Dynamic spatial partitioning and coexistence among tall grass grazers in an African savanna

Abstract

Competitive relationships among mobile animals may be expressed through dynamically changing spatial relationships over different time frames. Less common species that are apparently inferior competitors may be able to coexist with more abundant species by concentrating in regions of the landscape little utilized by the former at spatio-temporal scales from annual or seasonal ranges to the specific foraging localities exploited at different stages of the annual cycle. Spatial relationships may be influenced further by dependencies on other resources, predation risks and facilitatory interactions under certain conditions. Our study aimed to determine whether competition with more abundant zebra and buffalo restricted the abundance of sable antelope in a region where these three tall-grass grazers overlapped in their herd distributions. We tracked the simultaneous movements of animals representing herds of these species over two dry seasons and one wet season using GPS-GSM collars, and estimated seasonal or monthly range extents and their overlap. We also compared daily separation distances between these animals against the null pattern expected if their movements had been independent, and assessed how prior grazing by buffalo influenced the subsequent use of these localities by sable. The range of the sable herd was mostly separated from the seasonal range of the buffalo herd during the late dry season of 2006 and throughout the dry season of 2007. Seasonal home ranges of zebra herds overlapped partially with the range of the sable herd during most of the year. Even during times when their ranges overlapped, sable were rarely recorded within  1 km of the buffalo herd. Prior grazing by buffalo beyond a threshold level inhibited later use of these localities by sable, but the sable were nevertheless able to exploit places that were little utilized by buffalo at that time. Sable were less able to evade overlap with the small, mobile zebra herds, and hence more vulnerable to competitive exclusion by zebra than by buffalo. Our findings demonstrate how less abundant species can restrict competition from more abundant competitors through dynamic spatial partitioning in regions where their home ranges overlap.

Postrelease monitoring habitat selection by reintroduced burchell's zebra and blue wildebeest in southern Mozambique

“…Freshly grazed grass was identified by the lighter, more vivid coloration of the broken leaves and stems in comparison with older grazing (Macandza, Owen-Smith, & Cain, 2012). In these, plots were searched for freshly grazed grass to examine if the herds were grazing or not.…”

Mechanisms of coexistence in diverse herbivore–carnivore assemblages: demographic, temporal and spatial heterogeneities affecting prey vulnerability

“…When the expanded distribution of artificial water points drew more zebra into the northern region favoured by the rare antelope, populations of the latter went into decline (Harrington et al , Owen‐Smith and Mills ). These rare antelope seem inherently less agile at evading lion attacks, and also occupy habitats were there is much cover for stalking lions (Macandza et al ). Similarly, an increase in zebra on ranches in the Laikipia region of Kenya led to increased predation on hartebeest and other less common herbivore species (Georgiadis et al ).…”

Spatially nested niche partitioning between syntopic grazers at foraging arena scale within overlapping home ranges

“…Sable herds similarly restrict their foraging to localized arenas located within the annual ranges of more mobile zebra and buffalo herds (Owen-Smith and Martin 2015). Sable herds dynamically avoided close proximity to buffalo herds, but not zebra herds, but possibly due to avoidance of a shared predator rather than competitive displacement for resources (Macandza et al 2012b). Moreover, the regional distribution of sable antelope (Hippotragus niger) herds within KNP appears to be governed primarily by how the abundance of wildebeest and impala affects the risk of predation (Chirima et al 2013).…”