Shrinking sable antelope numbers in Kruger National Park: what is suppressing population recovery?

Abstract

Sable antelope numbers in the Kruger National Park have declined substantially since the mid-1980s and have shown little recovery despite improved rainfall conditions. We used aerial survey records to investigate how changes in herd numbers, herd sizes, calf proportions and consequent changes in the distribution range of breeding herds contributed to this situation. Both herd sizes and herd numbers decreased in the drier northern half of the park, coupled with low calf proportions, especially during and following a severe drought, while declines in these measures became evident later in the wetter southern half. Herd extirpations led to a 25% contraction in the local distribution range of sable antelope, although some of these herds had only become established during the high rainfall period in the late 1970s. Local population trends were not related to increases in zebra numbers potentially attracting more predators. Compensatory density dependence in the population growth rate was no longer evident after 1986 during the period of the population decline. The population trajectory modelled from rainfall relationships indicated that the depressed population growth rate relative to rainfall apparent after 1986 has persisted, implicating either enduring habitat degradation or continuing high predation pressure. The pattern of the population decline and subsequent lack of recovery raises the possibility that an Allee effect could be operating, mediated through the consequences of reduced herd sizes for exposure to predation. Our findings suggest how delays in responding to declining population numbers could prejudice population recovery well before the overall population size drops to levels threatening population viability. bs_bs_banner Animal Conservation. Print ISSN 1367-9430 Animal Conservation 15 (2012) 195-204 Shrinking sable antelope numbers N. Owen-Smith et al.

Roan antelope Hippotragus equinus in Africa: a review of abundance, threats and ecology

“…The continued inability of sable antelope populations to recover may be due to an Allee affect, where reduced herd vigilance causes increased juvenile mortality from predation and declining population numbers (Owen‐Smith et al. ). A similar Allee effect may be causal in the roan antelope population's inability to recover.…”

“…Continuous pressure on reproductive success could result in an Allee effect caused by decreased individual fitness as a result of decreased herd vigilance due to small population size (Stephens & Sutherland , Owen‐Smith et al. ).…”

Habitat differences do not explain population declines of sable antelope in an African savanna

“…Furthermore, while the habitat assessments provided by the remotely sensed imagery available to us represented only the woody component of the vegetation structure, other studies indicate that the grass species favored by sables remain available (Owen‐Smith et al ., ). Our findings leave heightened predation as the only tenable causal mechanism of the sable population decline (Owen‐Smith & Mills, ; Owen‐Smith et al ., ). Although artificial water sources established in the Pretoriuskop region were represented by merely three dams, the number of buffalo, Syncerus caffer , counted here increased substantially between 1980 and 1987 (South African National Parks, unpublished data), perhaps attracting more lions and hence elevating predation on sables (Owen‐Smith et al ., ).…”

“…Hence, locally deteriorating habitat appears not to be the determinant of herd disappearances. Recolonization has not occurred perhaps because the remaining herds are small in size, possibly as a result of past predation (Owen‐Smith et al ., ), meaning that there is little pressure for these animals to expand their ranges or form breakaway groups.…”

Drivers of population dynamics in sable antelope: forage, habitat or competition?

“…Evidence from Kruger suggests a likely effect of phase on the relationships between population growth and environmental covariates (Owen-Smith and Mills 2006;Owen-Smith et al 2012). All continuous covariates were standardized by subtracting their mean and dividing by their standard deviation.…”