Large herbivore assemblages in a changing climate: incorporating water dependence and thermoregulation.

The coexistence of different species of large herbivores (ungulates) in grasslands and savannas has fascinated ecologists for decades. However, changes in climate, land-use and trophic structure of ecosystems increasingly jeopardise the persistence of such diverse assemblages. Body size has been used successfully to explain ungulate niche differentiation with regard to food requirements and predation sensitivity. But this single trait axis insufficiently captures interspecific differences in water requirements and thermoregulatory capacity and thus sensitivity to climate change. Here, we develop a two-dimensional trait space of body size and minimum dung moisture content that characterises the combined food and water requirements of large herbivores. From this, we predict that increased spatial homogeneity in water availability in drylands reduces the number of ungulate species that will coexist. But we also predict that extreme droughts will cause the larger, water-dependent grazers as wildebeest, zebra and buffalo-dominant species in savanna ecosystems - to be replaced by smaller, less water-dependent species. Subsequently, we explore how other constraints such as predation risk and thermoregulation are connected to this two-dimensional framework. Our novel framework integrates multiple simultaneous stressors for herbivores and yields an extensive set of testable hypotheses about the expected changes in large herbivore community composition following climate change

Response of different-sized herbivores to fire history

Retrieve original file from: http://edepot.wur.nl/121801 High herbivore densities and re-occurring fires are natural phenomenons that determine the structure and functioning of African savannas. Traditional burning practices have been intensified over the past years due to increased herbivore numbers. Insight how animals respond to fire is important to understand ecosystem functioning and for the use of fire as management tool. Until now studies have centred mainly on spatial relationships between herbivores and fires. Very little information exists on effects of temporal fire patterns on herbivore communities. To assess the effect of fire succession on herbivore species greatly differing in body mass we conducted experiments and observational studies on smaller scales and landscape scales. We investigated the response of herbivores on both short-term and long-term succession patterns. Rodent densities were affected by the presence of larger herbivores rather than by seasonal patterns and increased with the absence of large herbivores and increasing vegetation height. Both middle-sized and large herbivore species responded on short-term post-fire patterns with large species being the pioneer species on burnt patches, whereas buffalo was the only species that also responded on long-term post-fire patterns. Our results indicate that larger herbivores in interaction with fire influence rodents by both increasing and prolonging the predation risk they experience in these habitats. Furthermore grazer presence and the amount of rainfall may be two interacting factors that prolong fire patterns in the landscape. Particularly buffalo responded to fire patterns, most likely due to the higher intake levels of digestible plant material and increased forage efficiency. Herbivore response to fire histor

Determinants of patchiness of woody vegetation in an African savanna

Questions: How is woody vegetation patchiness affected by rainfall, fire and large herbivore biomass? Can we predict woody patchiness and cover over large-scale environmental gradients? Location: Hluhluwe-iMfolozi Park, South Africa. Methods: We quantified variation in local patchiness as the lacunarity of woody cover on satellite-derived images. Using Random Forest regression we analysed how both average woody cover and its patchiness depend on annual rainfall, fire frequency and grazer and browser metabolic biomass densities. Results: Fire frequency and rainfall were the clearest predictors, whereas effects of large herbivores on woody vegetation were smaller and more complex. Under low rainfall conditions (500 mm·y

Evaluating herbivore management outcomes and associated vegetation impacts

African savannas are characterised by temporal and spatial fluxes that are linked to fluxes in herbivore populations and vegetation structure and composition. We need to be concerned about these fluxes only when management actions cause the system to shift towards a less desired state. Large herbivores are a key attribute of African savannas and are important for tourism and biodiversity. Large protected areas such as the Kruger National Park (KNP) manage for high biodiversity as the desired state, whilst private protected areas, such as those adjacent to the KNP, generally manage for high income. Biodiversity, sustainability and economic indicators are thus required to flag thresholds of potential concern (TPCs) that may result in a particular set of objectives not being achieved. In large conservation areas such as the KNP, vegetation changes that result from herbivore impact, or lack thereof, affect biodiversity and TPCs are used to indicate unacceptable change leading to a possible loss of biodiversity; in private protected areas the loss of large herbivores is seen as an important indicator of economic loss. Therefore, the first-level indicators aim to evaluate the forage available to sustain grazers without deleteriously affecting the vegetation composition, structure and basal cover. Various approaches to monitoring for these indicators were considered and the importance of the selection of sites that are representative of the intensity of herbivore use is emphasised. The most crucial step in the adaptive management process is the feedback of information to inform management decisions and enable learning. Feedback loops tend to be more efficient where the organisation’s vision is focused on, for example, economic gain, than in larger protected areas, such as the KNP, where the vision to conserve biodiversity is broader and more complex. Conservation implications: In rangeland, optimising herbivore numbers to achieve the management objectives without causing unacceptable or irreversible change in the vegetation is challenging. This manuscript explores different avenues to evaluate herbivore impact and the outcomes of management approaches that may affect vegetation