Mapping and assessing the impact of small‐scale ephemeral water sources on wildlife in an African seasonal savannah

Abstract

In many savannah regions of Africa, pronounced seasonal variability in rainfall results in wildlife being restricted to floodplains and other habitats adjacent to permanent surface water in the dry season. During the wet season, rainfall fills small-scale, ephemeral water sources that allow wildlife to exploit forage and other resources far from permanent surface water. These water sources remain difficult to quantify, however, due to their small and ephemeral nature, and as a result are rarely included in quantitative studies of wildlife distribution, abundance, and movement. Our goal was to map ephemeral water in Bwabwata National Park in Namibia using two different approaches and to relate measures of ephemeral water to the abundance, distribution, and movement of two large wildlife species. We used high-resolution Google Earth and Esri World imagery to visually identify waterholes. Additionally, we used Sentinel-2 satellite imagery to map ephemeral water across the study area using the Normalized Difference Water Index. With these mapped waterhole layers and data from GPS-collared individuals of African elephant (Loxodonta africana) and African buffalo (Syncerus caffer), we evaluated the importance of ephemeral water in conditioning abundance and movement of these two species. The two approaches to mapping ephemeral water resulted in the visual identification of nearly 10,000 waterholes, and a predicted ephemeral water layer of~76% accuracy. The inclusion of ephemeral water into models of abundance and movement resulted in improved goodness of fit relative to those without water, and water impacts on abundance and movement were among the strongest of all variables considered. The potential importance of ephemeral water in conditioning the movements and distributions of large herbivores in African savannahs has been difficult to quantify relative to vegetation drivers. Our results suggest research into ephemeral water impacts deserves more attention.

COMBINED USE OF REMOTE SENSING AND SPATIAL MODELLING: WHEN SURFACE WATER IMPACTS BUFFALO (SYNCERUS CAFFER CAFFER) MOVEMENTS IN SAVANNA ENVIRONMENTS

“…However, increase availability of free medium-resolution satellite sensors such as Sentinel-2 provides potentialities to characterize, via supervised classification of combined MNDWI and NDWI indices, surface water presence and dynamics at landscape scale (Du et al, 2016). Even if most studies focusing on buffalo movements only use in-situ observations of surface water (Zvidzai et al, 2013), SRS is increasingly used (Naidoo et al, 2020) and can be a valuable asset in areas that are difficult to access and where it is almost impossible to collect in-situ data. The surface water classification methodology developed in this study is efficient (c.f.…”

“…This is particularly true in the current context of increasing number and variety of SRS sensors (Paganini et al, 2018). For example, several studies have been combining GPS telemetry data with SRS in savanna environments to investigate the relationship between resource gradients and overlap between wild and domestic herbivores (Zengeya et al, 2015) or to assess the impact of small-scale ephemeral water sources on wildlife (Naidoo et al, 2020), greatly expanding our understanding of ecological functioning in relation to animal movement as a result. Since 2015, Sentinel-2 satellites provide 10m spatial resolution SRS images with a revisit frequency of 5 days that can potentially be combined with GPS telemetry data to conduct landscape scale ecological analysis.…”

Remote Sensing of Environmental Drivers Influencing the Movement Ecology of Sympatric Wild and Domestic Ungulates in Semi-Arid Savannas, a Review

“…So far, water spectral indexes in combination with supervised classification have hardly been used in direct relation with buffalo and cattle movements, although their potential within this framework have already been stressed [212]. Recently, Naidoo et al (2020) used the NDWI calculated from Sentinel-2 images to detect ephemeral water source in relation with buffalo and elephant movements in Namibia [91]. However, most of the reviewed studies integrating water into their analysis only used on-site observations of surface water [88,147,167] and natural or artificial waterholes [17,[213][214][215][216].…”

“…However, the numerous remote sensing-based water products presented in Table 5 have not been used in the different buffalo and cattle movement reviewed studies. Similarly, water spectral indexes listed in Table 4 and SAR images (Table 1), with the exception of one study that used NDWI derived from Sentinel-2 images in relation with buffalo movements [91], have not been used despite their potential to improve classification algorithms and water detection in savanna environments [177,203,204,207,212]. This may partly result from a lack of knowledge about the existence and availability of SRS products in the movement ecology community, a major gap that this review aims to fill.…”

African elephants can detect water from natural and artificial sources via olfactory cues

“…For instance, large mammalian herbivores (hereafter, large herbivores) living in arid environments are challenged by the limited distribution of water sources across the landscape. Permanent water bodies are rare, and the many ephemeral water sources such as pans, streams, shallow pools, and puddles dry up as the dry season progresses (Chamaillé-Jammes et al 2007;Naidoo et al 2020;Redfern et al 2005). Many species in these regions have evolved behavioural, physiological, and morphological mechanisms to reduce water loss or obtain more water from food (see Cain et al 2006;Kihwele et al 2020 for details).…”