Vertebrate Community Responses to Livestock Grazing in an Ancient Mediterranean Rangeland System: Rethinking the Role of Grazing in Biodiversity Conservation

Abstract

Despite claims that livestock grazing may be a useful conservation tool for promoting biodiversity, the effect of livestock grazing on vertebrate populations remains controversial and poorly understood. This is particularly the case for Mediterranean ecosystems that have been grazed by livestock for thousands of years. The aim of my thesis is to understand: (1) the nature of the relationship between livestock grazing and vertebrate abundance and species richness and; (2) the mechanisms that drive this relationship. Methods. I sampled small mammal, reptile, and passerine bird species across a range of livestock grazing intensities in a mediterranean pastoral system in the Aegean Sea (Greece). This study was conducted in a mediterranean summer-deciduous scrubland habitat with limestone substrate and shallow soils, which is widespread throughout the region. Using Generalized Linear Modeling (GLM), I determined the nature of the relationship between livestock grazing and vertebrate abundance and richness metrics. An information-theoretic approach was used to elucidate which habitat characteristics mediated by livestock grazing best predicted vertebrate responses. Results. I found that terrestrial (small mammal and reptile) vertebrate responses to changing livestock stocking rate were different from those of passerine birds. Terrestrial species abundance decreased exponentially with increasing livestock stocking rate; however species richness exhibited a unimodal relationship with stocking rate, peaking at intermediate stocking rates. The response of terrestrial assemblages was best predicted by invertebrate food availability. Passerine bird species richness and abundance showed no relationship with levels of grazing, however avian population abundance and species richness were significantly correlated with vegetation structural heterogeneity. Conclusions. My findings indicate that there is no ideal grazing level that optimizes vertebrate abundance and richness across different vertebrate groups. However light to intermediate grazing disturbance can be important in promoting terrestrial species richness, thus indicating that agricultural policy encouraging a mosaic of grazing intensities at the regional level could support diverse vertebrate assemblages.