Impacts of African elephants and other environmental drivers on trees nested in by critically endangered white-backed vultures

The decline of white-backed vultures (Gyps africanus, hereafter termed vultures) across Africa highlights the need to understand their habitat and nesting requirements, especially in protected areas where African elephants (Loxodonta africana) can impact the trees in which vultures build their nests. Our study aimed to assess the impact that elephants have on trees containing vulture nests in the Associated Private Nature Reserves (APNR) of South Africa’s savanna system through three separate but interlinked assessments. We first assessed the tree species used by vultures for nesting and compared their size and elephant impact scores between riparian and woodland habitats. We assessed how elephant presence or absence affects the size of Senegalia nigrescens, a key tree species, and compared vulture nests in an adjacent elephant-free area. Lastly, we modeled environmental factors influencing vulture nest and tree persistence using data from 2008-2020. Vultures utilised 10 tree species, with riparian trees supporting nests being significantly taller, with larger DBHs, and experienced lower elephant impact compared to woodland trees, which were more heavily impacted by elephants. Less robust species like S. nigrescens were more vulnerable to elephant damage, primarily bark-stripping, and less likely to host vulture nests. Our results show that vultures prefer the largest, least impacted trees for nesting, favouring those with greater stability and longevity. We suggest that although elephants influence the overall height range of trees to vultures, strong gusts of wind have a strong negative contribution on vulture nest persistence and that only a relatively small number of trees died during the 12-year study in comparison to fallen nests. We recommend further research into elephant impact thresholds on trees and vulture nest selection. Monitoring treefall and regeneration rates will help predict when vultures may face a shortage of suitable nesting trees.

Methods
Dataset Collection:

The dataset was collected through detailed field surveys conducted between 2008 and 2020 in the Associated Private Nature Reserves (APNR) and the adjacent Air Force Base Hoedspruit (AFB), located in South Africa’s savanna system. The surveys were designed to assess the impact of African elephants on trees utilized by white-backed vultures (Gyps africanus) for nesting.

The surveys focused on three objectives:

Measuring the morphometric traits (height, diameter at breast height [DBH]) and elephant impact scores of various tree species used for nesting by vultures, across both riparian and woodland habitats.
Comparing the characteristics and impact levels of nesting and control trees (non-nesting) of the key species Senegalia nigrescens in areas with and without elephants.
Investigating the long-term persistence of S. nigrescens trees with vulture nests under varying ecological and environmental pressures.

Data collection involved marking tree locations using GPS, measuring tree height with VolCalc software, and recording DBH using a standardized approach. Elephant impact was scored on a scale of 0-5 using an established framework (Walker, 1976; Helm & Witkowski, 2013) to classify damage ranging from minor bark-stripping to complete tree mortality. Additional covariates were recorded, including the presence of ants, termites, and shelf-bracket fungus, fire scars, distance to the nearest road and water source, and annual rainfall and wind gust data obtained from local weather stations. Historical data from the Elephants Alive NGO and aerial census records provided complementary information for long-term analysis.

Dataset Processing:

The collected data were processed and analyzed using R software (version 4.3.0). A suite of statistical tests and models was employed to address specific research objectives. These included:

Descriptive Statistics: Summarizing morphometric and environmental variables for trees in riparian versus woodland habitats.

Comparative Analyses: Mann-Whitney U-tests and Kruskal-Wallis one-way ANOVA were used to identify significant differences in tree characteristics, elephant impact scores, and habitat effects.

Persistence Analyses: Kaplan-Meier survival analyses were conducted to compare tree and nest survival rates over the 12-year study period. Mortality rates for trees and nests were calculated to identify key periods of decline.

Hazard Modeling: Cox proportional-hazards models were applied to explore the effects of environmental covariates (e.g., wind gusts, tree height, DBH) on tree and nest persistence. Continuous variables were scaled and centered to improve interpretability and model convergence. Stepwise backward selection, followed by forward validation, was used to refine models for robustness.

The resulting dataset provides a comprehensive view of the relationships between elephants, trees, and white-backed vulture nesting ecology, facilitating insights into tree persistence and nest-site suitability under environmental pressures. The data were cleaned and verified for consistency and accuracy before final analysis.

References

Helm, C.V. and Witkowski, E.T.F. 2013. Continuing decline of a keystone tree species in the Kruger National Park, South Africa. African Journal of Ecology 51(2), pp. 270–279. doi:
10.1111/aje.12032.

Walker, S.H. 1976. An approach to the monitoring of changes in the composition and utilization of woodland and savanna vegetation. South African Journal of Wildlife Research 6(1), pp. 1–32.