Elephant Herbivory Research Articles

Modelling Tree Allometries to Understand the Impact of African Savannah Elephant Herbivory Dynamics on the Vegetation Structure and Tree Cover Change in a Protected Area

ABSTRACTIn landscapes with high elephant density, trees often exhibit more open canopies with fewer branches and foliage due to browsing pressure. This can result in altered tree morphology, with trees exhibiting stunted growth, multiple stems or unusual branching patterns in response to repeated damage from browsing. The objectives of this research were to (i) model the vegetation structure allometries, (ii) assess the impact of African savannah elephant (Loxodonta africana) herbivory on the vegetation structure and (iii) assess tree cover change and vegetation performance over time in Mana Pools National Park in Zimbabwe. We established 26 plots of 30 × 30 m size. Selection of sampling plots was done following several steps. First, a fish net grid with 30 × 30 m polygons was created and projected on the polygon of Mana Pools National Park. The polygons for exclusion zones were then clipped from the fish net grid using the clip tool in ArcGIS Pro 3.0. Then, selection of sampling plots was done initially by stratified random sampling using the Sampling Design Tool add in for ArcGIS Pro 3.0. Landsat images for the years 2003, 2013 and 2023 were used to assess land use land cover (LULC) time series and to calculate Normalised Difference Vegetation Index (NDVI) and Soil Adjusted Vegetation Index (SAVI) for the period. A generalised linear model (GLM) was used to analyse tree allometries. Further statistical investigations were performed using Bayesian piecewise regression (BPR) and Bayesian regression modelling (BRM). Basal area, number of stems, height, long canopy, diameter and basal circumference were all significantly different (p < 0.05) across all sampled plots. The change in growing conditions occurring as a tree grows beyond the reach of the African savannah elephant browsing indicates a natural system breakpoint. The best‐fitting models were a simple linear model and a two breakpoint model for the plant population exposed to elephant herbivory. LULC, NDVI and SAVI confirm evidence of high tree regeneration over 2 decades. Understanding the dynamics in vegetation and LULC changes is critical for effective conservation and management of the habitats for African savannah elephants, as well as for maintaining the health and resilience of forest ecosystems.

The importance of history in understanding large tree mortality in African savannas

Fire and elephant herbivory are major drivers of large tree mortality in savanna ecosystems. Although the spatial variation of these agents is well studied, less attention has been paid to how disturbance history influences tree mortality over time. In a long‐term cohort study, we examined how the sequence of fire‐ and elephant‐induced damage influences mortality of trees and determined whether risk of mortality is compounded over time. Data on over 2500 large trees were collected from 22 transects in Kruger National Park, South Africa, in 2006, and trees were resampled in 2008, 2011, 2015 and 2018. Over the 12‐year period, we recorded a cumulative mortality of 47.6% with an estimated annual mortality of 3–5% from 2006 through 2015 and an increase to 8.8% in 2018. The main attributed agent of tree mortality was elephant damage, occurring either once or across multiple census periods. A classification tree analysis partitioned over different census periods showed that the probability of mortality of the 10 most common tree species depended not only on the type and intensity of elephant‐induced damage and fire but also on the historical sequence of damage by these agents. Elephant damage to the main stem incurred up to 12 years earlier increased risk of mortality for large trees, especially in combination with fire damage. As expected, vulnerability to damage and risk of mortality varied among species, resulting in the potential for long‐term changes in species composition over large areas. Overall, this study highlights how multiple interacting agents cause emergent and lagging patterns of mortality risk for large trees in savanna ecosystems, a result that only becomes apparent through fine resolution and long‐term tracking of cohorts.

Mass sterilization of a common palm species by elephants in Kruger National Park, South Africa

Chronic herbivory by elephants rarely eliminates any species of woody savanna plants because these plants are typically vigorous basal resprouters after damage by fire or herbivory. In some instances, resprouting after elephant herbivory even increases stem numbers per unit area compared to protected areas. It is thus difficult to know whether an area has been severely degraded by elephant herbivory or not because although trees may be severely reduced in size, they will still be present and may even be relatively dense. By using an elephant exclosure in the Kruger National Park, South Africa, we demonstrate that this resprouting ability masks the fact that entire populations of a widespread African palm, Hyphaene petersiana, are prevented from reaching sexual maturity by chronic elephant herbivory. Besides sterilizing these palms and thus preventing their evolution and seed dispersal, the absence of the palm fruits, flowers and tall stems has other negative biodiversity impacts on their associated fauna. We suggest that to determine sustainable elephant impacts on savanna plants, conservation managers also use the reproductive condition of savanna plants rather than their presence, height or stem density.

Elephant herbivory of knob‐thorn (Senegalia nigrescens) and ivory palm (Hyphaene petersiana) in Bwabwata National Park, Caprivi, Namibia: The role of ivory palm as a biotic refuge

AbstractAfrican elephants have major impacts on vegetation, particularly at high densities. Knob‐thorns (Senegalia nigrescens) are typically ring‐barked by elephant, and high levels of mortality are common at high elephant densities. Our study aimed to test whether ivory palm clusters (Hyphaene petersiana) form a biotic refuge for knob‐thorn against elephant herbivory. We measured the density, damage and mortality of knob‐thorns in sites differing according to ivory palm presence and elephant density, and thus, the probability of knob‐thorn encounter by elephants. The site with palms and low elephant density, had a high density of knob‐thorns, but lower proportions of damaged and dead trees, than sites without palms but with similar or higher elephant density. In the former, knob‐thorns were associated with palm clusters, particularly saplings and young adults. In this site, low proportions of damaged and dead knob‐thorns were recorded in palm clusters, compared with outside clusters, and to those in the other sites. Our study also showed that juvenile palms which protected knob‐thorns, suffered low mortality in contrast to subadult palms. We have no evidence but implicate elephants and suggest that in palm clusters, subadult palms are more accessible to elephants than knob‐thorns because of the different methods of utilisation.

Elephants, termites and mound thermoregulation in a progressively warmer world

With global change, microclimates become important refuges for temperature-sensitive, range-restricted organisms. In African savannas, woody vegetation on Macrotermes mounds create widely-dispersed microclimates significantly cooler than the surrounding matrix, which buffer against elevated temperatures at the finer scale of mounds, allowing species to persist at the landscape scale. Termite colonies cultivate symbiotic fungi to digest lignin, but the fungi require temperatures between 29 and 32 °C, which termites strive to maintain. Mound-associated vegetation is a hot-spot for elephant herbivory, so removal of woody species cover by elephants could influence mound-associated microclimates, impacting temperature regulation by termites. We explored the interaction between two prominent ecosystem engineers (termites and elephants) to ascertain whether elephant removal of mound woody cover affects (1) external mound-associated microclimate and (2) internal mound temperature. We surveyed 44 mounds from three sites in Kruger National Park, South Africa, during an El Nino/Southern Oscillation-induced drought and heatwave, recording whether sub-canopy, external, mound-surface and internal mound temperatures varied with vegetation removal by elephant. Elephant damage to mound-associated vegetation reduces the fine-scale microclimate effect provided by vegetation on Macrotermes mounds. Despite this, termites were able to regulate internal mound temperatures, whereas internal temperatures of abandoned mounds increased with elevated surface temperatures. Termites can persist despite loss of mound-associated microclimates, but the loss likely increases energetic costs of mound thermoregulation. Since mound vegetation buffers against drought, loss of widely-dispersed, fine-scale microclimates could increase as megaherbivores remain constrained to protected areas, impacting climate-sensitive organisms and ecosystem function at a range of scales.

The ecological impact of elephant herbivory on vegetation of Atherstone Collaborative Nature Reserve, Limpopo Province, South Africa

The ecological impact of elephant herbivory on vegetation of Atherstone Collaborative Nature Reserve, Limpopo Province, South Africa

Precipitation and elephants, not fire, shape tree community composition in Serengeti National Park, Tanzania

AbstractSavannas are spatially diverse, variable and are susceptible to high rates of disturbance from fire and herbivory. There is significant interest in woody cover dynamics in relation to disturbance regimes. Less effort has been devoted to understand processes that drive tree community composition. In this study, tree species composition data collected at the landscape scale in the Serengeti were used to identify key environmental factors driving variation in species composition. A system of 38 plots clustered within 10 sites spanning the mean annual precipitation (MAP) gradient was used to assess the relative role of bottom‐up (precipitation, soil nutrients and soil texture) vs. top‐down factors (fire and elephant herbivory) on tree community composition. We developed candidate models relating tree species composition (based on multivariate community analysis) to different combinations of plot‐level environmental covariates. Results suggest that tree community composition is largely driven by MAP and is associated with elephant population density. Strikingly, we found no evidence that fire influences species compositional turnover. In a second analysis, we used structural equation model (SEM) to explore the possible direction of association between elephant density and tree species composition. We compared a model that included elephant effects on composition to one that included community composition effects on elephant density. Results suggest that variation in elephant population density across space is more likely to drive tree community composition and not vice versa. We propose that precipitation and herbivory, rather than fire, determine tree species composition in Serengeti Acacia tree community.

Elephant damage, not fire or rainfall, explains mortality of overstorey trees in Serengeti

Summary Generalizations about the drivers of tree demography in tropical savannas continue to prove difficult because of the complex and dynamic interactions involved, and because multi‐year data sets spanning meaningful gradients in potential drivers are lacking. Overstorey trees play disproportionate roles in the long‐term dynamics and functioning of savanna ecosystems. Understanding demographic patterns in these trees is complicated by their resprouting ability after being top‐killed and few studies have attempted to separate top‐kill from true mortality events. We examined the interactive effects of fire frequency, mean annual precipitation and elephant herbivory on overstorey (>2 m) tree mortality between 2009 and 2014 across 32 permanent vegetation plots in the Serengeti Ecosystem, Tanzania. Mean tree mortality over the study period was 0.28 ± 0.02 (0.07 year−1). Among trees that were top‐killed (19.1% of all individuals), 31.2% resprouted. Mortality was driven largely by elephant herbivory, though mortality rates varied considerably across space and tree species. Fire frequency and mean annual rainfall were weak predictors of tree mortality. Over the 5 year period, chronic elephant herbivory (i.e. repeated through time) was a stronger predictor of tree mortality than maximum elephant herbivory, suggesting that repeated, low‐intensity damage from elephants was more important to mortality than acute, but infrequent, damage. Elephants disproportionately damaged certain tree species relative to their availability on the landscape, and the tolerance to damage differed by species as well. Acacia senegal was strongly selected and appeared to have low tolerance to damage when it occurred, resulting in a mortality rate of 0.736 ± 0.052 (0.234 year−1) over the study period. Top‐killing and resprouting rates varied across species, with Acacias generally having low resprout rates. Synthesis. Our study highlights the considerable role that chronic herbivory plays in structuring savanna tree populations, irrespective of prevailing fire and rainfall conditions, and provides important vegetative context to the dramatic recent declines and (in the case of Serengeti) increases in savanna elephant densities in sub‐Saharan Africa.

Status of African baobab (<i>Adansonia digitata</i>) across Gonarezhou National Park, Zimbabwe

An assessment was done to determine the abundance and structure ofbaobab (Adansonia digitata) across Gonarezhou National Park, Zimbabwe. Baobabs were sampled on fifteen belt transects of constant width of 300 m with fifteen baobabs in each belt transect determined the length of a particular belt transect between May and June 2012. Our results showed that there were no significant differences in basal area, height and density of baobabs across Gonarezhou. Moreover, elephant (Loxodonta africana) dung counts and damaged baobabs were similar across Gonarezhou. Our findings suggest a relatively similar spatial effect of elephant herbivory and other disturbance regimes on baobabs in Gonarezhou. We recommend the continuous monitoring of baobab woodland stands across Gonarezhou. © JASEM

Termite Mounds Increase Functional Diversity of Woody Plants in African Savannas

Fine-scale spatial heterogeneity influences biodiversity and ecosystem productivity at many scales. In savanna systems, Macrotermes termites, through forming spatially explicit mounds with unique woody plant assemblages, emerge as important sources of such heterogeneity. Despite a growing consensus regarding the importance of functional diversity (FD) to ecosystem processes, no study has quantified how termite mounds affect woody plant FD. We address whether termite mounds alter the distribution of functional traits, and increase FD of woody plant communities within Africa’s largest savanna woodland, the 2.7 million km2 miombo system. Using plant traits that change according to soil resources (for example, water and nutrients), and disturbance (for example, fire and elephant herbivory), we identified response functional groups and compared relative representation of these groups between mound and matrix habitats. We also asked whether mound and matrix habitats differed in their contribution to FD within the system. Although species representing most functional groups were found in both mound and matrix habitats, relative abundance of functional groups differed between mound and matrix. Mound plant assemblages had greater response diversity to soil resources than matrix plots, but there was no difference in response diversity to disturbance. High trait values on mounds included tree height, leaf nitrogen, phosphorus, and palatability. Species with root ectomycorrhizae dominated the matrix. In conclusion, these small patches of nutrient-enriched substrate emerge as drivers of FD in above-ground woody plant communities.

Seasonal Selection Preferences for Woody Plants by Breeding Herds of African Elephants (Loxodonta africana) in a Woodland Savanna

To evaluate dynamics of elephant herbivory, we assessed seasonal preferences for woody plants by African elephant breeding herds in the southeastern part of Kruger National Park (KNP) between 2002 and 2005. Breeding herds had access to a variety of woody plants, and, of the 98 woody plant species that were recorded in the elephant's feeding areas, 63 species were utilized by observed animals. Data were recorded at 948 circular feeding sites (radius 5 m) during wet and dry seasons. Seasonal preference was measured by comparing selection of woody species in proportion to their estimated availability and then ranked according to the Manly alpha (α) index of preference. Animals demonstrated a selection preference in feeding on woody vegetation, andGrewia hexamita, Grewia bicolor, Grewia flavescens, andGrewia monticolawere selected consistently more over all seasons. In addition, our results indicate that elephant herds have a low preference for at least some of the woody species prone to extirpation and that feeding preferences for woody plants do not account for the association of elephants and riparian fringe habitat.

Relevance of elephant herbivory as a threat to Important Plants in the Addo Elephant National Park, South Africa

AbstractAlthough 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.

Elephant herbivory, frost damage and topkill in Kalahari sand woodland savanna trees

AbstractQuestions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime?Location: Western Zimbabwe.Methods: Elephant and frost damage, topkill, and whole‐plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two‐year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality.Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and ‐ against expectations ‐ prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees.Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors ‐ such as elephant herbivory ‐ that reduce woodland canopy cover.

Elephant herbivory, frost damage and topkill in Kalahari sand woodland savanna trees

Questions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime? Location: Western Zimbabwe. Methods: Elephant and frost damage, topkill, and whole-plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two-year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality. Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and - against expectations - prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees. Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors - such as elephant herbivory - that reduce woodland canopy cover.

Stem Mortality Following Fire in Kalahari Sand Vegetation: Effects of Frost,Prior Damage, and Tree Neighbourhoods

Fire is a key factor affecting the survivorship and dynamics of woody plants in savannas, but few empirical studies in savanna vegetation have investigated correlates of mortality following fire at the level of individual stems. A study of stem mortality as a function of size, neighbourhood effects, and prior damage (mainly caused by elephants) was undertaken in an area of Kalahari sand vegetation in western Zimbabwe. Stem and whole-plant mortality were quantified for the dominant stems of 557 trees in 4 plots following dry season unplanned fires in 2001. Two plots were located in areas that had been affected by frost earlier in the season, and 2 in areas that had not. Mortality was also recorded for 1762 trees in 20 unburned reference plots, also classified according to the presence of frost damage. Mortality estimators were constructed with a maximum likelihood regression method. Whole-plant mortality was low (on the order of 1–2%) compared with stem mortality, which in burned plots approximated 100% for the smallest size classes and declined as a function of stem diameter. Fire-driven mortality was lower in stems protected by the crowns of larger trees than in stems that were in the open. There was also evidence to suggest that the effects of fire are exacerbated by the prior action of frost and elephant herbivory.

The impact of elephants on the marula tree Sclerocarya birrea

AbstractThis study determined the abundance, density and population structure of the marula tree, Sclerocarya birrea, in three game reserves in South Africa, and assessed patterns and amounts of new and cumulative impact of elephants. Elephant feeding was very patchy so several attributes of individual trees, sampled transects and communities that might influence elephant herbivory were investigated. The incidence and type of elephant impact (bark, branch or stem breakage) were significantly related to tree diameter, but not to fruiting nor proximity to roads. At the transect level, elephant impact was influenced by density of marula trees, but was not influenced by proximity to roads, nor proportion of marula trees bearing fruits in the vicinity. At the community level, elephant impact was higher on reserves with higher total marula densities. Fourfold differences in elephant densities (0.08–0.30 elephants km−2) did not explain marula consumption: the percentage of trees with branch damage was similar across reserves and bark damage was inversely proportional to elephant density. Variation across reserves may reflect local and landscape‐level marula tree abundance, differences in alternative food plants and individual feeding habits. The recorded levels of impact appeared to be sustainable because mortality rates were low, affected trees often recovered, and small trees were not preferentially preyed upon.

Small-Scale Variations in Species Composition of Miombo Woodland in Sengwa, Zimbabwe: The Influence of Edaphic Factors, Fire and Elephant Herbivory

Small-Scale Variations in Species Composition of Miombo Woodland in Sengwa, Zimbabwe: The Influence of Edaphic Factors, Fire and Elephant Herbivory

The winter diet of elephant in Eastern Cape Subtropical Thicket, Addo Elephant National Park

Direct observational methods were used to establish the winter diet of elephants in Eastern Cape Subtropical Thicket in the Addo Elephant National Park, thereby determining which plant species were most at risk from elephant herbivory. A total of 70 species were identified as food plants for elephants, with the grass Cynodon dactylon and the succulents Portulacaria afra and Platythyra haeckeliana dominating, both in terms of frequency of feeding events and volume consumed. In view of the fact that elephants represent 78 of the herbivore biomass in the park, it appears likely that elephant feeding restricts the availability of forage for other browsers. Due to the limited time frame of this study, further research is needed to provide a comprehensive record of the elephant diet for all seasons of the year.