Long-Term Consequences of Feral Sheep Grazing on Herbaceous Cover of Santa Cruz Island, California

Understanding how insular ecosystems recover after the eradication of an invasive species is crucial in prioritizing island conservation and restoration efforts. Invasive grazers, such as the feral sheep (Ovis aries) that occupied Santa Cruz Island after the late 1800s, suppress plant growth and recruitment, perturb ecological succession, and alter competitive interactions between species, thereby altering entire plant communities. Eradication of the sheep removes these pressures and allows ecosystems to return to a pre-disturbed state. In the absence of sheep we anticipated an increase in woody plant cover and a subsequent decrease in other vegetation types. To examine recovery after the removal of sheep in 2001, we determined the floral cover before the eradication in 1980 and again after the eradication in 2012 using a point-intercept method to assess percent cover of grasses, forbs, succulents, thatch, soil/rocks and outcrop in both grazed and non-grazed areas. Additionally we determined tree and shrub cover from landscapes photographed pre- and post-eradication. Before eradication grazed areas were characterized by decreased grass cover and increased exposure of bare ground due to consumption and erosion caused by the sheep. These data demonstrate the serious threats posed by feral sheep to insular plant communities. A decade after the removal of feral sheep our analysis showed a decrease in forbs and an increase in trees and shrubs, which highlights the transition from a predominantly grassland community to a coastal scrubland and suggests the ability of an island ecosystem to recover through passive restoration after release from invasive grazing pressures.