This article also exists in French ("Alors, c 'est qui le plus fort : le rhinocéros ou l'éléphant ?"), translated by Timothée Froelich.
Black rhinoceros are endangered species and victims of poachers, but many factors must be considered to preserve them. A new publication in PLOS One [available on MyScienceWork] shows that we also need to consider elephants’ presence before reintroducing rhinoceros into an area. For the first time, researchers have proved that the two compete for food.
Rhinoceros vs. Elephant
Megaherbivores dominate the biomass due to their great ability to digest even low quality food and they eat a majority of the available resources. Between two of them, African elephants and black rhinoceros, no one had proved, until now, that there was competition for food. Based on early results obtained during her PhD studies, Marietjie Landman from the Centre for African Conservation Ecology at the Nelson Mandela Metropolitan University had “the feeling that there was competition but didn’t know how to prove it.” One line of evidence, already known, was that the two species are aggressive toward each other and elephants may kill rhinoceros. Elephant: 1, Rhino: 0. After investigating the two species’ diets over three years, the researchers found a new element to support the idea of competition: rhinoceros shift their diet in the presence or absence of elephants.
515 rhinoceros were killed by poachers since January in South Africa.
One possible explanation could have been that, due to evolutionary pressures in the past, elephants and rhinos adapted their feeding behavior long ago, the authors acknowledge. However, the current study was based on two different sites: one where the two species cohabitate, and one where no elephants were present. The fact that the black rhino diet is not stable, but changes between the two sites, shows that the competition is affecting their behavior today.
Reading poop like a crystal ball
By collecting fresh fecal samples, the researchers were able to determine what kind of vegetation elephants and rhinoceros eat. First, they dried the excrement in an oven, then ground the coarse material (thanks for the details!) and analyzed the resulting 2,500 fragments of epidermis, the exterior layer of plants. These fragments are like fingerprints that are compared to reference samples of local vegetation. The result was clear: rhinoceros shift their diet in the presence of elephants. Considered strict browsers, rhinos do not usually eat grass, preferring things like woody shrubs and flowering plants. This was the case where the animals were isolated from elephants. But, surprisingly, with elephants in their neighborhood, rhinoceros eat much more grass.
An important result for preserving biodiversity
Elephants consume most of the resources in their environment and have a great impact on the ecosystem. They can eat around 300 kg of biomass per day and can forage up to 8 meters above the ground. “Because of their size they have a large impact on spatial distribution of vegetation, and so, on biodiversity,” explains Mariejie Landman. “This data [regarding competition between rhinos and elephants] shows we need to be careful when managing these populations.” In small reserves, the impact of elephants increases, and rhinoceros should not be reintroduced too close to their giant cousins.
Elephant effects on the woody community are dramatic and significant declines in species richness, density and biomass have been recorded.” Shift in Black Rhinoceros Diet in the presence of Elephant: evidence for Competition?, PLOS ONE, July 2013.
Credit: Flickr/Abir Anwar
At the same time, elephants are not to be blamed for challenges facing black rhino conservation. Not all of their effects are negative: elephants are a natural component of the system. They may change a woody habitat to a grassy habitat, for instance. Mariejie Landman explains that “some of their effects actually increase the amount of food available to rhinos, so elephants are probably essential to black rhino populations. It’s a dual effect. The key is to manage this.”
Reference: Landman M., et al., (2013). Shift in Black Rhinoceros Diet in the Presence of Elephant: Evidence for Competition?, PLOS ONE., DOI: 10.1371/journal.pone.0069771