Feeding Ecology of the Galapagos Hawk After the Eradication of Goats on Santiago Island
The Peregrine Fund and the Galapagos National Park Directorate
Project began in 2010; GC funded in 2011; completed in 2012
Scientists are studying the feeding ecology of hawks on Santiago. Photo by Joe Moore.
The removal of invasive species typically has a positive impact on biological communities as they recover, and there are many good examples of native species recoveries. However, there are often unforeseen negative consequences, particularly when the eradicated exotic species had replaced the function of an extinct component of the ecosystem. This is the case on Santiago Island, which is now largely without vertebrate herbivory (plant-eating animals) after the successful eradication of goats in 2006.
Within this eradication effort, the largest island in the world ever cleaned of exotic ungulates (animals with hooves) is Santiago, where native herbivores had been driven extinct (land iguanas) or nearly so (giant tortoises) leaving the island largely without vertebrate herbivory. On Santiago, a well-studied population of the Galapagos Hawk (Buteo galapagoensis) resides as the apex terrestrial predator (meaning few or no predators of its own) in the simple Galapagos ecosystem. This study proposes long-term monitoring of this population and its prey populations to test several hypothetical consequences of the eradication of ungulates and the subsequent recovery of the biological community.
Information from this project has impacts beyond strengthening our understanding of population dynamics of the Galapagos Hawk. It will contribute greatly to conservation and management decisions by the Galapagos National Park in their efforts to preserve this Natural World Heritage Site with direct benefits to local residents, tourism, conservation, and scientific sectors. The study also provides direct education and training opportunities for Ecuadorian students and park rangers.
Funding from Galapagos Conservancy and Swiss Friends of Galapagos provided support for the final field season of a 2.5-year project that was the masters thesis project of an Ecuadorian national. The original project aimed to assess the effects of goat eradication on Santiago Island, Galapagos, on the feeding ecology of the Galapagos Hawk.
The removal of goats in 2006 left the island largely without herbivores. The native land iguana is extinct on Santiago and the tortoise population was decimated by whalers and greatly impacted by the goats. As a result of the goat eradication, the vegetation on Santiago experienced explosive growth.
The project goal was to assess the impact of this recovery on the Galapagos Hawk, a species that primarily hunts in open habitat. This project benefited from an extensive data set of feeding deliveries to 19 nests of Galapagos Hawks on Santiago in 1999-2000, prior to goat eradication. The MS student completed a comparable set of observations to analyze differences in prey before and after goat eradication. In the 2010 season, she stayed the entire year, yet was only able to observe 11 nests, due to the extreme irregularity of nesting during that year. The additional funding from GC and the Swiss FOGO allowed an extension of her observations into the 2011 nesting season, thus completing the remainder of the observations.
2011 Field Season: During the 2011 field season (May 31st - Aug 10th,) diet observations were completed at 7 nests with a total of 137 prey items delivered. Rat capture data was collected from 14 territories. In addition, activities related to the larger monitoring effort of the Galapagos Hawk on Santiago were completed (census information for all known territories at James and Sullivan Bay and juvenile scans to estimate the population of juvenile floaters at James Bay).
The project was able to complete the original objectives to compare the prey items brought to nests following goat eradication to those brought to nests prior to goat eradication. Key results included:
As vegetation recovered, the hawks did not switch to more arboreal prey as predicted, although there were significant effects of vegetation on prey;
The habitat of the hawks’ territory (whether in coastal, transition, or lava zones) played a significant part in the impact of goat eradication on feeding ecology;
Rats were a significantly greater proportion of the hawks’ diet after goat eradication than before, suggesting an explosion of the rat population as vegetation recovered, and
No significant increase in rat density was found when measured directly – suggesting that the hawks may be controlling rat numbers.
In addition, the project achieved the stated training objectives. The Ecuadorian student graduated in December 2011 and earned an MSc degree at the University of Missouri — Saint Louis. The project also provided training opportunities in field research techniques to 8 students from Ecuadorian universities and CDF volunteers. The student has submitted an article to Conservation Biology for publication.
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