradication of Invasive Animals on Isabela and Santiago Islands Restore the Fragile Native Ecosystem of the Galápagos Islands, or will Interactions between Invasive Plants, Invasive Animals, and the Human Dimension Create New Challenges for Conservation and Restoration of Protected Areas?

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,” (the “Isabela” Project), a multi-institutional project carried out in partnership with 12 national and international institutions, as well as the Galápagos National Park and the Charles Darwin Research Station. This Project is the largest invasive species eradication project ever undertaken anywhere in the World. The Project was financed primarily by the United Nations and the World Bank through the Global Environmental Facility (GEF). The goals of the Project 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 almost 8-years they have achieved the elimination of the feral goats from northern Isabela Island, the largest island of the Galápagos archipelago, and neighboring Santiago Island. Now, not only are the feral goats no longer a threat to the fragile native ecosystem and endemic flora and fauna, but also gone are the other invasive species such as donkeys, pigs, and several invasive plant species. The goats were introduced in centuries past as a food source for pirates, whalers, sealers, and early colonists. Due to their great adaptability, goats quickly began to proliferate, eating native and endemic plants and were often seen in great herds along the flanks of volcanoes where they destroyed the protective cover of vegetation causing erosion and land degradation. The altered and disturbed ecosystem had disastrous effects on the rare native and endemic flora and fauna of the Galápagos Islands. The invasive animals were largely responsible for the 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. Breeding and release programs were developed as a direct response to the dramatic reductions in tortoise populations caused by the invasive animals. The effects of the invasive animals were not restricted to the giant tortoises alone, but also to other animal species, such as the Darwin Finches as a consequence of habitat change through the degradation of native and endemic animal and plant species. So too, human populations were affected through widespread land degradation by invasive plant and animal species within and adjacent to agricultural areas that resulted in land abandonment and population consolidation in nearby communities. Further, fragile marine environments were degraded through soil erosion, runoff, and sedimentation. The feedbacks between land degradation, fisheries resources, agriculture, and the eco-tourism industry are also of great significance to the socio-economic and ecological systems of the Galápagos Islands.

Today, recovery of vegetation has begun on Isabela and Santiago Islands without the pressure of the goats, and some preliminary observations related to succession of vegetation and ecosystem restoration is suggested: (1) over-grazing by the goats and the other eradicated invasive animals suppressed invasive plant populations and restricted their areal distributions, (2) elimination of the goat population is correlated to the initial stages of an extensification process of invasive plant species that is now being observed, (3) feedbacks exist between land degradation caused by the goats, the trophic cascade effects produced by the eradication of the goats, and the occurrences and spatial patterns of invasive plant species, with space and time lags, (4) historical land use practices and land recovery rates from disturbances imposed by the invasive animals are spatially and temporally mediated, (5) repatriation of native and endemic flora and fauna to areas disturbed by invasive plants and animals is dependent upon local site conditions, terrain settings, timing and severity of the land disturbances, interactions among natural disturbances (e.g., El Niño events), severity of damage caused by invasive animals, as well as habitat requirements of native and endemic plant and animal species, and the spatial-temporal scales of ecosystem restoration efforts, (6) a complex interplay occurs among people, place, and the environment related to pattern-process relationships involved in invasion, eradication, and restoration of native and endemic flora and fauna, and (7) eradication of invasive plants and animals is best conducted at early stages of invasion for effective habitat restoration.

To conduct a preliminary analysis, a series of intensive study areas will be strategically located on Isabela and Santiago Islands that sample across the “disturbance gradient.” Sites will be selected that represent “minimal-to-severe” disturbances for “historical-to-contemporary” periods of animal and plant invasions and eradication. Hyper-spatial (i.e., Quickbird) and hyper-spectral (i.e., Hyperion) satellite data will be acquired for these sites and periods. In addition, historical aerial photography and a Landsat/SPOT image time-series will be generated for the test sites as regional and temporal context. Image processing will be used to assess the state and condition of the invasion, re-vegetation, and ecosystem restoration. Linear mixture modeling will be used to map land use/land cover fractions within pixels; object-oriented classifications will be generated to emphasize ecological variations at the patch and for ecological zones; and vegetation indices will be computed to assess biomass conditions and ecological settings. Field studies will be conducted in the test areas to assess re-vegetation, plant abundances, and ecosystem recovery of invasive, native, and endemic plants on affected and unaffected sites. Longitudinal sampling of invasive and native plants will be conducted along transects positioned along the “disturbance gradient.” Also, a hand-held spectral radiometer and a plant canopy analyzer (i.e., near-real time calculation of the Leaf Area Index) will be used to provide a field-to-satellite link for validating the image processing and assessing spectral characteristics of the field samples. The basic intent is to assess the ability of remote sensing systems (i.e., hyper-spatial, hyper-spectral, and moderate scale Landsat/SPOT systems and an assembled image time-series) and image processing approaches to describe the severity of land degradation, the type, pattern, and abundance of invasive plant species, key feedbacks between animal disturbance and invasive plant patterns, and the level of ecosystem recovery and restoration of native and endemic vegetation in the test sites. This preliminary project will facilitate future research aimed at addressing the intersection of geographic techniques and biogeography to optimize eradication and restoration of habitats affected by invasive plants and animals in fragile and protected environments.