Indirect effects of cattle grazing on shrub spatial pattern in a mediterranean scrub community

Identifying the mechanisms and interactions that influence the spatial structure of vegetation is important for both scientific and practical purposes. Grazing is one of the most fundamental interactions in ecology but so far its effect on vegetation spatial pattern received little attention. In this study we propose a conceptual model that can be used to predict the effect of grazing on shrub spatial pattern in water-limited ecosystems where shrubs grow within a matrix of annual vegetation. According to the model, grazing may increase or decrease clumping in shrub distribution, depending on (1) the relative palatability of shrubs vs. annual plants to the herbivores, and (2) the manner (negative or positive) by which adult shrubs and annual plants affect the establishment of shrub seedlings. We tested our model in a Mediterranean scrub ecosystem by analyzing the development of shrub spatial pattern over a period of 40 years in plots characterized by contrasting intensities of cattle grazing. As predicted by the model, all plots showed a clumped pattern of shrub distribution in the absence of cattle grazing while intense cattle grazing reduced the clumpiness of the vegetation and generated a more random pattern of shrub distribution. Interestingly, plots representing the two grazing regimes did not differ significantly in their shrub cover, suggesting that shrub spatial pattern may be more sensitive to grazing than overall shrub cover.