Ungulates are famous for large-scale movements in response to local and regional changes in plant productivity. However, it is poorly understood how non-migratory ungulates respond towards shortages in plant availability. In the arid Kunene Region of Namibia, we studied the variation of home ranges and habitat use of gemsbok during a period of severe drought that led to a critical food shortage. We then related the spatiotemporal behaviour of gemsbok to relative plant greenness and local plant productivity, using local Normalized Differenced Vegetation Indices (NDVI) as a proxy, and to satellite images to assess habitat types and topography. Core home range sizes of gemsbok did not vary with decreasing local NDVI values, while total home-ranges increased with decreasing NDVI values. Gemsbok responded to resource fluctuations by using a combination of habitats differing in plant productivity, topography and vegetation cover. Gemsbok increased the number of habitats used when NDVI values decreased. Individuals remained non-migratory even during the period of peak drought. Non-migratory gemsboks are capable of exploiting local resources efficiently by adjusting their habitat use, when facing declining resources and unpredictable food shortages. Our study provides important information on the habitat use of gemsbok for the sustainable management of populations in arid environments.

Ungulates often adjust their diet when food availability varies over time. However, it is poorly understood when and to what extent individuals change their diet and, if they do so, if all individuals of a population occupy distinct or similar dietary niches. In the arid Namibian Kunene Region, we studied temporal variations of individual niches in grazing gemsbok (Oryx gazella gazella) and predominantly browsing springbok (Antidorcas marsupialis). We used variation in stable C and N isotope ratios of tail hair increments as proxies to estimate individual isotopic dietary niches and their temporal plasticity. Isotopic dietary niches of populations of the two species were mutually exclusive, but similar in breadth. Isotopic niche breadth of gemsbok was better explained by within-individual variation than by between-individual variation of stable isotope ratios, indicating that gemsbok individuals were facultative specialists in using isotopically distinct local food resources. In contrast, inter- and intra-individual variations contributed similarly to the isotopic niche breadth of the springbok population, suggesting a higher degree of individual isotopic segregation in a more generalist ungulate. In both species, between-individual variation was neither explained by changes in plant primary productivity, sex, geographical position nor by group size. Within species, individual dietary niches overlapped partially, suggesting that both populations included individuals with distinct isotopic dietary niches. Our study provides the first evidence for isotopic dietary niche segregation in individuals of two distinct desert ungulates. Similar, yet isotopically distinct dietary niches of individuals may facilitate partitioning of food resources and thus individual survival in desert ecosystems.