The influence of tree canopies and elephants on sub‐canopy vegetation in a savannah

Abstract

The apparent influence of elephants on the structure of savannahs in Africa may be enhanced by management activities, fire and other herbivores. We separated the effect elephants have on grasses, woody seedlings (<0.5 m) and saplings (0.5-2 m) from the effect of tree canopies (canopy effect), and herbivory (park effect). We defined the canopy effect as the differences between plant abundances and diversity indices under tree canopies and 20 m away from these. Our testing of the park effect relied on the differences in the sub-canopy plant indices inside and outside a protected area that supported a range of herbivores. We based our assessment of the elephant effect on sub-canopy vegetation indices associated with elephant induced reductions in tree canopies. The park and canopy effects were more pronounced than the elephant effect. The park effect suppressed the development of woody seedlings into saplings. Conditions associated with tree canopies benefited woody plants, but not the grasses, as their indices were lower under trees. Elephants reducing canopies facilitated grass species tolerant of direct solar radiation. We concluded that management should consider other agents operating in the system when deciding on reducing the impact that elephants may have on vegetation.

Elephant herbivory in Majete Wildlife Reserve, Malawi

“…Rising elephant populations densities, coupled with a contraction in the size of their home range has made elephants responsible for changing vegetation structure and composition, a situation that requires new management strategies especially in Southern Africa, where populations are among the highest on the continent (Guldemond & Van Aarde, , ). Conservation measures that focus on reducing habitat conversion, cost of protection and/or biological value are becoming increasingly implemented in conservation practice and research (Mills, Frederickson & Moorhead, ; Pearce, Burgman & Franklin, ; Stephens et al ., ).…”

Complexity in African savannas: Direct, indirect, and cascading effects of animal densities, rainfall and vegetation availability

“…Elephants can open up a landscape and thereby increase fire frequencies. This means that they impact large trees, and through competition release, grass availability increases [ 5 ]. Therefore higher elephant densities are expected to be correlated with a higher fuel load and a higher frequency of fires.…”

“…direct effects) can trigger far reaching, cascading effects [ 2 ]. However, many studies only focussed on simple one-to-one relationships in their analysis, such the correlation between vegetation biomass and herbivore densities [ 3 – 5 ], or the effect of rainfall on vegetation biomass [ 4 ], while ignoring the direct and indirect effects of other variables in the system, such as competitive and facilitative effects that, together, structure the vegetation and herbivore community composition. Hence, in this study, a PLS-PM is used that is able to address this analysis bias, quantifying the direct and indirect effects of multiple variables in a single approximate analysis.…”

“…The competition between trees and grass for water is probably not large because of niche differentiation, but many tall trees increase the canopy cover, so that sunlight does not reach the grass layer [ 7 ]. If trees are uprooted or heavily browsed, the landscape becomes more open, which can have a positive influence on the grass layer, triggering a change in species composition of the vegetation [ 5 ].…”

Heavy impact on seedlings by the impala suggests a central role in woodland dynamics

“…Where home-ranges of different browser species overlap, separating the species-specific impacts on woody vegetation becomes difficult (Guldemond & van Aarde 2009). Three broad approaches have been used: visual separation based on characteristic markings left by different browsers on plants (Barnes 2001, Sharam et al 2006, correlating episodic growth of woodlands with population collapses of specific browser species (Prins & Van der Jeugd 1993) and, most commonly, the use of exclosures that exclude particular browser species (Augustine & McNaughton 2004, Belsky 1984, Goheen et al 2004, Moe et al 2009.…”