Elephants limit aboveground carbon gains in African savannas

Abstract

Understanding the drivers of vegetation carbon dynamics is essential for climate change mitigation and effective policy formulation. However, most efforts focus on abiotic drivers of plant biomass change, with little consideration for functional roles performed by animals, particularly at landscape scales. We combined repeat airborne Light Detection and Ranging with measurements of elephant densities, abiotic factors, and exclusion experiments to determine the relative importance of drivers of change in aboveground woody vegetation carbon stocks in Kruger National Park, South Africa. Despite a growing elephant population, aboveground carbon density (ACD) increased across most of the landscape over the 6‐year study period, but at fine scales, bull elephant density was the most important factor determining carbon stock change, with ACD losses recorded only where bull densities exceeded 0.5 bulls/km2. Effects of bull elephants were, however, spatially restricted and landscape dependent, being especially pronounced along rivers, at mid‐elevations, and on steeper slopes. In contrast, elephant herds and abiotic drivers had a comparatively small influence on the direction or magnitude of carbon stock change. Our findings demonstrate that animals can have a substantive influence on regional‐scale carbon dynamics and warrant consideration in carbon cycling models and policy formulation aimed at carbon management and climate change mitigation.

Heterogeneity in African savanna elephant distributions and their impacts on trees in Kruger National Park, South Africa

“…Secondly, we find that the distribution of elephant damage is largely inexplicable by any of the factors we analyze here, even by the distribution of elephants themselves. This suggests that there is no obvious silver bullet for why elephants are where they are and why their preferences are what they are, consistent with a large body of literature (Anderson & Walker, 1974;Asner et al, 2016;Davies & Asner, 2019;Duffy et al, 2002;Guldemond et al, 2017;Van Wyk & Fairall, 1969). This does present an intriguing question, however: What decouples elephants from their impacts?…”

“…Finally, elephant densities at the VCA sites were estimated based on megaherbivore helicopter aerial censuses performed by park management each year from 1985 through 2008. Aerial censuses were performed during the late dry season (August) when elephant impacts on trees are thought to be most severe (Davies & Asner, 2019; Smit & Ferreira, 2010) due to their increased utilization of trees for forage (Codron et al., 2006). Transects were flown across the park following drainage lines, with four expert observers noting the position, number of individuals, and composition of all elephant groups (see Smit et al., 2007 for further details).…”

“…To account for movement, parkwide density layers were generated from point counts for each year from 1985 to 2008; separate layers were generated for bulls and for mixed herds (comprising females, young males, and calves) (Figure 1c and d). Point‐based censuses were interpolated using a 5 km Gaussian kernel smoother with the function density.ppp in the “spatstat” package (Baddeley & Turner, 2004) to generate maps of elephant density across the park for each census year (per Davies & Asner, 2019; MacFadyen et al., 2019). Parkwide density layers from 1985 to 2008 were aggregated across years to yield average distribution estimates for bulls and mixed herds (Figure 1c and d).…”

“…This debate is exacerbated by general uncertainty about what underlies heterogeneity both in elephant density and in how their impacts are distributed in savanna landscapes, which can vary dramatically in space (Anderson & Walker, 1974; Codron et al., 2006). Many studies have attempted to identify consistent patterns of impact (Anderson & Walker, 1974; Asner et al., 2016; Davies & Asner, 2019; Duffy et al., 2002; Van Wyk & Fairall, 1969), but results are so often uncertain or contradictory that meta‐analysis has concluded there is no universal predictor that generalizes well across study areas (Guldemond et al., 2017).…”

“…As such, even places where elephants are not foraging intensely may still suffer severe impacts. Indeed, the assumption that elephant impacts are most severe where elephants are most densely concentrated has limited empirical support (Asner et al., 2016; Davies & Asner, 2019), in part because (a) spatially explicit data on both elephants and their impacts across sufficiently large scales are challenging to collect; and (b) few places have stable enough elephant populations and management histories to reasonably draw such a comparison (Bradshaw et al., 2005; Chase et al., 2016; Guldemond et al., 2017; Smit & Ferreira, 2010). Altogether, it remains unclear whether the distribution of elephants informs the distribution of their impacts.…”