Searching for cover: soil enrichment and herbivore exclusion, not fire, enhance African savanna small-mammal abundance

Large mammalian herbivores (LMH) are known to suppress populations of small mammals in African savanna ecosystems; whether this suppression is driven by depletion of nutrients and food resources, or of cover, is poorly understood. Cattle management creates scattered, persistent, nutrient-enriched areas (glades). Similarly, prescribed fire may enhance forage nutrition of burned patches. Both enriched microhabitats attract wild and domestic LMH and are fertilized by their wastes, but removal of vegetative cover by LMH or fire may negate the benefits of enhanced nutrition to risk-averse small-mammal species or individuals. We used replicated LMH exclusion experiments on red sandy loam and black-cotton clay soils, and a multi-scale burn experiment on black cotton to examine abundance, diversity, and biomass of small mammals across sites varying in vegetation cover and enrichment. Small-mammal species composition varied dramatically among habitats. Species diversity and density on red sands were roughly double that of black cotton soils. Small-mammal densities and diversity were dramatically greater inside LMH exclosures; maximal densities occurred inside fenced, nutrient-rich, tall-grass glades. Small-mammal density was similar between black-cotton burn sites and unburned matrix habitat but was significantly greater on unburned than burned patches within the burn sites. Burned patches contained less grassy cover up to 50 months post-burn, although vegetation differences diminished following significant rains. Mice captured on burned patches traveled farther but were not heavier than mice captured on unburned patches. Small mammals were nearly 20-fold more abundant on totally fenced glades than matrix habitat on both soils and ninefold more abundant on fenced bushland (non-glades) on red sands. Unfenced glades supported intermediate densities in black cotton but lower densities in red sands because of close cropping by LMH. Total small-mammal biomass tended to be greater on red sands (though mean body mass was greater on black cotton for two common species), within exclosures, and on glades. Both the grass rat and pouched mouse reached maximal densities within glade LMH exclosures on both soils. This and the positive association of small-mammal densities with grass height and dead-stem density suggest loss of cover is a primary driver of large-herbivore suppression of certain savanna small mammals.