Jason P. Marshal

Population dynamics and vital rates of savanna large herbivores, such as the black (Diceros bicornis) and white rhinoceros (Ceratotherium simum), are influenced by intrinsic and extrinsic factors, among which environmental fluctuations play an important role as they affect food resources. Given the differences in diets between the two rhino species and the differences in relationships between rainfall and woody versus herbaceous plants, there should also be differences in their survival and other vital rates in response to environmental stochasticity. We used historical rhino-monitoring, rainfall and vegetation greenness data to investigate rhino survival and other vital rates of two rhino populations at Lapalala Wilderness Nature Reserve. Mean population growth rates during the study were 4.8% for black rhinos and 8.9% for white rhinos, and both populations illustrated no evidence of density-dependence. Cox proportional hazard models showed that survival of both species was stage-specific and increased with rainfall. Vegetation greenness models similarly showed that survival increased with integrated annual greenness for all stages and proved to be the better proxy for white rhino survival. Additionally, density effects were somewhat trivial for both black and white rhinos as the populations seem to be rather resource limited. With droughts expected to be more common in an already variable system, species-specific assessment of how environmental stochasticity affects vital rates will be necessary for effective conservation of species in semi-arid regions.

Costs of reproduction and reproductive success depend on the timing of reproduction with favourable environmental conditions. For species like the black and white rhinoceros, which live in seasonal environments, changing climate might desynchronize reproduction and periods of favourable conditions. Additionally, fluctuations in environmental conditions can affect calf sex ratios. We thus investigated the reproductive life histories of black and white rhinoceroses at Lapalala Wilderness Nature Reserve, and their response to variations in rainfall, vegetation greenness and population density. We used historical rhino-monitoring data to assess age at primiparity, inter-calving intervals and offspring sex ratios. White rhinos had their first calf earlier than black rhinos (9.4 versus 11.7 years, respectively) but the inter-calving intervals of white rhino were longer than those of black rhino, averaging 38 and 33 months, respectively. Conceptions for both species peaked during the wet season and were associated with periods of high forage availability. Effects of greenness on age at primiparity differed between species, being positive for white rhinos and negative for black rhinos. We suggest that increases in greenness during conception periods might indicate increased grass production which the black rhinos do not have access to and might not consume. The birth sex ratio was male-biased for both species; however, there was evidence for an environmental effect on calf sex allocation. In particular, black rhinos favoured male calves during periods of decreased vegetation greenness. Although reproduction for both species appears to be related to resources, observed differences are possibly influenced at least in part by differences in the dynamics of plant growth forms that each consumes and the responses of each rhino to those dynamics.

Extreme weather events perturb ecosystems and increasingly threaten biodiversity¹. Ecologists emphasize the need to forecast and mitigate the impacts of these events, which requires knowledge of how risk is distributed among species and environments. However, the scale and unpredictability of extreme events complicate risk assessment^1,2,3,4—especially for large animals (megafauna), which are ecologically important and disproportionately threatened but are wide-ranging and difficult to monitor⁵. Traits such as body size, dispersal ability and habitat affiliation are hypothesized to determine the vulnerability of animals to natural hazards^1,6,7. Yet it has rarely been possible to test these hypotheses or, more generally, to link the short-term and long-term ecological effects of weather-related disturbance^8,9. Here we show how large herbivores and carnivores in Mozambique responded to Intense Tropical Cyclone Idai, the deadliest storm on record in Africa, across scales ranging from individual decisions in the hours after landfall to changes in community composition nearly 2 years later. Animals responded behaviourally to rising floodwaters by moving upslope and shifting their diets. Body size and habitat association independently predicted population-level impacts: five of the smallest and most lowland-affiliated herbivore species declined by an average of 28% in the 20 months after landfall, while four of the largest and most upland-affiliated species increased by an average of 26%. We attribute the sensitivity of small-bodied species to their limited mobility and physiological constraints, which restricted their ability to avoid the flood and endure subsequent reductions in the quantity and quality of food. Our results identify general traits that govern animal responses to severe weather, which may help to inform wildlife conservation in a volatile climate.

Droughts have become more prevalent in recent years, causing problems for large herbivores such as the black (Diceros bicornis) and white (Ceratotherium simum) rhinoceros, whose populations are influenced by variation in climatic conditions. This variation acts primarily through the effects of rainfall on forage availability but acts differently on browse or grass. Consequently, browsing black rhinos and grazing white rhinos might show differing responses to changes in rainfall. Moreover, young large herbivores have a more limited capacity to survive environmental stressors than do adult animals that are more resilient to environmental variation. Thus, our aim was to investigate the relationship between rainfall and the survival of young (<3 years old) black and white rhinos at Lapalala Wilderness Nature Reserve, South Africa. Using stratified Cox models, we found that both species had higher survival with more rain, but the effect was more similar than expected given the differences in diets. Moreover, there was evidence of an effect of age class for white rhinos but not for black rhinos. Additionally, most rhinos that died did so during the dry season when scarce forage and limited fat reserves likely constrained their ability to tolerate periodically low-quality diets. Despite the effect of rainfall operating through substantially different plant growth forms, young animals of both rhino species showed sensitivity to variation in rainfall that could ultimately affect population abundance and conservation status.

Geographic gradients in plant productivity can present foraging large herbivores with varying resource levels across the range of a population, to which individuals can demonstrate varying intensity of selection. To investigate individually-specific responses of nyala antelopes (Tragelaphus angasii ) to vegetation greenness, we monitored the landscape use of eight telemetry-collared adult females, half at the wetter end and half at the drier end of Hluhluwe–iMfolozi Park, South Africa. We used remotely-sensed greenness as an index for plant productivity and data on canopy structure, and we compared the strength of selection for greenness between animals having different greenness averages within their home ranges. We found a decreasing pattern between home-range average greenness and strength of selection, which depended on season and presence of tree canopies. Animals in low-greenness home ranges tended to have stronger positive selection in closed-canopy vegetation in the dry season; those in high-greenness home ranges tended to have the stronger negative selection in open-canopy vegetation in the wet season. Our study emphasizes the possibility that a spatial resource gradient on a landscape might provide a valuable model of temporal change in a resource and an indication of potential for foraging animals to adapt to those changes.

The coexistence of a large herbivore community depends on the degree of overlap in the resource used by the different species composing it. We investigated the role of grass greenness, height, biom...

Across southern Africa, many populations of the highly-mobile common eland (Tragelaphus oryx) are limited to small, fenced protected areas. We studied such a population at Kgaswane Mountain Reserve (KMR), South Africa, using radio-collared animals to better understand eland range use in a landscape with restricted movement. Placement of home ranges within the reserve changed from favouring woodland to dry grassland after the beginning of the wet season, while wet grassland was favoured all year. Habitat use within home ranges indicated an interaction between vegetation types and elevation that changed with season: low-elevation selection for dry grassland, open shrubland and woodland changed to selection for wet and dry grassland; high-elevation selection for open shrubland alone changed to selection for both open shrubland and woodland. Despite the fenced nature of KMR, eland appeared to have sufficiently flexible behaviour to follow landscape use and seasonal home range changes demonstrated by eland in less restrictive environments. Management activities that promote fine-scale heterogeneity and varying levels of woody cover might compensate for lack of mobility where fences limit broad-scale movements.