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Feral sheep on Socorro Island: facilitators of alien plant colonization and ecosystem decay

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The paper examines the role of feral sheep (Ovis aries) in facilitating the naturalization of alien plants and degrading a formerly robust and stable ecosystem of Socorro, an isolated oceanic island in the Mexican Pacific Ocean. Approximately half of the island is still sheep-free. The other half has been widely overgrazed and transformed into savannah and prairie-like open habitats that exhibit sheet and gully erosion and are covered by a mix of native and alien invasive vegetation today. Vegetation transects in this moderately sheep-impacted sector show that a significant number of native and endemic herb and shrub species exhibit sympatric distribution patterns with introduced plants. Only one alien plant species has been recorded from any undisturbed and sheep-free island sector so far.

Socorro Island provides support for the hypothesis that disturbance of a pristine ecosystem is generally required for the colonization and naturalization of alien plants. Sheep are also indirectly responsible for the self-invasion of mainland bird species into novel island habitats and for the decline and range contraction of several endemic bird species.
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Keywords: Biological invasions; Socorro Island; community resistance; ecosystem decay; endemic biota; feral sheep; habitat disturbance; island biogeography

Document Type: Research Article

Affiliations: Division of Biodiversity and Ecological Entomology, Illinois Natural History Survey, Champaign, IL 61820, USA

Publication date: 01 March 2008

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