Does competition or facilitation regulate migrant ungulate populations in the Serengeti? A test of hypotheses

Abstract

Interspecific competition and facilitation have both been proposed as processes promoting species separation and co-existence in African ungulates. In one group of grazers on the Serengeti plains, comprising wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni), these processes have also been suggested to regulate the populations. Censuses of these populations over 20 years have shown changes that allow a test of which, if either, process regulates population numbers. Wildebeest numbers have levelled off as a result of intraspecific competition for food following a five-fold increase due to release from disease and an increase in food supply. Zebra numbers have remained stationary throughout the same period. Gazelle numbers have declined in the last 10 years. These results are contrary to the facilitation hypothesis, which predicts that wildebeest numbers should not have increased if those of zebra did not, and that gazelle numbers should have increased since the wildebeest population increased. The gazelle results are consistent with the interspecific competition hypothesis, but the zebra results are contrary to it. We propose an alternative hypothesis that predation regulates the zebra population, and we suggest ways of testing this.

The spatial distribution of African savannah herbivores: species associations and habitat occupancy in a landscape context

“…In the first category, large-bodied herbivores such as zebras and wildebeest may facilitate consumption by smaller bodied herbivores such as Thomson's gazelle, topi and hartebeest [45,63]. Others have rejected this hypothesis using population [64] and habitat overlap data [65]. In our study, the strong spatial association between species that are unlikely to elicit facilitation, such as between impala and topi, for example, also suggest that an explanation rooted in purely facilitation cannot account for our results.…”

Fire history and management as determinant of patch selection by foraging herbivores in western Serengeti, Tanzania

“…There are 27 species of large mammal herbivores in the Serengeti ecosystem (Sinclair and Norton-Griffiths 1982), of which wildebeest, zebra, Thomson's gazelle, buffalo (Syncerus Figure 1. Position of four wildlife protected areas in the Serengeti-Mara ecosystem that formed study area: Serengeti National Park (SNP), Ngorongro Conservation Area (NCA), Ikorongo Game Reserve (IGR) and Gurumenti Game Reserve (GGR), and nine areas in which animals were counted.…”

Winning by a Neck: Tall Giraffes Avoid Competing with Shorter Browsers

“…In this system, the larger species (buffalo Syncerus caffer, zebra Equus burchelli, wildebeest Connochaetes taurinus) consume less leaf and more stem than the smaller-bodied species (Grant's and Thomson's gazelles, Gazella granti and Gazella thomsonii), which results in a grazing succession, as described for the Serengeti Plains of East Africa (Gwynne and Bell 1968;Bell 1970Bell , 1971. It was initially hypothesized that the larger species facilitated the grazing opportunities of the smaller species (Bell 1970(Bell , 1971, but long-term data did not support the facilitation hypothesis (Sinclair and Norton-Griffiths 1982). Rather, the smaller grazers are more specialized feeders, using their narrower muzzles to deplete individual grass tufts of the highest-quality parts and thereby reducing the overall quality of the sward for the larger generalist grazers Gordon 1987, 1992;Murray and Illius 2000).…”