The return of the giants: ecological effects of an increasing elephant population

Are hippos Africa's most influential megaherbivore? A review of ecosystem engineering by the semi-aquatic common hippopotamus

Megaherbivores perform vital ecosystem engineering roles, and have their last remaining stronghold in Africa. Of Africa's remaining megaherbivores, the common hippopotamus (Hippopotamus amphibius) has received the least scientific and conservation attention, despite how influential their ecosystem engineering activities appear to be. Given the potentially crucial ecosystem engineering influence of hippos, as well as mounting conservation concerns threatening their long-term persistence, a review of the evidence for hippos being ecosystem engineers, and the effects of their engineering, is both timely and necessary. In this review, we assess, (i) aspects of hippo biology that underlie their unique ecosystem engineering potential; (ii) evaluate hippo ecological impacts in terrestrial and aquatic environments; (iii) compare the ecosystem engineering influence of hippos to other extant African megaherbivores; (iv) evaluate factors most critical to hippo conservation and ecosystem engineering; and (v) highlight future research directions and challenges that may yield new insights into the ecological role of hippos, and of megaherbivores more broadly. We find that a variety of key life-history traits determine the hippo's unique influence, including their semi-aquatic lifestyle, large body size, specialised gut anatomy, muzzle structure, small and partially webbed feet, and highly gregarious nature. On land, hippos create grazing lawns that contain distinct plant communities and alter fire spatial extent, which shapes woody plant demographics and might assist in maintaining fire-sensitive riverine vegetation. In water, hippos deposit nutrient-rich dung, stimulating aquatic food chains and altering water chemistry and quality, impacting a host of different organisms. Hippo trampling and wallowing alters geomorphological processes, widening riverbanks, creating new river channels, and forming gullies along well-utilised hippo paths. Taken together, we propose that these myriad impacts combine to make hippos Africa's most influential megaherbivore, specifically because of the high diversity and intensity of their ecological impacts compared with other megaherbivores, and because of their unique capacity to transfer nutrients across ecosystem boundaries, enriching both terrestrial and aquatic ecosystems. Nonetheless, water pollution and extraction for agriculture and industry, erratic rainfall patterns and human-hippo conflict, threaten hippo ecosystem engineering and persistence. Therefore, we encourage greater consideration of the unique role of hippos as ecosystem engineers when considering the functional importance of megafauna in African ecosystems, and increased attention to declining hippo habitat and populations, which if unchecked could change the way in which many African ecosystems function.

Looking into the world's largest elephant population in search of ligninolytic microorganisms for biorefineries: a mini-review

Gastrointestinal tracts (GIT) of herbivores are lignin-rich environments with the potential to find ligninolytic microorganisms. The occurrence of the microorganisms in herbivore GIT is a well-documented mutualistic relationship where the former benefits from the provision of nutrients and the latter benefits from the microorganism-assisted digestion of their recalcitrant lignin diets. Elephants are one of the largest herbivores that rely on the microbial anaerobic fermentation of their bulky recalcitrant low-quality forage lignocellulosic diet given their inability to break down major components of plant cells. Tapping the potential of these mutualistic associations in the biggest population of elephants in the whole world found in Botswana is attractive in the valorisation of the bulky recalcitrant lignin waste stream generated from the pulp and paper, biofuel, and agro-industries. Despite the massive potential as a feedstock for industrial fermentations, few microorganisms have been commercialised. This review focuses on the potential of microbiota from the gastrointestinal tract and excreta of the worlds' largest population of elephants of Botswana as a potential source of extremophilic ligninolytic microorganisms. The review further discusses the recalcitrance of lignin, achievements, limitations, and challenges with its biological depolymerisation. Methods of isolation of microorganisms from elephant dung and their improvement as industrial strains are further highlighted.

Assessing the Impact of Wildlife on Vegetation Cover Change, Northeast Namibia, Based on MODIS Satellite Imagery (2002–2021)

Human–wildlife conflict in the Zambezi region of northeast Namibia is well documented, but the impact of wildlife (e.g., elephants) on vegetation cover change has not been adequately addressed. Here, we assessed human–wildlife interaction and impact on vegetation cover change. We analyzed the 250 m MODIS and ERA5 0.25° × 0.25° drone and GPS-collar datasets. We used Time Series Segmented Residual Trends (TSS-RESTREND), Mann–Kendall Test Statistics, Sen’s Slope, ensemble, Kernel Density Estimation (KDE), and Pearson correlation methods. Our results revealed (i) widespread vegetation browning along elephant migration routes and within National Parks, (ii) Pearson correlation (p-value = 5.5 × 10−8) showed that vegetation browning areas do not sustain high population densities of elephants. Currently, the Zambezi has about 12,008 elephants while these numbers were 1468, 7950, and 5242 in 1989, 1994, and 2005, respectively, (iii) settlements and artificial barriers have a negative impact on wildlife movement, driving vegetation browning, and (iv) vegetation greening was found mostly within communal areas where intensive farming and cattle grazing is a common practice. The findings of this study will serve as a reference for policy and decision makers. Future studies should consider integrating higher resolution multi-platform datasets for detailed micro analysis and mapping of vegetation cover change.

Naïve plant communities and individuals may initially suffer in the face of reintroduced megafauna: An experimental exploration of rewilding from an African savanna rangeland

Excluding large native mammals is an inverse test of rewilding. A 25-year exclosure experiment in an African savanna rangeland offers insight into the potentials and pitfalls of the rewilding endeavor as they relate to the native plant community. A broad theme that has emerged from this research is that entire plant communities, as well as individual plants, adjust to the absence of herbivores in ways that can ill-prepare them for the return of these herbivores. Three lines of evidence suggest that these "naïve" individuals, populations, and communities are likely to initially suffer from herbivore rewilding. First, plots protected from wild herbivores for the past 25 years have developed rich diversity of woody plants that are absent from unfenced plots, and presumably would disappear upon rewilding. Second, individuals of the dominant tree in this system, Acacia drepanolobium, greatly reduce their defences in the absence of browsers, and the sudden arrival of these herbivores (in this case, through a temporary fence break), resulted in far greater elephant damage than for their conspecifics in adjacent plots that had been continually exposed to herbivory. Third, the removal of herbivores favoured the most palatable grass species, and a large number of rarer species, which presumably would be at risk from herbivore re-introduction. In summary, the native communities that we observe in defaunated landscapes may be very different from their pre-defaunation states, and we are likely to see some large changes to these plant communities upon rewilding with large herbivores, including potential reductions in plant diversity. Lastly, our experimental manipulation of cattle represents an additional test of the role of livestock in rewilding. Cattle are in many ways ecologically dissimilar to wildlife (in particular their greater densities), but in other ways they may serve as ecological surrogates for wildlife, which could buffer ecosystems from some of the ecological costs of rewilding. More fundamentally, African savannah ecosystems represent a challenge to traditional Western definitions of "wilderness" as ecosystems free of human impacts. We support the suggestion that as we "rewild" our biodiversity landscapes, we redefine "wildness" in the 21st Century to be inclusive of (low impact, and sometimes traditional) human practices that are compatible with the sustainability of native (and re-introduced) biodiversity.

Feasibility of reintroducing grassland megaherbivores, the greater one-horned rhinoceros, and swamp buffalo within their historic global range

Reintroduction of endangered species is an effective and increasingly important conservation strategy once threats have been addressed. The greater one-horned rhinoceros and swamp buffalo have declined through historic hunting and habitat loss. We identify and evaluate available habitat across their historic range (India, Nepal, and Bhutan) for reintroducing viable populations. We used Species Distribution Models in Maxent to identify potential habitats and evaluated model-identified sites through field visits, interviews of wildlife managers, literature, and population-habitat viability analysis. We prioritize sites based on size, quality, protection, management effectiveness, biotic pressures, and potential of conflict with communities. Our results suggest that populations greater than 50 for rhinoceros and 100 for buffalo were less susceptible to extinction, and could withstand some poaching, especially if supplemented or managed as a metapopulation. We note some reluctance by managers to reintroduce rhinoceros due to high costs associated with subsequent protection. Our analysis subsequently prioritised Corbett and Valmiki, for rhino reintroduction and transboundary complexes of Chitwan-Parsa-Valmiki and Dudhwa-Pilibhit-Shuklaphanta-Bardia for buffalo reintroductions. Establishing new safety-nets and supplementing existing populations of these megaherbivores would ensure their continued survival and harness their beneficial effect on ecosystems and conspecifics like pygmy hog, hispid hare, swamp deer, hog deer, and Bengal florican.

The management dilemma: Removing elephants to save large trees

The loss of large trees ( 5 m in height) in Africa’s protected areas is often attributed to the impact by savanna elephants (Loxodonta africana). Concerns have been raised over large tree mortality levels in protected areas such as South Africa’s Kruger National Park (KNP) and in the past, the need to manage its elephant population in order to preserve large trees and biodiversity as a whole. Our review aims to synthesise and discuss the complexities of managing elephants’ effects on the landscape to ensure the survival of large trees, as well as the application purposes of the various lethal and non-lethal elephant mitigation strategies. We further critically evaluate past management strategies, which have solely focused on controlling elephant numbers to protect large trees. Past mitigation strategies focused on managing elephant impact by directly reducing elephant numbers. However, maintaining elephant numbers at a pre-determined carrying capacity level did not prevent the loss of large trees. Research on large tree survival in African savannas has continually exposed the complexity of the situation, as large tree survival is influenced at various demographic stages. In some cases, a coalescence of historical factors may have resulted in what could be perceived as an aesthetically appealing savanna for managers and tourists alike. Furthermore, the past high density of surface water within the KNP homogenised elephant impact on large trees by increasing the encounter rate between elephants and large trees. Our review evaluates how current mitigation strategies have shifted from purely managing elephant numbers to managing elephant distribution across impact gradients, thereby promoting heterogeneity within the system. Additionally, we discuss each mitigation strategy’s occurrence at various landscape scales and its advantages and disadvantages when used to manage impact of elephant on large trees.Conservation implications: A variety of options exist to manage the effects that elephants have on large trees. These options range from large-scale landscape manipulation solutions to small-scale individual tree protection methods. Interactions between elephants and large trees are complex, however, and conservation managers need to consider the advantages and disadvantages of each mitigation strategy to protect large trees.

Complexity in African savannas: Direct, indirect, and cascading effects of animal densities, rainfall and vegetation availability

Savanna ecosystems are popular subjects for interaction studies. Multiple studies have been done on the impact of elephants on vegetation, the impact of grass and browse availability on animal densities or on competition between herbivore species. Previous studies showed that elephant densities are frequently negatively correlated with densities of tall trees, and that browse and grass availability are correlated with browser and grazer density respectively. Additionally, a competition effect between browse and grass availability has been reported. These relationships are usually analysed by testing direct relationships between e.g., herbivore densities and food availability, without addressing competition effects or other indirect effects. In this study, multiple interactions in a savanna system have been analysed simultaneously using Partial Least Square-Path Modelling (PLS-PM) using mammal and vegetation data from three different wildlife reserves in southern KwaZulu-Natal. The results showed that the processes that three separate models for the three areas provided the best understanding of the importance of the different interactions. These models suggest that elephants had a negative impact on trees, but also on grass availability. The impact is stronger when elephants are not able to migrate during the dry season. Browsers and grazers were correlated with browse and grass availability, but competition between browse and grass was not detected. This study shows that due to the complexity of the interactions in an ecosystem and differences in environmental factors, these interactions are best studied per area. PLS-PM can be a useful tool for estimating direct, indirect, and cascading effects of changing animal densities in conservation areas.

Assessing multi-decadal land-cover - land-use change in two wildlife protected areas in Tanzania using Landsat imagery

Landscape change in and around protected areas is of concern worldwide given the potential impacts of such change on biodiversity. Given such impacts, we sought to understand the extent of changes in different land-cover types at two protected areas, Tarangire and Katavi National Parks in Tanzania, over the past 27 years. Using Maximum Likelihood classification procedures we derived eight land-cover classes from Landsat TM and ETM+ images, including: woody savannah, savannah, grassland, open and closed shrubland, swamp and water, and bare land. We determined the extent and direction of changes for all land-cover classes using a post-classification comparison technique. The results show declines in woody savannah and increases in barren land and swamps inside and outside Tarangire National Park and increases in woody savannah and savannah, and declines of shrubland and grassland inside and outside Katavi National Park. The decrease of woody savannah was partially due to its conversion into grassland and barren land, possibly caused by human encroachment by cultivation and livestock. Based upon these changes, we recommend management actions to prevent detrimental effects on wildlife populations.

A systematic review of elephant impact across Africa

Contradictory findings among scientific studies that address a particular issue may impede the conversion of science to management implementation. A systematic review of peer-reviewed studies to generate a single outcome may overcome this problem. The contentious topic of the impact that a megaherbivore such as the savanna elephant have for other species and their environment can benefit from such an approach. After some 68 years, 367 peer-reviewed papers covered the topic and 51 of these papers provided sufficient data to be included in a meta-analysis. We separated the direct impact that elephants had on trees and herbs from the indirect effects on other vertebrates, invertebrates, and soil properties. Elephants have an impact on tree structure and abundance but no overall negative cascading effects for species that share space with them. Primary productivity explained a small amount of variation of elephant impact on vegetation. Elephant numbers (density), study duration, rainfall, tree cover, and the presence of artificial water and fences failed to describe patterns of impact. We conclude that published information do not support the calls made for artificially manipulating elephant numbers to ameliorate elephant impact, and call for the management of space use by elephants to maintain savanna heterogeneity.

Seasonal habitat selection by African buffalo Syncerus caffer in the Savuti–Mababe–Linyanti ecosystem of northern Botswana

This study aimed to establish seasonal movement and habitat selection patterns of African buffalo Syncerus caffer in relation to a detailed habitat map and according to seasonal changes in forage quality and quantity in the Savuti–Mababe–Linyanti ecosystem (Botswana). Two buffalo were collared in November 2011 and another in October 2012. All three buffalo had greater activities in the mopane–sandveld woodland mosaic during the wet season, which provided high-quality leafy grasses and ephemeral water for drinking, but moved to permanent water and reliable forage of various wetlands (swamps and floodplains) and riverine woodlands during the dry season. Wetlands had higher grass greenness, height and biomass than woodlands during the dry season. Buffalo had similar wet season concentration areas in the 2011–2012 and 2012–2013 wet seasons and similar dry season concentration areas over the 2012 and 2013 dry seasons. However, their dry season location of collaring in 2011 differed dramatically from their 2012 and 2013 dry season concentration areas, possibly because of the exceptionally high flood levels in 2011, which reduced accessibility to their usual dry season concentration areas. The study demonstrates that extremely large and heterogeneous landscapes are needed to conserve buffalo in sandy, dystrophic ecosystems with variable rainfall.Conservation implications: This study emphasises the importance of large spatial scale available for movement, which enables adaptation to changing conditions between years and seasons.

Influence of cattle on browsing and grazing wildlife varies with rainfall and presence of megaherbivores

In many savanna ecosystems worldwide, livestock share the landscape and its resources with wildlife. The nature of interactions between livestock and wildlife is a subject of considerable interest and speculation, yet little controlled experimental research has been carried out. Since 1995, we have been manipulating the presence and absence of cattle and large mammalian herbivore wildlife in a Kenyan savanna in order to better understand how different herbivore guilds influence habitat use by specific wildlife species. Using dung counts as a relative assay of herbivore use of the different experimental plots, we found that cattle had a range of effects, mostly negative, on common mesoherbivore species, including both grazers and mixed feeders, but did not have significant effects on megaherbivores. The effect of cattle on most of the mesoherbivore species was contingent on both the presence of megaherbivores and rainfall. In the absence of megaherbivores, wild mesoherbivore dung density was 36% lower in plots that they shared with cattle than in plots they used exclusively, whereas in the presence of megaherbivores, wild mesoherbivore dung density was only 9% lower in plots shared with cattle than plots used exclusively. Cattle appeared to have a positive effect on habitat use by zebra (a grazer) and steinbuck (a browser) during wetter periods of the year but a negative effect during drier periods. Plots to which cattle had access had lower grass and forb cover than plots from which they were excluded, while plots to which megaherbivores had access had more grass cover but less forb cover. Grass cover was positively correlated with zebra and oryx dung density while forb cover was positively correlated with eland dung density. Overall these results suggest that interactions between livestock and wildlife are contingent on rainfall and herbivore assemblage and represent a more richly nuanced set of interactions than the longstanding assertion that cattle simply compete with (grazing) wildlife. Specifically, rainfall and megaherbivores seemed to moderate the negative effects of cattle on some mesoherbivore species. Even if cattle tend to reduce wildlife use of the landscape, managing simultaneously for livestock production (at moderate levels) and biodiversity conservation is possible.

In the elephant's seed shadow: the prospects of domestic bovids as replacement dispersers of three tropical Asian trees

As populations of the world's largest animal species decline, it is unclear how ecosystems will react to their local extirpation. Due to the unique ecological characteristics of megaherbivores such as elephants, seed dispersal is one ecosystem process that may be affected as populations of large animals are decimated. In typically disturbed South Asian ecosystems, domestic bovids (cattle, Bosprimigenius, and buffalo, Bubalus bubalis) may often be the species most available to replace Asian elephants (Elephas maximus) as endozoochorous dispersers of large-fruited mammal-dispersed species. We use feeding trials, germination trials, and movement data from the tropical moist forests of Buxa Tiger Reserve (India) to examine whether domestic bovids are viable replacements for elephants in the dispersal of three large- fruited species: Dillenia indica, Artocarpus chaplasha, and Careya arborea. We find that (1) once consumed, seeds are between 2.5 (C. arborea) and 26.5 (D. indica) times more likely to pass undigested into elephant dung than domestic bovid dung; and (2) seeds from elephant dung germinated as well as or better than seeds taken from bovid dung for all plant species, with D. indica seeds from elephant dung 1.5 times more likely to germinate. Furthermore, since wild elephants have less constrained movements than even free-roaming domestic bovids, we calculate that maximum dispersal by elephants is between 9.5 and 11.2 times farther than that of domestic bovids, with about 20% of elephant-dispersed seeds being moved farther than the maximum distance seeds are moved by bovids. Our findings suggest that, while bovids are able to disperse substantial numbers of seeds over moderate distances for two of the three study species, domestic bovids will be unable to routinely emulate the reliable, long-distance dispersal of seeds executed by elephants in this tropical moist forest. Thus while domestic bovids can attenuate the effects of losing elephants as dispersers, they may not be able to prevent the decline of various mammal-dispersed fruiting species in the face of overhunting, habitat fragmentation, and climate change.

The Asian elephant is amongst the top three frugivores of two tree species with easily edible fruit

Large animal species are prone to local extirpation, but ecologists cannot yet predict how the loss of megaherbivores affects ecosystem processes such as seed dispersal. Few studies have compared the quantity and quality of seed dispersal by megaherbivores versus alternative frugivores in the wild, particularly for plant species with fruit easily consumed by many frugivorous species. In a disturbed tropical moist forest in India, we examine whether megaherbivores are a major frugivore of two tree species with easily edible, mammal-dispersed fruit. We quantify the relative fruit removal rates of Artocarpus chaplasha and Careya arborea, by the Asian elephant (Elephas maximus) and alternative dispersers. Through focal watches and camera trapping, we found the elephant to be amongst the top three frugivores for each tree species. Furthermore, seed transects under A. chaplasha show that arboreal frugivores discard seeds only a short distance from the parental tree, underscoring the elephant's role as a long-distance disperser. Our data provide unprecedented support for an old notion: megaherbivores may be key dispersers for a broad set of mammal-dispersed fruiting species, and not just fruit inaccessible to smaller frugivores. As such, the elephant may be particularly important for the functional ecology of the disturbed forests it still inhabits across tropical Asia.

Modeling elephant-mediated cascading effects of water point closure

Wildlife management to reduce the impact of wildlife on their habitat can be done in several ways, among which removing animals (by either culling or translocation) is most often used. There are, however, alternative ways to control wildlife densities, such as opening or closing water points. The effects of these alternatives are poorly studied. In this paper, we focus on manipulating large herbivores through the closure of water points (WPs). Removal of artificial WPs has been suggested in order to change the distribution of African elephants, which occur in high densities in national parks in Southern Africa and are thought to have a destructive effect on the vegetation. Here, we modeled the long-term effects of different scenarios of WP closure on the spatial distribution of elephants, and consequential effects on the vegetation and other herbivores in Kruger National Park, South Africa. Using a dynamic ecosystem model, SAVANNA, scenarios were evaluated that varied in availability of artificial WPs; levels of natural water; and elephant densities. Our modeling results showed that elephants can indirectly negatively affect the distributions of meso-mixed feeders, meso-browsers, and some meso-grazers under wet conditions. The closure of artificial WPs hardly had any effect during these natural wet conditions. Under dry conditions, the spatial distribution of both elephant bulls and cows changed when the availability of artificial water was severely reduced in the model. These changes in spatial distribution triggered changes in the spatial availability of woody biomass over the simulation period of 80 years, and this led to changes in the rest of the herbivore community, resulting in increased densities of all herbivores, except for giraffe and steenbok, in areas close to rivers. The spatial distributions of elephant bulls and cows showed to be less affected by the closure of WPs than most of the other herbivore species. Our study contributes to ecologically informed decisions in wildlife management. The results from this modeling exercise imply that long-term effects of this intervention strategy should always be investigated at an ecosystem scale.

Elevated elephant density does not improve ecotourism opportunities: convergence in social and ecological objectives

In order to sustainably conserve biodiversity, many protected areas, particularly private protected areas, must find means of self-financing. Ecotourism is increasingly seen as a mechanism to achieve such financial sustainability. However, there is concern that ecotourism operations are driven to achieve successful game-viewing, influencing the management of charismatic species. An abundance of such species, including the African elephant (Loxodonta africana), has been stocked in protected areas under the assumption that they will increase ecotourism value. At moderate to high densities, the impact of elephants is costly; numerous studies have documented severe changes in biodiversity through the impacts of elephants. Protected areas that focus on maintaining high numbers of elephants may therefore face a conflict between socioeconomic demands and the capacity of ecological systems. We address this conflict by analyzing tourist elephant-sighting records from six private and one statutory protected area, the Addo Elephant National Park (AENP), in the Eastern Cape Province of South Africa, in relation to elephant numbers. We found no relationship between elephant density and elephant-viewing success. Even though elephant density in the AENP increased over time, a hierarchical partitioning analysis indicated that elephant density was not a driver of tourist numbers. In contrast, annual tourist numbers for the AENP were positively correlated with general tourist numbers recorded for South Africa. Our results indicate that the socioeconomic and ecological requirements of protected areas in terms of tourism and elephants, respectively, converge. Thus, high elephant densities and their associated ecological costs are not required to support ecotourism operations for financial sustainability. Understanding the social and ecological feedbacks that dominate the dynamics of protected areas, particularly within private protected areas, can help to elucidate the management challenges of minimizing ecological trade-offs while meeting ecotourist demands and achieving sustainability.

Elephant movement closely tracks precipitation-driven vegetation dynamics in a Kenyan forest-savanna landscape

BACKGROUND

This study investigates the ranging behavior of elephants in relation to precipitation-driven dynamics of vegetation. Movement data were acquired for five bachelors and five female family herds during three years in the Marsabit protected area in Kenya and changes in vegetation were mapped using MODIS normalized difference vegetation index time series (NDVI). In the study area, elevations of 650 to 1100 m.a.s.l experience two growth periods per year, while above 1100 m.a.s.l. growth periods last a year or longer.

RESULTS

We find that elephants respond quickly to changes in forage and water availability, making migrations in response to both large and small rainfall events. The elevational migration of individual elephants closely matched the patterns of greening and senescing of vegetation in their home range. Elephants occupied lower elevations when vegetation activity was high, whereas they retreated to the evergreen forest at higher elevations while vegetation senesced. Elephant home ranges decreased in size, and overlapped less with increasing elevation.

CONCLUSIONS

A recent hypothesis that ungulate migrations in savannas result from countervailing seasonally driven rainfall and fertility gradients is demonstrated, and extended to shorter-distance migrations. In other words, the trade-off between the poor forage quality and accessibility in the forest with its year-round water sources on the one hand and the higher quality forage in the low-elevation scrubland with its seasonal availability of water on the other hand, drives the relatively short migrations (the two main corridors are 20 and 90 km) of the elephants. In addition, increased intra-specific competition appears to influence the animals' habitat use during the dry season indicating that the human encroachment on the forest is affecting the elephant population.

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.

Variable-source flood pulsing in a semi-arid transboundary watershed: the Chobe River, Botswana and Namibia

The Chobe River, characterized by an unusual flood pulsing regime and shared between Botswana and Namibia, lies at the heart of the world's largest transfrontier conservation area (the Kavango-Zambezi Transfrontier Conservation Area). Significant ecological changes and vegetation conversions are occurring along its floodplains. Various scenarios for agricultural and urban water use are currently being proposed by the government of Botswana. However, the understanding of the river's annual flow regime and timing of the relative contributions of water from three different sources is relatively poor. In light of past and future climate change and variability, this means that allocating water between ecological flows and economic and domestic uses will become increasingly challenging. We reconstruct the inundation history in this basin to help ease this challenge. This paper presents a spatiotemporal approach to estimate the contribution of water from various sources and the magnitude of changes in the flooding extent in the basin between 1985 and 2010. We used time series analysis of bimonthly NOAA AVHRR and NASA MODIS data and climatologic and hydrologic records to determine the flooding timing and extent. The results indicate that between 12 and 62 % of the basin is flooded on an annual basis and that the spatial extent of the flooding varies throughout the year as a function of the timing of peak discharge in two larger basins. A 30-year trend analysis indicates a consistent decline in the average monthly flooded area in the basin. The results may prove useful in future water utilization feasibility studies, in determining measures for protecting ecological flows and levels, and in ecosystem dynamics studies in the context of current and future climate change and variability.

A spatio-temporal analysis of fire recurrence and extent for semi-arid savanna ecosystems in Southern Africa using moderate-resolution satellite imagery

Savanna ecosystems are semi-arid and fire-prone. Increasing temperatures and decreasing precipitation in Southern Africa will probably have a series of strong impacts on the various components of fire regimes in these ecosystems that will, in turn, affect their ecology, structure, and function. This paper presents a geospatial analysis to quantify changes in fire frequency, seasonality and spatial distribution during the last decade and creates a fire return interval map for the core area of the Kavango-Zambezi Transfrontier Conservation Area, which spans five Southern African countries and is the largest cooperative multistate conservation region in the world. To disentangle the relative contribution of environmental variability from country-specific land management decisions in driving changes in fire regimes, we use two different products from the MODIS Terra platform (Active Fire and Burned Area products), TRMM precipitation data and the Multivariate ENSO Index data to analyze change in fire regimes among the five countries, differentiating between different land uses such as protected areas, forest reserves, and communal lands and accounting for specific changes in fire management policies. There are significant differences in fire frequencies between countries with more effective fire management (Botswana and Zimbabwe) and countries where anthropogenic, mainly early-dry season, burning is largely uncontrolled (Namibia, Angola, and Zambia), both within and outside protected areas, while all countries and land-use units show an overall increasing trend in fire occurrences. Large fire occurrences increased up to 200% in the period before the beginning of the natural fire season in Namibia, where a new prescribed burn policy was introduced in 2006, while the other countries show a slightly different shift in seasonality of increasing fire occurrences mainly during the dry season. The mean size of fires also increases significantly across all land uses despite increasing fire prevention efforts in most protected areas in the five countries. These findings can contribute to more effective transboundary natural resource and wildlife habitat management by providing a baseline assessment of fire return intervals across five countries with different fire management policies and have implications in the climate change arena.

Physiological stress and refuge behavior by African elephants

Physiological stress responses allow individuals to adapt to changes in their status or surroundings, but chronic exposure to stressors could have detrimental effects. Increased stress hormone secretion leads to short-term escape behavior; however, no studies have assessed the potential of longer-term escape behavior, when individuals are in a chronic physiological state. Such refuge behavior is likely to take two forms, where an individual or population restricts its space use patterns spatially (spatial refuge hypothesis), or alters its use of space temporally (temporal refuge hypothesis). We tested the spatial and temporal refuge hypotheses by comparing space use patterns among three African elephant populations maintaining different fecal glucocorticoid metabolite (FGM) concentrations. In support of the spatial refuge hypothesis, the elephant population that maintained elevated FGM concentrations (iSimangaliso) used 20% less of its reserve than did an elephant population with lower FGM concentrations (Pilanesberg) in a reserve of similar size, and 43% less than elephants in the smaller Phinda reserve. We found mixed support for the temporal refuge hypothesis; home range sizes in the iSimangaliso population did not differ by day compared to nighttime, but elephants used areas within their home ranges differently between day and night. Elephants in all three reserves generally selected forest and woodland habitats over grasslands, but elephants in iSimangaliso selected exotic forest plantations over native habitat types. Our findings suggest that chronic stress is associated with restricted space use and altered habitat preferences that resemble a facultative refuge behavioral response. Elephants can maintain elevated FGM levels for ≥6 years following translocation, during which they exhibit refuge behavior that is likely a result of human disturbance and habitat conditions. Wildlife managers planning to translocate animals, or to initiate other management activities that could result in chronic stress responses, should consider the potential for, and consequences of, refuge behavior.

A Biomass Flow Approach to Population Models and Food Webs

The dominant differential equation paradigm for modeling the population dynamics of species interacting in the framework of a food web retains at its core the basic prey-predator and competition models formulation by Alfred J. Lotka (1880-1945) and Vito Volterra (1860-1940) nearly nine decades ago. This paradigm lacks a trophic-level-independent formulation of population growth leading to ambiguities in how to treat populations that are simultaneously both prey and predator. Also, this paradigm does not fundamentally include inertial (i.e. change resisting) processes needed to account for the response of populations to fluctuating resource environments. Here I present an approach that corrects both these deficits and provides a unified framework for accounting for biomass transformation in food webs that include both live and dead components of all species in the system. This biomass transformation formulation (BTW) allows for a unified treatment of webs that include consumers of both live and dead material-both carnivores and carcasivores, herbivores and detritivores-and incorporates scavengers, parasites, and other neglected food web consumption categories in a coherent manner. I trace how BTW is an outgrowth of the metaphysiological growth modeling paradigm and I provide a general compact formulation of BTW in terms of a three-variable differential equation formulation for each species in the food web: viz. live biomass, dead biomass, and a food-intake-related measure called deficit-stress. I then illustrate the application of this new paradigm to provide insights into two-species competition in variable environments and discuss application of BTW to food webs that incorporate parasites and pathogens.