Habitat patchiness promotes invasion by alien grasses on serpentine soil

Serpentine soils are considerably less invaded by alien species than nonserpentine soils in California's North Coast Ranges. However, alien species are significantly more prevalent on small patches of serpentine (< 5 ha) than in the interiors (> 100 m from edges) of very large ones (< 1 km(2)). We studied reasons for this pattern, focusing on the Mediterranean annual grasses Avena fatua L. and Bromus hordeaceus L. Using field transects, we found that both Avena and Bromus were more common at the edges of large patches than in large-patch interiors, even though soils from edges were not significantly different from soils in interiors. Using a growth experiment, we found that soils from small patches did not support better growth of either grass than soils from large-patch interiors. We conclude that the observed pattern is probably caused by small patches and large-patch edges receiving a higher flow of propagules from the nonserpentine matrix, and not by differences in habitat suitability. In both species, populations from small serpentine patches outperformed nonserpentine populations when grown on small-patch soil. This suggests that small serpentine patches may promote the evolution of serpentine-tolerant ecotypes of these alien grasses. (C) 2001 Elsevier Science Ltd. All rights reserved.