Document Type

Thesis

Degree

Master of Science

Major

Biology

Date of Defense

11-18-2011

Graduate Advisor

Patricia Parker, PhD

Committee

Vargas, Hernan

Deem, Sharon

Marquis, Robert

Abstract

The eradication of invasive species is becoming a common approach for the conservation of native communities around the world. The current study is one component of a long-term monitoring of the Galapagos Hawk (Buteo galapagoensis) before, during and after the eradication of goats (Capra hircus) on Santiago Island. As the herbivory pressure was released, we foresaw that the rapid vegetation recovery would affect the hunting success of the Galapagos Hawk on its preferred terrestrial prey. We performed a comparative study on the feeding ecology of the hawk by direct observation of prey delivered to nests pre- (1999-2000) and post-goat eradication (2010-2011). We predicted that the Galapagos Hawk would adapt to its new environment by shifting its diet composition, from predominantly terrestrial prey before goat removal to a more arboreal prey base after goat removal, and that the effect would differ across habitat types. Additionally, we were interested in assessing the response of introduced rats (Rattus rattus) to the removal of goats. Contrary to our primary hypothesis, we were unable to find overall changes diet composition of terrestrial and arboreal prey. Nonetheless, the consumption of terrestrial prey did vary between vegetation types, confirming the influence of vegetation on the amount of prey consumed. Even though terrestrial prey consumption did not change much, it consisted of a much higher proportion of introduced rats. However, rat-trapping indicated no increases in abundance of rat populations, which coupled with the increased consumption of rats by hawks, suggests top predator control on the rodent invader. Moreover, it appears that the hawk’s ability to hunt arboreal prey is hampered in areas with high vegetation. Consequently, hawks in densely vegetated territories now depend largely on introduced rodents as a food source. Overall, we observed how the territorial population of the top predator in this community has, so far, been able to withstand these changes by adjusting to a new diet and is possibly exerting top down control on other potentially threatening invaders. Thus, we have learned that special consideration should be given to natural trophic interactions to understand the potential effects of invasive species eradication.

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