Potential of grass invasions in desert shrublands to create novel ecosystem states under variable climate

The invasion of exotic grasses into shrublands is a major disturbance to dryland ecosystems. The presence of exotic grasses enhances the occurrence of wildfire in landscapes that had not evolved in the presence of fire, leading to high rates of mortality of the native vegetation. Exotic grasses could be more prone to water stress and mortality than the shrubs they replaced and may not establish during drought, facts that are crucial in ecosystems undergoing increased climatic variability. Here, we develop a process-based modelling framework to investigate the complex dynamics resulting from the introduction of exotic grasses under variable climate. We find that the system converges towards different steady states, depending on the magnitude of climatic variability. While in the absence of climate fluctuations the shrubland state is replaced by an exotic grassland, interannual climate variability may inhibit grass invasion and stabilize the shrubland state. However, climatic variability also gives rise to a novel third, unvegetated state, with grass invasion being followed by drought, grass mortality and intense soil erosion. Most of the research on climate change effects on ecosystems has historically concentrated on the ecological impact of shifts in mean climate conditions. This study shows that changes in the variance are also important when shifts in vegetation composition (e.g. species invasions) result in different susceptibility to climatic variability. In the presence of random climate fluctuations, ecosystems can display steady states that differ from those that would exist under a constant climate or with a climate trend.