Moisture, nutrients, fire and herbivory are the principal factors governing tree -grass cover ratios of savannas. We investigated tree (Acacia mellifera) recruitment after fire and under conditions of maximumrecorded rainfall, nitrogen addition and grazing in a completely-crossed field experiment. We employed a similar garden experiment with the exception of the fire treatment. Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that only received natural rain, and insufficient watering frequency in irrigated plots. Due to low germination in the field experiment, no treatment significantly affected tree recruitment. In the garden experiment, frequent watering, nutrient control (i.e. no nitrogen addition) and grazing enhanced tree recruitment with significant interactions between rain, nitrogen and grazing. We infer that above-average rainfall years with frequent rainfall events are required for mass tree recruitment. Grass defoliation makes space and resources available for tree seedlings. Nitrogen enrichment increases the competitive ability of fast-growing grasses more than that of the N 2 -fixing tree component. In contrast to conventional wisdom that grazing alone causes encroachment, we suggest that there are complex interactions between the above-mentioned factors and 'triggering' events such as unusually high rainfall.

Understanding how predation risk and plant defenses interactively shape plant distributions is a core challenge in ecology. By combining global positioning system telemetry of an abundant antelope (impala) and its main predators (leopards and wild dogs) with a series of manipulative field experiments, we showed that herbivores' risk-avoidance behavior and plants' antiherbivore defenses interact to determine tree distributions in an African savanna. Well-defended thorny Acacia trees (A. etbaica) were abundant in low-risk areas where impala aggregated but rare in high-risk areas that impala avoided. In contrast, poorly defended trees (A. brevispica) were more abundant in high- than in low-risk areas. Our results suggest that plants can persist in landscapes characterized by intense herbivory, either by defending themselves or by thriving in risky areas where carnivores hunt.

Bush encroachment affects the agricultural productivity and biodiversity of 10-20 million ha of South Africa. Many people believe that we understand the causes of bush encroachment. We do not. Many people believe that either fire or heavy grazing by domestic livestock is the sole cause of bush encroachment. This is wrong. Bush encroachment occurs in many arid regions where fuel loads are insufficient for fires to be an important causal factor. Belief in grazing as the sole cause of bush encroachment stems from Walter's two-layer model. This model states that grasses typically outcompete trees in open savannas by growing fast and intercepting moisture from the upper soil layers, thereby preventing trees from gaining access to precipitation in the lower soil layers where their roots are mostly found. When heavy grazing occurs, grasses are removed and soil moisture then becomes available to the trees, allowing them to recruit en masse. The fact that many bush-encroached areas are heavily grazed means neither that grazing causes encroachment nor that Walter's model is correct. Bush encroachment is widespread in areas where there is a single soil layer and where grazing is infrequent and light. We need to move away from observational studies and these single-factor explanations. If we are to understand the causes of bush encroachment, we need mechanistic models to guide us and multi-factorial experiments to tease out the interactions among causal factors. Current disturbance-based models have many of the right elements necessary to make mechanistic predictions but need to be appropriately parameterised. Some patch dynamic models also appear to hold great promise in this regard. Field experiments carried out to date show that support for factors conventionally claimed to cause bush encroachment is underwhelming, and that rainfall amount and frequency, coupled with specific soil nutrient levels, may drive this phenomenon.

The stability of ecological communities is critical for the stable provisioning of ecosystem services, such as food and forage production, carbon sequestration, and soil fertility. Greater biodiversity is expected to enhance stability across years by decreasing synchrony among species, but the drivers of stability in nature remain poorly resolved. Our analysis of time series from 79 datasets across the world showed that stability was associated more strongly with the degree of synchrony among dominant species than with species richness. The relatively weak influence of species richness is consistent with theory predicting that the effect of richness on stability weakens when synchrony is higher than expected under random fluctuations, which was the case in most communities. Land management, nutrient addition, and climate change treatments had relatively weak and varying effects on stability, modifying how species richness, synchrony, and stability interact. Our results demonstrate the prevalence of biotic drivers on ecosystem stability, with the potential for environmental drivers to alter the intricate relationship among richness, synchrony, and stability.