Feral sheep on Socorro Island: facilitators of alien plant colonization and ecosystem decay
Authors: Walter, Hartmut S.; Levin, Geoffrey A.
Source: Diversity & Distributions, Volume 14, Number 2, March 2008 , pp. 422-431(10)
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.
Document Type: Research Article
Affiliations: Division of Biodiversity and Ecological Entomology, Illinois Natural History Survey, Champaign, IL 61820, USA
Publication date: March 2008