Browsing by Author "Henley, Michelle"

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Impacts of African elephants and other environmental drivers on trees nested in by critically endangered whitebacked vultures
(Nordic Society Oikos) Cook, Robin; Witkowski, Edward; Henley, Michelle
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.
The value of large trees and their protection where elephants and trees co-exist
(University of the Witwatersrand, Johannesburg, 2024) Cook, Robin Michael; Henley, Michelle; Witkowski, Edward
Increasing African elephant (Loxodonta africana) numbers in many southern African protected areas have raised concerns about their impact on large tree species and subsequent effects on biodiversity. However, sustainable strategies for managing elephant impact on particular large trees of concern require stakeholder support. Concerned stakeholders in South Africa's Associated Private Nature Reserves (APNR), a protected area containing a high density of waterholes and elephants, are implementing tree protection methods to protect large trees from elephant impact, but few studies have assessed these methods’ efficacy. This thesis aimed to understand stakeholder perception of the value of large trees. Thereafter, I investigate elephant impact on the large tree component within the APNR over a 12-year period while also assessing the relationship between the persistence of the nests of large tree nesting birds and that of the trees themselves. Lastly, I assessed the effectiveness of various implemented tree protection methods to mitigate against elephant impact. A combination of qualitative and quantitative analysis was used to measure stakeholders’ values on large tree and elephant population dynamics, as well as management strategies to reduce the impact on large trees. The results show that stakeholders were concerned about the loss of large trees and its impact on other species. However, they disagreed on the most effective management strategy to minimise elephant impact, with varying values across stakeholder generations and professions. The persistence trends of 2,758 large trees comprising three species of concern were analysed between 2008-2020 to understand the impact of elephants and other environmental factors on tree mortality. The annual large tree mortality rate from 2008-2020 was 5.6%, with varying declines among tree species and the most significant declines occurring during dry periods. Furthermore, the long-term impact of elephants on trees nested in by the critically endangered white-backed vulture (WbV, Gyps africanus) was studied in riparian and woodland habitats to investigate both tree and nest persistence. Ten tree species were utilised for nesting sites, with woodland trees more at risk to elephant impact verses those in the riparian habitat. However, there was no direct correlation between WbV nest loss and tree fall. Lastly, when considering tree protection methods, about half of the 2,758 trees surveyed were wire-netted as potential elephant impact mitigation strategy. Trees with a stem diameter >40 cm that were wire-netted had the highest persistence rates, but the wire-netting needed replacing after four years. Overall, wire-netting was the most practical method for larger- scale implementation, particularly on larger trees elephants cannot push over. Of the four tree protection methods evaluated in this study in terms of their effectiveness and practicality as a mitigation strategy, beehives were found to be the most effective at protecting trees but proved to be the most expensive in comparison to wire-netting, concrete pyramids and creosote jars