Shift in black rhinoceros diet in the presence of elephant: evidence for competition? - PubMed (original) (raw)

Shift in black rhinoceros diet in the presence of elephant: evidence for competition?

Marietjie Landman et al. PLoS One. 2013.

Abstract

In African large herbivore assemblages, megaherbivores dominate the biomass and utilise the greatest share of available resources. Consequently, they are considered a separate trophic guild that structures the food niches of coexisting large herbivores. However, there exists little empirical evidence on how food resources are shared within this guild, and none for direct competition for food between megaherbivores. Using the histological analysis of faeces, we explore this phenomenon for African elephant Loxodonta africana and black rhinoceros Diceros bicornis in the Addo Elephant National Park, South Africa, where the accumulated impacts of elephant have reduced browse availability. Despite being unable to generalise beyond our study sites, our observations support the predictions of competition theory (as opposed to optimality theory) by showing (1) a clear seasonal separation in resource use between these megaherbivores that increased as resource availability declined, and (2) rhinoceros changed their selectivity in the absence of elephant (using an adjacent site) by expanding and shifting their diet along the grass-browse continuum, and in relation to availability. Although black rhinoceros are generally considered strict browsers, the most significant shift in diet occurred as rhinoceros increased their preferences for grasses in the presence of elephant. We speculate that the lack of specialised grazing adaptations may increase foraging costs in rhinoceros, through reduced harvest- and handling-efficiencies of grasses. In the short-term, this may be off-set by an enhanced tolerance for low quality food and by seasonally mobilising fat reserves; however, the long-term fitness consequences require further study. Our data suggest that managing elephant at high densities may compromise the foraging opportunities of coexisting browsers. This may be particularly important in small, fenced areas and overlapping preferred habitats where impacts intensify.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Non-metric Multidimensional Scaling ordinations of principal dietary items identified in the diet of elephant (E) and black rhinoceros (R) in the Addo Main Camp section.

ANOSIM (Analysis of Similarity) R values indicate the degree of diet separation across seasons; values approaching unity indicate clear separation.

Figure 2

Figure 2. Seasonal diet, grouped into broad growth form categories (mean ± SD), of elephant (shaded bars) and black rhinoceros (clear bars) in the Addo Main Camp section.

Figure 3

Figure 3. Normalized diet, grouped into broad growth form categories (mean ± 95% confidence interval), of elephant and black rhinoceros in the Addo Elephant National Park.

Positive values (i.e. use>availability) with lower confidence limits greater than zero indicate preference; negative values (use

<availability) indicate="" avoidance;="" at="" zero,="" use =" availability.</div"> </availability)>

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Grants and funding

The International Fund for Animal Welfare, Centre for the Reproduction of Endangered Species (San Diego), National Research Foundation and Nelson Mandela Metropolitan University provided financial support. Mazda Wildlife Fund provided field transportation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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