Consequences of climate-induced vegetation changes exceed those of human disturbance for wild impala in the Serengeti ecosystem

Abstract

In East Africa, climate change is predicted to reduce vegetation quality, and pervasive human disturbance has already resulted in significant declines in biodiversity. We studied the combined effects of reduced forage quality and human disturbance on faecal glucocorticoid metabolite (FGM) concentrations. We predicted that decreasing nutritional quality and increasing human disturbance would have an additive positive effect on FGM levels in wild impala (Aepyceros melampus). Employing a space-for-time approach, we used normalized difference vegetation index (NDVI) as a measure of forage quality, combined with spatially explicit proxies of human disturbance across areas of different protection management strategies in the Serengeti ecosystem. We collected 639 faecal samples, spread over 4 years, including both wet and dry seasons. Impala FGM levels increased significantly with declining NDVI and, to a lesser extent, with increasing proxies for human disturbance. However, we found no interaction between the two, such that impala had elevated FGM levels with low NDVI and low FGM levels with high NDVI regardless of human disturbance levels. This implies that impala will have high FGM levels if forage quality is poor, even with significant protection and reduced human disturbance. Understanding how animals respond to and cope with changes in forage quality and human land use across different protected areas is important for conservationists and managers to better protect species at risk and predict population viability.

Keywords: NDVI; conservation; cortisol; forage quality; protected areas; stress; ungulate.

Figure 1

The Serengeti ecosystem. Map of the Serengeti ecosystem, consisting of seven areas with different management strategies and different human land uses. Serengeti National Park (SNP) is subdivided in four areas by dashed lines (see Methods). Areas with darker fills are areas with higher predicted intensities of human disturbance. Locations where samples were collected are in solid (blue) circles, while settlement locations are represented as open triangles

Figure 2

Changes in impala FGM concentrations due to environmental factors. The effect (blue line) of (A) the normalized difference vegetation index (NDVI), and (B) rainfall on impala faecal glucocorticoid metabolite (FGM) concentrations. Adjusted response values are represented as points; 95% confidence interval is the shaded blue area. On panel B, star denotes significant difference from no rainfall category (dashed line; P < 0.05).

Figure 3

Changes in impala FGM concentrations due to environmental factors. The effect (blue line) of (A) Settlement density (kernel density estimate; scaled), (B) shortest Euclidean distance to nearest boundary of Serengeti National Park (SNP; dashed line) in kilometre and (C) land use area on impala faecal glucocorticoid metabolite (FGM) concentration. Adjusted response values are represented as points; 95% confidence interval is the shaded blue area or the error bars. On panel C, stars denote significant difference from cSNP (dashed line; P < 0.05).

Figure 4

Interaction between NDVI and settlement density on impala FGM concentrations. The effect of high and low normalized difference vegetation index (NDVI) values on impala faecal glucocorticoid metabolite (FGM) concentrations when in high (black line) and low (light coloured line) settlement density (SD). High values are those higher than third quantile, and low values are those lower than first quantile. Includes 176 data points: 45 high SD and NDVI; 34 high SD, low NDVI; 40 low SD and high NDVI; 53 low SD and NDVI. Error bars show standard error of the estimate.