Mass Movement Effects on Grassland Vegetation and Soils on Santa Cruz Island, California

Two types of mass movement are studied to test the hypothesis that spatial heterogeneity would affect species richness of grassland communities. Rotational slumps are low-moderate intensity disturbances that generate a variety of new microhabitats (scarp and toe versus flatter middle). Concave shallow slips are higher intensity disturbances that create one new type of soil microhabitat (uniform exposed subsoil inside). Plant species response fails to conform to predictions that heterogeneity would foster greater species richness. The more intense slip disturbance provides sites for four new species to colonize, but the overall landscape richness is lower than elsewhere. The new colonizers are mostly shrubs characteristic of nearby coastal sage communities. Most native perennials are only in non-slip areas. The most successful colonizers are the dominant introduced European annuals, which occur everywhere. The greater heterogeneity of slumps does not affect richness of the grassland community. All species found inside slumps are in outside areas, but within slumps the species are spatially segregated. The middle relatively undisturbed zone inside slumps has fewer species than the outside area, indicating a loss of species after slump formation, perhaps due to competition. Low disturbance frequency and pre-movement conditions may confound the role of heterogeneity and disturbance in these grasslands.