Fluctuations in abundance of large herbivore populations: insights into the influence of dry season rainfall and elephant numbers from long‐term data

Abstract

International audienceIn some African protected areas, elephant populations have reached high densities causing concern about their influence on other forms of biodiversity, and in particular, other large herbivores. This has led to a debate whether management of elephant populations might need to be implemented. Surprisingly, few studies have focused on the influence of elephants on other herbivores and the existing results are conflicting. We investigated such influence in Hwange National Park, Zimbabwe, where elephants were culled and maintained at low density up to 1986, and have more than doubled since then, thus providing the opportunity to explore their possible influence on other herbivore species. Trends of different herbivore populations were obtained using long-term waterhole census data, with these results also being correlated to changes in dry season rainfall, as another fundamental determinant of herbivore population fluctuations. While our study did not allow us to properly disentangle the role of elephants and rainfall, it provided valuable insights. For five out of the 13 herbivore species studied, the population increased between 1973 and 1986 (high dry season rainfall; relatively low elephant abundance), then decreased between 1987 and 1995 (extremely low dry season rainfall; increase in the elephant population), and finally have increased since 1996 (high dry season rainfall; high elephant abundance). These results are consistent with an important role of dry season rainfall on herbivore population dynamics. Additionally, for four other species, the population increased between 1973 and 1986 and has decreased since then. As most herbivore declines occurred simultaneously with increase in the elephant population in the 1987–1995 period, and with the recent increase in most populations happening at a lower rate than the 1973–1985 period, our results suggest the possible existence of a negative influence of high elephant densities on other herbivores. We discuss what the possible underlying mechanisms can be and how to investigate them

Interspecific interference competition at the resource patch scale: do large herbivores spatially avoid elephants while accessing water?

“…) and where most of the other herbivore species have shown declines in their populations (Valeix et al . ) rising concerns about the impact of elephants on other herbivore species. Our work shows that while there seem to be some costs associated to cryptic interference competition with elephants, these costs are not high enough to prevent other herbivores from accessing high‐quality resources at the peak of the dry season when water requirements are crucial.…”

Elephant-mediated habitat modifications and changes in herbivore species assemblages in Sabi Sand, South Africa

“…According to Fritz et al (2002) and Valeix et al (2008), elephants may compete with mesomixed feeders (such as impala), depending on the abundance and quality of the forage. However, Sinclair et al (2003) showed that predation is a factor defining herbivore densities on African savannas, particularly for small herbivores (<150 kg).…”

“…Fritz et al (2002) concluded that elephants compete with the smaller mesobrowser species for browse resources, as the relative contribution of these species declined with increasing elephant densities. Valeix et al (2008) also reported a possible negative influence of elephants on other herbivore species. Elephant impact, especially on the larger trees, could also have negative consequences for large browsing species such as giraffe (Fritz et al 2002).…”

“…The rainy season is from October to April and the region's mean annual precipitation calculated from July in the preceding year to June in the current year averaged 615 mm (1990, three rainfall gauges at Gowrie, Shaws and Newington with monthly data cover the entire study . The mean total dry season rainfall, which has been correlated with elephant population growth and densities of other herbivore species (Ogutu and Owen-Smith 2003;Valeix et al 2008), was calculated from May to September, averaging 38 mm (SD=24) over the study period. The complex is characterized by an Acacia and Combretum woodland, and more than 50 % of all trees belong to only 8 species (in order of abundance): Combretum apiculatum, Dalbergia melanoxylon, Acacia exuvialis, Dichrostachys cinerea, Acacia nigrescens, Terminalia sericea, Ormocarpum trichocarpum, Albizia harveyi.…”

Influence of increasing dryness, animal feeding strategy and human hunting on large ungulates abundance: A first approach in West Africa

“…Several studies try to demonstrate that populations of animals-from herbivores to top predators, verte brates and invertebrates-are limited by their food, and that the availability of this food is correlated with the weather (Ottichilo et al, 2001;Ogutu et al, 2003;Dunham et al, 2004;White, 2008;Valeix et al, 2008). Estes et al (2006) point out that research in this area is urgently needed to increase understanding of savana complex ecosystems.…”