Many large plant-eating mammals have evolved to live in multispecies assemblages, with species competing for food and other resources. Through domestication and animal husbandry, however, humans have enabled a few species of livestock, such as cattle, to dominate such assemblages. One standard practice in livestock production on rangelands, espoused by commercial ranchers and subsistence pastoralists alike, is the eradication of large, indigenous herbivores that are believed to compete with livestock for food. These eradication efforts have increasingly problematic implications for biodiversity conservation (1). So it is timely that on page 1753 of this issue, Odadi et al. (2) report on a relatively simple experiment that tested the assumption that cattle and wildlife compete for food. Their study, conducted in an East African savanna renowned for its large herbivore diversity, revealed that cattle do compete with herbivores such as zebras and gazelles during the dry season, when food quantity is low. In contrast, during the wet season, when food quantity is high, grazing by wildlife benefits cattle by improving the quality of forage. The findings highlight ecological processes that promote coexistence among large herbivores in grasslands and savannas, and hence could be useful for conservation.Large herbivores (>5 kg) generally belong to either a grazing guild (eating mostly grass) or a browsing guild (eating mostly foliage on trees and shrubs); a few are “mixed feeders” that alternate in response to seasonal changes in food plants (3). This grazer-browser dichotomy is a key factor promoting resource partitioning, with coexisting herbivores feeding on different plants or plant parts in the same area (4). In addition, coexisting species within each guild often differ in body size and/or digestive morphology; the result is that they also differ in feeding behavior and in the range of diet quality they can tolerate (see the figure) (4–6). For example, in the African savanna and other ecosystems characterized by a patchwork of ungulate habitats (7), smaller-bodied species typically occupy habitats that provide high-quality food throughout the year, whereas larger-bodied species can tolerate feeding in a wider range of habitats (8–10). This pattern applies generally to ruminants such as antelopes and deer (which have multicompartment stomachs for digesting food). In contrast, nonruminants such as horses and zebras have faster-throughput digestive systems that enable them to ingest a more fibrous diet and thereby occupy a somewhat wider range of habitats than ruminants of similar size (10). This diversity within trophic guilds (organisms feeding at the same level of the food web) can generate complex interspecific interactions—including competition (interactions in which one species “loses”) and facilitation (in which at least one species benefits and none lose)—that contribute to ecosystem stability.Odadi et al. examined how large grazers, including buffalo and elephants, influenced diet and weight gain in cattle. Their results suggest that cattle-wildlife interactions can be both competitive and facilitative, with the net effect determined by the relative densities of each herbivore type and how their populations vary by space and time across a shared rangeland. Their insights offer a refreshing view of how we might manage multispecies animal production systems in which domesticated and wild species are managed not only to reduce competition, but also to capitalize on the benefits of facilitation.
Facilitation among ungulates is best known from studies of “grazing succession” on the Serengeti (5, 11). Researchers observed buffalo and zebras feeding on and trampling taller grasses, clearing the way for other herbivores to feed on exposed plant parts of higher nutritional quality. In this example, the feeding actions of individuals of one species (species A) improved the feeding efficiency of individuals of one or more coexisting species (species B, C, D, etc.). As originally conceived, grazing succession implied that species A is a bulk grazer moving across the landscape seeking ungrazed swards, with its movements independent of species B following behind. Subsequent modeling, however, found that the movement of species A into taller, ungrazed swards might not be independent of species B; it showed that if B is smaller-bodied than A, then B is likely to displace A because smaller grazers can feed more efficiently on shorter swards (12). Now, grazing succession is seen as a sequence of pull-push interactions, with larger grazers facilitating sward conditions that attract smaller grazers, which rapidly remove higher-quality plant parts and cause the larger species to move on. The larger species, having a wider dietary tolerance, can meet nutritional requirements while moving at the front of the grazing succession.
Smaller species in grazing and browsing guilds typically have a competitive advantage over larger species (13). There is not always a facilitating species, however, and if there is, it is not necessarily larger than the species it facilitates. For instance, Odadi et al. present evidence that zebras facilitate grazing conditions for cattle during the wet season, despite similarities in body size and competitive interactions in the dry season. They suggest that differences in anatomy and physiology enable zebras to remove fibrous grass stems, benefiting cattle during the wet season.
Competitive interactions between livestock and wildlife, whether perceived or real, constitute the main source of human-wildlife conflict, and wildlife populations are inevitably on the losing side (14). But mixed grazing systems involving livestock and wildlife can be beneficial for biodiversity (15) where veterinary restrictions do not preclude it, and if management is attentive. A mix of herbivores that differ in body size, trophic guild, and digestive system should provide managers with opportunities to capitalize on facilitative interactions, intervene against competitive ones, and enhance animal production overall. In developed countries, it is standard practice for rangeland managers to propagate a mix of native and nonnative plants. The next step, which requires bold experimentation and a break from orthodoxy, is to actively manage interspecific interactions within mixes of native and non-native herbivores to enhance the delivery of ecosystem goods and services from rangelands.