Centre for African Conservation Ecology
The impacts of elephants on biodiversity in Eastern Cape Subtropical Thickets
We review available information on the impact of elephants on the Subtropical Thickets of the Eastern Cape province as a contribution to the current debate around biodiversity and the need to manage elephant populations. This ecologically diverse region historically supported an abundance of elephants that was incrementally reduced to a single population limited to the Addo Elephant National Park (AENP). The results of research on elephant impacts associated with this population has shown that these animals influence many ecological processes, and patterns, including soil features, landscape patchiness and plant biomass and diversity. Furthermore, elephants influence insect, bird and antelope abundances and reduce browse availability for black rhinoceros. We conclude that elephants affect biodiversity at all levels investigated but that further research is necessary to identify the mechanisms responsible. Of specific concern is the observation that the AENP represents the only current example where elephants may be driving many endemic plants to extinction. This suggests that managing elephant impacts in Subtropical Thickets, specifically, is a matter of urgency.
Relevance of elephant herbivory as a threat to Important Plants in the Addo Elephant National Park, South Africa
Although elephants are recognized as keystone species, the mechanisms of their impacts on biodiversity and community structure are rarely identified. In the Addo Elephant National Park (AENP), South Africa, elephant Loxodonta africana herbivory is apparently responsible for a significant reduction in plant richness, especially among the regionally rare and endemic small succulent shrubs and geophytes (Important Plants). We used faecal analysis to investigate the utilization of Important Plants in elephant diet in the AENP. Ninety plant species were identified in the diet. Only 14 of the 77 (c. 18%) Important Plants previously thought particularly vulnerable to elephant browsing occurred in the diet, while at least 6% of species for which there are data were avoided. This refutes the generally held belief that elephant herbivory is the major driver of decline among Important Plants, and emphasizes the likely contribution of other mechanisms (e.g. knock-on effects, trampling, zoochory, etc.) to this phenomenon. The accurate prediction of impacts caused by elephants in the AENP and elsewhere, therefore requires an understanding of these previously marginalized mechanisms. By demonstrating appropriate cause-and-effect relationships between elephants and ecosystem change, we will be able to move beyond assuming that all the observed changes are due to elephant herbivory.
Landman et al. 2008. Relevance of elephant herbivory as a threat to Important Plants in the Addo Elephant Nation Park, South Africa. Journal of Zoology, London 274: 51-58.
Understanding long-term variations in an elephant piosphere effect to manage impacts
Surface water availability is a key driver of elephant impacts on biological diversity. Thus, understanding the spatio-temporal variations of these impacts in relation to water is critical to their management. However, elephant pioshere effects (i.e. the radial pattern of attenuating impact) are poorly described, with few long-term quantitative studies. Our understanding is further confounded by the complexity of systems with elephant (i.e. fenced, multiple water points, seasonal water availability, varying population densities) that likely limit the use of conceptual models to predict these impacts. Using 31 years of data on shrub structure in the succulent thickets of the Addo Elephant National Park, South Africa, we tested elephant effects at a single water point. Shrub structure showed a clear sigmoid response with distance from water, declining at both the upper and lower limits of sampling. Adjacent to water, this decline caused a roughly 300-m radial expansion of the grass-dominated habitats that replace shrub communities. Despite the clear relationship between shrub structure and ecological functioning in thicket, the extent of elephant effects varied between these features with distance from water. Moreover, these patterns co-varied with other confounding variables (e.g. the location of neighbouring water points), which limits our ability to predict such effects in the absence of long-term data. We predict that elephant have the ability to cause severe transformation in succulent thicket habitats with abundant water supply and elevated elephant numbers. However, these piosphere effects are complex, suggesting that a more integrated understanding of elephant impacts on ecological heterogeneity may be required before water availability is used as a tool to manage impacts. We caution against the establishment of water points in novel succulent thicket habitats, and advocate a significant reduction in water provisioning at our study site, albeit with greater impacts at each water point.
Landman et al. 2012. Understanding long-Term variations in an elephant piosphere effect to manage impacts. PloS One 7(9).
Shift in black rhinoceros diet in the presence of elephant: evidence for competition?
In African large herbivore assemblages, megaherbivores dominate the biomass and utilise the greatest share of available resources. Consequently, they are considered a separate trophic guild that structures the food niches of coexisting large herbivores. However, there exists little empirical evidence on how food resources are shared within this guild, and none for direct competition for food between megaherbivores. Using the histological analysis of faeces, we explore this phenomenon for African elephant Loxodonta africana and black rhinoceros Diceros bicornis in the Addo Elephant National Park, South Africa, where the accumulated impacts of elephant have reduced browse availability. Despite being unable to generalise beyond our study sites, our observations support the predictions of competition theory (as opposed to optimality theory) by showing (1) a clear seasonal separation in resource use between these megaherbivores that increased as resource availability declined, and (2) rhinoceros changed their selectivity in the absence of elephant (using an adjacent site) by expanding and shifting their diet along the grass-browse continuum, and in relation to availability. Although black rhinoceros are generally considered strict browsers, the most significant shift in diet occurred as rhinoceros increased their preferences for grasses in the presence of elephant. We speculate that the lack of specialised grazing adaptations may increase foraging costs in rhinoceros, through reduced harvest-and handling-efficiencies of grasses. In the short-term, this may be off-set by an enhanced tolerance for low quality food and by seasonally mobilising fat reserves; however, the long-term fitness consequences require further study. Our data suggest that managing elephant at high densities may compromise the foraging opportunities of coexisting browsers. This may be particularly important in small, fenced areas and overlapping preferred habitats where impacts intensify.
Landman et al. 2013. Shift in black rhinoceros diet in the presence of elephant: evidence for competition? PloS One 8(7).
Impact of elephant on two woody trees, Boscia oleoides and Pappea capensis, in an arid thicket-Nama Karoo mosaic, Greater Addo Elephant National Park
Despite extensive evidence of the influences of elephant on woody trees in savannah habitats, effects on trees in the succulent thickets of the Eastern Cape are relatively poorly described. Our study investigates the role and intensity of elephant impacts on Pappea capensis and the relatively rare Boscia oleoides in an arid thicket-Nama Karoo mosaic habitat of the Greater Addo Elephant National Park. We show that roughly 19% of the B. oleoides and nearly half of the P. capensis individuals recorded showed signs of elephant impact. Elephant often toppled our study trees, and where these individuals were uprooted, mortalities occurred: B. oleoides ~ 44% of the impacted trees (4 individuals); P. capensis ~22% of the impacted trees (29 individuals). Whilst this study is restricted by limited spatial and temporal replication, P. capensis mortalities caused by elephant occurred at a rate exceeding that of other processes. Our results provide insight into the severity of the measured changes and the need to reduce the impacts. However, it would be critically important to establish the specific driver of elephant-tree interactions before any management intervention is implemented.
Landman et al. 2014. Impact of elephant on two woody trees, Boscia oleoides and Pappea capensis in an arid thicket-Nama Karoo mosaic ...Koedoe 56(1).
Long-term monitoring reveals differing impacts of elephants on elements of a canopy shrub community
The conservation management of southern Africa's elephants focuses on identifying and mitigating the extent and intensity of impacts on biological diversity. However, variation in the intensity of elephant effects between elements of biodiversity is seldom explored, which limits our ability to interpret the scale of the impacts. Our study quantifies >50 years of impacts in the succulent thickets of the Addo Elephant National Park, South Africa, contrasting hypotheses for the resilience of the canopy shrubs (a key functional guild) to elephants with those that argue the opposite. We also assess the impacts between elements of the community, ranging from community composition and structure to the structure of individual canopy species. We show the vulnerability of the canopy shrubs to transformation as the accumulated influences of elephants alter community composition and structure. The pattern of transformation is similar to that caused by domestic herbivores, which leads us to predict that elephants will eventually bring about landscape-level degradation and a significant loss of biodiversity. While we expected the canopy species to show similar declining trends in structure, providing insight into the response of the community as a whole, we demonstrate an uneven distribution of impacts between consistuent elements; most of the canopy dominants exhibited little change, resisting removal. This implies that these canopy dominants might not be useful indicators of community change in thickets, a pattern that is likely repeated among the canopy trees of savanna systems. Our findings suggest that predicting elephant impacts, and finding solutions to the so-called "elephant problem", require a broader and more integrated understanding of the mechanisms driving the changes between elements of biodiversity at various spatial and temporal scales.
Landman et al. 2014. Long-term monitoring reveals differing impacts of elephants on elements of a canopy shrub community. Ecological Applications 24(8): 2002-2012.
Expert-derived monitoring thresholds for impacts of megaherbivores on vegetation cover in a protected area
Monitoring is meant to inform conservation nauthoritioes, yet managers often don't know when to respond to monitoring results. One of the reasons is management often lacks consensus on monitoring thresholds for intervention. This result in aimless monitoring without a clear directive on when monitoring indicates a trajectory towards an unacceptable state or impending change, which possibly necessitates intervention. Although experts rarely provide simple, measurable and quantifiable monitoring thresholds as required by management, they are often more comfortable expressing opinions on whether a specific area is desirable or not. This allows threshold to be reverse engineered: by getting experts to identify sites as desirable and undesirable, field variables can subsequently be measured to derive the boundary between subjectively identified desirable states. Such a boundary provides a defendable point for management to assess and consider intervention. Here we describe the identification of monitoring thresholds by defining the limits of desirable canopy cover, derived from expert stakeholder preferences, in the Sundays Spekboom Thicket vegetation of the Addo Elephant National Park, South Africa. The park has experienced variable utilization intensity by large herbivores, especially elephant. For years, managers have grappled with the question of what percentage shrub canopy cover is desirable as a management target, but science has failed to provide this. Using experts to assess pre-selected sites as desireable or undesirable across a range of canopy covers, we showed that a canopy cover of ~65% (~15%) would be desirable for expert stakeholders. We then used satellite imagery to map canopy cover, providing managers for the first time with a large-scale map of canopy cover, indicating desirability status. This approach was useful for facilitating joint-decision making between conservation agencies and stakeholders on tangible indicators of achieving goals, and may be useful in foresting relationships, trust, mutual understanding and transparency, characteristics critical for managing complex socio-ecological systems.
Smit et al. 2016. Expert-derived monitoring thresholds for impacts of megaherbivores on vegetation cover ... Journal of Environmental Management 177: 298-305.
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