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

Authors: BERGSTROM, B - Valdosta State University; SENSENIG, R - Goshen College; Augustine, David; YOUNG, T - University Of California, Davis

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2018
Publication Date: 11/1/2018
Citation: Bergstrom, B.J., Sensenig, R.L., Augustine, D.J., Young, T.P. 2018. Searching for cover: soil enrichment and herbivore exclusion, not fire, enhance African savanna small-mammal abundance. Ecosphere. 9(11):e02519. https://doi.org/10.1002/ecs2.2519.
DOI: https://doi.org/10.1002/ecs2.2519

Interpretive Summary: In African savannas, grazing by domestic livestock and by wild herbivores such as zebra and antelope leads to reduced populations of many smaller mammal species (species less than 2 lbs in body size). Whether this suppression of small mammals is caused by a reduction in food availability, or by a loss of vegetation cover that hides small mammals from predators, is not known. In African savannas, the presence of domestic cattle leads to the creation of discrete, grassland patches (glades) where vegetation cover is very limited, but the grasses are enriched in nutrients that may benefit small mammals. Similarly, recently burned areas contain short but nutrient-enriched grasses. Both of these enriched microhabitats attract large herbivores and are fertilized by their wastes. The short vegetative cover may negate the benefits of higher forage quality to risk-averse, small mammals. We used replicated exclosure experiments on two different soil types (sands and clays), and a multi-scale burn experiment on clay soils to examine the abundance, diversity, and biomass of small mammals. Small-mammal species composition differed dramatically between treatments, but density and species diversity on sandy soils were roughly double that of clay soils. Small-mammal densities and diversity were dramatically higher wherever large herbivore grazing was prevented by exclosures. The highest small-mammal densities occurred inside fenced nutrient-rich, tall-grass glades. Small-mammal density was similar between burn sites and unburned surrounding grassland, but was significantly higher on unburned patches within the burn sites. Small mammals were nearly 20-fold more abundant on totally fenced glades than matrix habitat on both soil types, and 9-fold more abundant on fenced, nutrient-poor sites. Unfenced glades supported intermediate densities. Total small-mammal biomass tended to be greater on sandy soils, within exclosures, and on glades. Both the grass rat and pouched mouse reached maximal densities within glade total exclosures on both soils. This and the positive association of small-mammal densities with grass height and dead-stem density suggest that loss of cover is a primary reason that large herbivores suppress the abundance of certain small savanna mammals.

Technical Abstract: Large mammalian herbivores (LMH) are known to suppress populations of small mammals in African savanna ecosystems, but 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 concentrations of LMH and are fertilized by their wastes, but LMH removal of vegetative cover may negate the benefits of higher forage quality 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 the abundance, diversity, and biomass of small mammals across sites varying in vegetation cover and enrichment. Small-mammal species composition differed dramatically between habitats, but density and species diversity on red sands were roughly double that of black cotton. Small-mammal densities and diversity were dramatically higher inside LMH exclosures; highest 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 higher on unburned than burned patches within the burn sites; burned patches had less grassy cover up to 55 months post-burn, although vegetation differences diminished following significant rains. Mice traveled farther to burned patches 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 9-fold more abundant on fenced bushland (non-glades) on red sand. 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 2 common species), within exclosures, and on glades. Both the grass rat and pouched mouse reached maximal densities within glade total exclosures on both soils. This and the positive association of small-mammal densities with grass height and dead-stem density suggest that loss of cover is a primary driver of large-herbivore suppression of certain small savanna mammals.