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.
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