Savanna Tree Density, Herbivores, and the Herbaceous Community: Bottom-Up vs. Top-Down Effects

Abstract

Herbivores choose their habitats both to maximize forage intake and to minimize their risk of predation. For African savanna herbivores, the available habitats range in woody cover from open areas with few trees to dense, almost-closed woodlands. This variation in woody cover or density can have a number of consequences for herbaceous species composition, cover, and productivity, as well as for ease of predator detection and avoidance. Here, we consider two alternative possibilities: first, that tree density affects the herbaceous vegetation, with concomitant "bottom-up" effects on herbivore habitat preferences; or, second, that tree density affects predator visibility, mediating "top-down" effects of predators on herbivore habitat preferences. We sampled sites spanning a 10-fold range of tree densities in an Acacia drepanolobium-dominated savanna in Laikipia, Kenya, for variation in (1) herbaceous cover, composition, and species richness; (2) wild and domestic herbivore use; and (3) degree of visibility obstruction by the tree layer. We then used structural equation modeling to consider the potential influences that tree density may have on herbivores and herbaceous community properties. Tree density was associated with substantial variation in herbaceous species composition and richness. Cattle exhibited a fairly uniform use of the landscape, whereas wild herbivores, with the exception of elephants, exhibited a strong preference for areas of low tree density. Model results suggest that this was not a response to variation in herbaceous-community characteristics, but rather a response to the greater visibility associated with more open places. Elephants, in contrast, preferred areas with higher densities of trees, apparently because of greater forage availability. These results suggest that, for all but the largest species, top-down behavioral effects of predator avoidance on herbivores are mediated by tree density. This, in turn, appears to have cascading effects on the herbaceous vegetation. These results shed light on one of the major features of the "landscape of fear" in which African savanna herbivores exist.

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

“…Riginos and Grace (2008) reported a declining dung pile abundance of most herbivores with increasing tree densities and, hence, decreasing visibility with a concordant increase in predation risk. Riginos and Grace (2008) further argued that these areas were probably not being used by herbivores to avoid predators. By avoiding predators, prey species can shape the habitat vegetation structure of their refuge area (Ripple and Beschta 2004).…”

“…Areas where small trees are at high densities are relatively unattractive foraging sites for herbivores (Riginos and Grace 2008). Riginos and Grace (2008) reported a declining dung pile abundance of most herbivores with increasing tree densities and, hence, decreasing visibility with a concordant increase in predation risk. Riginos and Grace (2008) further argued that these areas were probably not being used by herbivores to avoid predators.…”

Ecosystem structure, function and composition in rangelands are negatively affected by livestock grazing

“…First, grazing effects are known to vary between native and domestic herbivores (Riginos and Grace 2008) and among different breeds of livestock, which have different foraging behaviors and patch preferences (Squires 1981). Grazing by sheep, for example, has been associated with increases in plant richness, but cattle grazing can substantially reduce plant diversity or ecosystem functioning (Letnic 2004, Socher et al 2013).…”

Conserving Wildlife in African Landscapes Kenya’s Ewaso Ecosystem

“…In the black cotton habitat, a recent study examined patterns of herbivore use in A. drepanolobium woodland habitats that varied from 240 to 2,784 trees per hectare, or 6 to 45% canopy cover. All but the largest herbivore species (elephants) spent significantly more time in areas with low tree density and avoided areas with high tree density (Riginos and Grace, 2008). This preference was independent of grass cover or species composition.…”