The role of elephants in the ecosystem

A large herbivore such as the elephant obviously makes a tremendous impact on the habitat through its feeding and other activities. This has several conse quences for the plant and animal communities in the ecosystem. The transformation of woodland into savanna favors the populations of grazing ungulates. Browsing by elephants on trees keeps the trees in a stunted, shrubby stage that facilitates feeding by smaller browsing mammals. The creation of gaps in forests likewise promotes the growth of herbaceous plants that provide forage for many other herbivores. The paths that elephants make through dense undergrowth also help smaller mammals. The holes that elephants dig in dry riverbeds for subsoil water are also used by other animals. Elephants eat soil or rock (geophagy) to satisfy their appetite for salt. This may result in considerable erosion of land and even the formation of caves along the steep-sided walls of river valleys or cliffs. Some of the larger caves discovered, such as at Mount Elgon in Kenya, have been formed through rock-crunching by elephants over many thousands of years. In semiarid East Africa, the Maasai have long recognized that an abundant elephant population that reduces the density of bushland also keeps down the population of tsetse flies that cause sleeping sickness in humans.

The role of elephants in dispersing fruits and their seeds is well documented, especially in rain forest habitats (see chapter 5). The key question, then, is to what extent is such dispersal important to the regeneration of these plants. One obvious advantage is the mere transport of seeds away from the vicinity of the parent tree, where they may suffer higher destruction from pathogens and seed predators. The passage of a seed through the gut of an animal is also known in several cases to improve the chances of germination by softening or scarification of the seed coat.

Seeds of leguminous plants such as acacias are known to germinate more successfully after being voided by a large herbivore. In semiarid African and Asian habitats, the seeds of such legumes are very common in the dung of elephants. Joseph Dudley found that the seeds of Acacia erioloba alone made up 12% of the dung weight, with some piles having over 5,000 seeds of this plant during the dry season in Zimbabwe's Hwange National Park. James Powell recorded 91 plant species germinating from the dung of forest elephants in Cameroon, these representing 14% of the species recorded in this region. Experiments by Diana Lieberman and associates on seeds of 11 species in Ghana's moist forests showed that, in three of these, the rate of germination was clearly higher in ingested seeds than in fresh seeds. In two species, there was no difference in germination rates, while in the rest the experiments were incomplete. While smaller fruits and their seeds may be dispersed by a variety of animals, a tree with large fruits and seeds may be exclusively dispersed by the megaherbivores.

Would such tree species fail to regenerate and decline over time if elephants disappear from an area? Daniel Janzen and Paul Martin proposed that the demography of many Neotropical plants that had evolved for dispersal by megafauna (e.g., gomphotheres) would have altered after the late Pleistocene extinctions (chapter 1). Several traits of extant plants, such as large fleshy fruits, large seeds protected by hard coats, or seemingly passive dispersal are "anach ronisms" in today's world. Henry Howe has questioned this hypothesis as being vague; in particular, it is not clear how so many species with "megafaunal syndromes" have persisted for over 10,000 years.

A careful inventory by William Hawthorne and Marc Parren of West African rain forest plants in Upper Guinea, where elephants have declined greatly, found little evidence for a collapse of plant populations, including those most likely to be dependent on elephants for dispersal. One exception was the forest date (Balanites wilsoniana), a dry forest tree with seeds that germinate much better and seedlings that grow faster after passage through an elephant's gut. This tree may suffer a decline over a centennial timescale if elephants disappear from a region.

To evaluate the role of elephants in seed dispersal and plant demography, one has to consider several factors, including seed survival from ingestion to voiding, dispersal distance, germination success, and the eventual growth and establishment of the seedlings up to at least the sapling stage. Only then can the role of elephants in seed dispersal be objectively described.

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