Differential impacts of grazing on vegetation patterning in drylands: A comparative study

The dryland ecosystems have received considerable attention due to the threat of land degradation and declining bio-productivity. The spatial self-organization of vegetation patterns, such as gaps, spots, stripes, or labyrinths, is driven by shifting precipitation rates and changing evaporation rates. These patterns enable ecosystems to adapt to climate changes like drought and increasing temperatures. It has been recognized that herbivore grazing has a crucial impact on the formation of these patterns. To have a deeper insight into the differential impact of different types of herbivore grazing on the formation and maintenance of vegetation patterns, here we have formulated a mathematical model of vegetation dynamics in dryland ecosystems characterized by uncrusted sandy soil, where vegetation biomass and soil water are approximately constant through the narrow root zone. We then incorporate linear and natural (sigmoidal) grazing into the vegetation equation to analyze their effects separately. The key finding of this study is that increasing soil evaporation, declining precipitation, and excessive grazing can drive a regime shift in vegetation biomass through climate-induced hysteresis. This can cause catastrophic collapses in ecosystems. Another notable finding is that natural (sigmoidal) grazing is more efficient for the long-term survival of vegetation with less precipitation than linear grazing. Depending upon grazing pressure, different types of vegetation patterns appear through Turing and Hopf-Turing bifurcations. These vegetation patterns are quite similar to the real-world patterns observed through a field study. Therefore, grazing-induced regime shifts of ecosystems are obvious from this study.