Discriminating direct and indirect effects of exotic grazers on native plant cover in the Monte desert of Argentina

Abstract

The relationship between exotic mammalian herbivores (EMH) and the native vegetation has often been focused on direct effects and viewed it as binary interactions. However, in arid environments indirect effects and the community context may also influence the effect of EMH on plants. We sampled the flora and soil nutrients (SN) in seven paddocks that differed in stocking rates, and used structural equation modeling (SEM) to partition the net effect of EMH into direct and indirect effects on the most abundant plant species of the Monte desert of Argentina. We proposed two indirect effects by which EMH can modify the cover of a focal plant species: the modification of the assemblage plant cover (APC) and alteration of SN. Indirect effects appear to be as important as direct ones. EMH did not affect the status of SN; thus indirect effects through this pathway were irrelevant. Conversely, EMH always affected the APC negatively, but the effect of APC on a focal plant varied according to their identity. Therefore, indirect effects through the APC were positive or negative depending on plant species. Several possible mechanisms explaining how changes in the cover of nearby plants can alter the cover of a focal plant species are discussed. These results emphasize the importance of indirect interactions to explain the complex relationship between EMH and native plant cover in arid ecosystems.