Individual-plant selectivity by sheep in drylands scales-up at plant population level and controls the forage supply and its accessibility

Abstract

Diet selectivity by domestic herbivores controls plant community structure and dynamics and may induce rangeland degradation, particularly in drylands. However, management decisions frequently ignore herbivore selectivity. Here, we studied how grass morphology controls sheep selectivity for individual plants, and how this selectivity interacts with grazing intensity to determine population plant-size distributions and the forage supply. In Patagonian steppes, we manipulated the plant morphology (size and standing-dead proportion) of three dominant grass species differentially preferred by sheep for 4 years. Then, we evaluated how these morphological alterations affected intra- and inter-specific preference patterns. We also evaluated how grazing intensity (ungrazed, moderate grazing and intensive grazing) affected the plant-size distribution of the three species, the forage supply, and its accessibility. For the highly preferred species, herbivores selected plants that were either naturally or experimentally short, with low proportion of standing-dead biomass. In contrast, morphological changes did not alter the within-species selectivity of the least preferred species. Grazing intensity strongly changed the population plant-size distribution of preferred species in ways that resembled the experimental manipulations of morphology. Moderate grazing showed the greatest morphological heterogeneity among individuals. When integrating the green biomass of forage species' individuals at ecosystem level, we found that the forage supply was the highest in ungrazed sites and decreased as grazing intensity increased. However, considering the dissuading effect of the standing-dead proportion of plants, the accessible forage was the highest under moderate grazing. Synthesis and applications. Our findings (i) showed that, within preferred species, sheep selectivity at individual-plant level is controlled by morphological characteristics that determine accessibility to green high-quality biomass. This effect was as important as that of species identity; (ii) empirically proved plant–animal positive feedback at individual level; and (iii) revealed how the individual-plant selectivity scales-up at population level and controls the forage supply, but also its accessibility. Our complementary approach generates critical knowledge for developing management practices to control key forage species defoliation and to adjust the grazing pressure to the offer of accessible forage, avoiding the common carrying capacity overestimation. These aspects are essential for sustainable production in grazed drylands.