Challenges of foraging on a high-quality but unpredictable food source: the dynamics of grass production and consumption in savanna grazing lawns

1. Grazing lawns are short grassland areas where intense grazing maintains grass in an early growth stage. These areas represent a source of high-quality forage for herbivores. However, as herbivores continually remove nearly all the newly accumulated biomass, instantaneous resource availability depends on the dynamics of grass growth. 2. In this study, we investigate how production and consumption inside grazing lawns are synchronized. We then explore how that synchronization affects the ability of large herbivores to use these lawns. We also provide a critical comparison between grazing lawns and intensively managed grasslands in livestock farms. 3. We investigated vegetation production and herbivore grazing activity during a wet and a dry season using clipping experiments and direct observation in two grazing lawns in a South African savanna. 4. Weekly total grazing activity by unit area was strongly and positively related to short-term primary production. This indicates a close synchronization between these two processes. In contrast, grazing activity was poorly related to standing biomass. Primary production had a threshold response to the weekly pattern of rainfall, implying a stochastic dynamics of grass growth. 5. The dynamics of grass production and consumption of grazing lawns is similar to the one of continuously stocked grazing systems from intensively managed grasslands. But the mechanisms regulating the two systems lead to different equilibrium points between production and consumption. The two systems also have opposed nutritional functions within the animal diet. 6. Synthesis. The close synchronization between resource production and consumption inside grazing lawns indicates that instantaneous resource availability is a direct function of the short-term rate of grass growth. In tropical savannas, the main source of variability of lawn grass primary productivity is the stochastic nature of short-term rainfall. As a result, herbivores' ability to use grazing lawns is poorly predictable in time. This has important consequences on the degree of information herbivores can use in the elaboration of their foraging strategies, and on the potential interest of grazing lawns.