Field Trip EarthField Trip Earth
Elephants as Keystone Species
 
Choose a Trip
Field Trip Earth Home

Field Trip Earth

Join Field Trip Earth
About Field Trip Earth
Interviews
Field Reports
What I Know About...
Educator Resources
Contact Field Trip Earth
Search

Home > What I Know About... > Elephants as Keystone Species

Elephants as Keystone Species

by Field Trip Earth staff

What is a Keystone Species?
Scientists understand that living communities of plants and animals depend keenly on one another for survival. Plants provide essential nutrients and energy to browsing and grazing animals and, ultimately, to the carnivores that feed on these herbivores. When plants and animals die, fungi, microbes and other organisms mine the nutrients from the dead tissues and return these chemicals to the soil. Newly enriched, the soil is ready to support yet another generation of plants and animals. As vital as these connections are, they are not the only ties that link animals and plants together. Plants and animals interrelate on many levels and in almost infinite ways. Various plant species, for example, rely on animals—insects, bats, rodents or others—to carry pollen and fertilize seeds. And, various animals count on plants for shelter—branches for nesting sites or tall grasses to hide from predators or ambush prey. By the same token, roots hold soils steady against erosion, keeping silt out of streams and water clean for fish and other organisms. And, so it goes. All living things are connected, and maintaining these connections promotes the health of species, populations, communities, habitats and ecosystems.

Within a habitat, each species connects to and depends on other species, and each species contributes to the overall integrity of the habitat. But, while each species contributes to habitat functioning, some species apparently do more than others in the overall scheme of things. Some species provide essential services that are also unique. Without the work of these key species, the habitat changes significantly. Scientists call these pivotal players keystone species. When a keystone species disappears from its habitat, that habitat changes dramatically. The keystone's disappearance triggers the loss of other resident species, and the intricate connections among the remaining residents begin to unravel. In a "domino effect," species losses cascade through the habitat, as the loss of one species prompts the loss of still others.

As resident species vanish, other species move in or become more abundant. The altered mix of species changes the habitat's appearance and character. The "new" habitat looks different from the original one, and it houses a new mix of plants and animals. Often, the new habitat supports fewer species and works less efficiently than the original one: nutrients and energy turn over more slowly and less efficiently, biological diversity dwindles and the landscape begins to change.

Example: Elephants as Keystone Species in Grasslands
Elephants appear to be keystone species in African grasslands. That is, without elephants (or some other player to fill the elephants' role), the grasslands actually cease to exist as grasslands. Take away the elephants, and the grasslands, which overgrow with woody plants, convert to forests or to shrub-lands.

This conversion begins when woody plants, particularly various species of acacias, sprout among the grasses. Left unchecked, these sprouts can grow and reproduce, eventually forming a closed stand of trees or shrubs. Once in place, the stand's interlocking branches and leaves shade out the grasses. Without enough sunlight to survive, the grasses dwindle, the grassland disappears, and a forest or shrub thicket grows in its place.

As the grasses disappear, so do the throngs of grazing antelopes that once massed on the grassland and, with them, go the former grassland's prides, packs and clans of great carnivores. The newly growing forest feeds fewer species than the former grassland. The forest supports a new web of life, but a web that is more impoverished and less productive than the one that preceded it.

As keystone species, elephants stop the progression of grassland to forest or thicket by by weeding out the trees and shrubs. Elephants browse on these woody plants, yanking young trees out by their roots or stunting their growth by eating the growth points on their branches. Even if a tree or two escapes the weeding, they won't last long. Sooner or later an elephant will push the plant over, yank it out of the ground, or kill it slowly by prying away its bark.

In return for these efforts, elephants feast on the grasses that flourish in the grassland. This grazing does not harm the grasses, for grasses are adapted to live in harmony with their grazers. Grasses live comfortably with grazers by sacrificing a few leaves in exchange for keeping their roots and growth points intact. Grasses ensure this compromise by forming leaves that connect to their roots through week and narrow bases. The weak bases snap when a grazer takes the leaves into its mouth. Because the leafs break off quickly, the animal does not pull out the roots or damage the grasses' growth points, which remain nestled safely below the ground.

Grass and grazer live in harmony. The grazer gains nourishment as it eats the grasses and then moves on. The grass keeps its roots and sprouts quickly in the sunlight that floods the recently cleared land. By feeding elephants and other large grazers, the grassland helps ensure its own future.

In grasslands throughout the world, fire provides some of the same services that elephants provide in African grasslands. If set regularly, the fires kill and clear away young trees before they become established in the grassland. Because grasses store their roots and growth points below the soil, the fires do not harm them.

Example: Elephants in Forests
Only one species of elephant, Loxodonta africana, is native to Africa. Most scientists, however, now agree, that Loxodonta africana appears in two subspecies: the savanna or bush elephant (L. a. africana) and the forest elephant (L. a. cyclotis).

The two subspecies differ slightly in form and size. The forest elephant is shorter than the savanna subspecies and has somewhat smaller and more rounded ears. In addition, the forest elephant's forehead is flatter, and its tusks are longer and thinner than the savanna subspecies. Otherwise, the differences between the two subspecies are small. Actually, adult forest elephants look very much like young savanna elephants, making the two subspecies difficult to tell apart. Forest elephants are thought to hold keystone status in some woodlands in western Africa. In these forests, elephants are the only species large enough to eat and disperse the seeds of some very important plant species. In fact, studies show that 30 percent of the larger tree species (and 40 percent of the tallest tree species) in these forests depend on elephants for seed dispersal. Without elephants, many of these plant species would be unable to reproduce.

Many trees that depend on elephants for seed dispersal produce large fruits with thick shells. The shells surrounding the fruits may be a quarter of an inch thick, and only elephants are large enough and strong enough to break open the fruit. The nutlike pits inside these seeds pass through an elephant's intestine unharmed. The seed is then deposited with the animal's dung, which fertilizes the new plant as it germinates and continues to grow.

Consequently, for the long-term, elephants are required to maintain species diversity within these forests. Without elephants to disperse their seeds, some 30 percent of the tree species will probably disappear, significantly changing the character of the forest. Elephants, then, are key to maintaining these habitats and ensuring the long-term survival of the other organisms that are adapted to live in these places.

Some plant species produce small seeds that can survive a trip through an elephant's digestive tract. These seeds benefit when they are deposited in an elephant's dung. The dung provides a rich source of nutrients.


Would you like to comment on this article?
---
(print) View printer-friendly version