Forest elephant family. Photo by USFWS, Public Domain
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A “keystone species” is a species that has a large influence on the other species within its biotic community. It is often defined in terms of loss: “if this species is removed from a community” or “because this species was removed from its community” the entire community, all of the populations of all of the other species, were seriously affected and degraded. There is a subjective tint to this definition due to the individual interpretation of how “large” the influence of a particular species might be or how “serious” a particular loss might reverberate through an ecosystem, but the term has come into widespread use especially in discussions of the rampant species extinctions going on in our Anthropocene times.
Forest elephants are a species that has recently been described as playing a keystone role in its African tropical rainforest habitat (The New York Times, August 19, 2019). The African rainforest is second only to the Amazon rainforest in its size and scope and also in its influence on global water and carbon cycles. The lack of large herbivores in the Amazon, though, is one of a number of interesting differences between these two great, tropical ecosystems. The Amazon’s “elephant-like” herbivores (which included giant ground sloths and gomphotheres and glyptodonts) went extinct 12,000 years ago. As a consequence, there is no animal in the Amazon biotic community capable of trampling or uprooting small trees, and, logically, there is a much higher percentage of small trees in the Amazon rainforest and a reduced overall mass of above ground vegetation compared to an elephant inhabited African rainforest.
Rain forest near Konimbo, Liberia (West Africa). Photo by J. Atherton, Flickr
Looking more closely at this, researchers studied the tree distributions and carbon compositions of the tropical rainforests in the Democratic Republic of Congo (which have not had forest elephants for the past 30 years) and in the nearby Republic of Congo (which has only recently lost its forest elephants ). They found that forest elephants regularly trample trees that are less than a foot in diameter, and that they primarily eat the soft-wooded, relatively fast growing tree species. As a consequence of their trampling and tree consumption more sunlight reaches the forest floor (stimulating tree seedling growth) and water availability to forest plants is increased. Larger, slower growing, longer-lived tree species were subsequently favored in an elephant impacted rainforest, and these trees store significantly more carbon than their smaller, faster growing counterparts.
Researchers estimate that the widespread extinction of forest elephants will result in a 7% loss in African rainforest vegetation (the equivalent of three billion tons of stored carbon). The storage of this carbon is valued at $43 billion!
Forest elephant populations have decreased by 62% in the first decade of the 21st Century! They are functionally extinct from most of their 850,000 square mile, natural African range. Poaching along with habitat loss are the main reasons behind the widespread decline of this critical, rainforest keystone species.
American alligator. Photo by D. DeLoach. Wikimedia Commons
Another keystone species found closer to home is the American alligator. The American Alligator is a large reptile that was once found abundantly from wetlands and lakes of Texas, all across the states bordering the Gulf of Mexico, and up the U.S. Atlantic coast as far north as North Carolina. Freshwater and brackish water open and marshy habitats could be occupied by this apex predator. By 1967, though, primarily due to widespread, uncontrolled hunting, the numbers of wild American alligators were so low that the species was teetering on extinction. Placement of the species on the endangered species list in 1967 and the passage of the full Endangered Species Act in 1973 provided sufficient protection to allow the American alligator to recover and to re-inhabit much of its former natural range.
Why was it important to re-establish this formidable reptile in its former ecosystems? The answer revolves around the keystone nature of this species.
First and foremost, the American alligator is a predator. It broadly and opportunistically takes almost any prey species that come available. It especially consumes relatively large prey species that live in their aquatic habitats (especially large fish), and they also take a wide range of terrestrial species that wade or swim across their aquatic habitats or come to the edge of their pools, ponds or streams to drink. American alligators have been known to take black bears, panthers, deer and wild boar but more commonly capture and eat smaller terrestrial prey like raccoons and muskrats. Wading birds may also be taken but they are not a typical part of an alligator’s diet. Control of these prey species is a very important ecological role of the American alligator.
One very interesting “unintended consequence” of the extirpation of the American alligator from wetlands in Florida (which was motivated in part at least to stop the alligator consumption of game fish) was the precipitous drop in game fish populations after the alligators were gone. Researchers determined that the alligators preferentially consumed the larger fish (like gars) that ate the game fish and that without the alligator control of the gar population, game fish numbers drastically declined.
Gator hole. Photo by A. Gould, Flickr
American alligators also construct “gator holes” or “gator ponds” in their wetland habitats. The alligator uses its snout and tail to dig down through the accumulated muck and vegetation to create a relatively deep water pool in which it can hide and hunt. These pools fill up with freshwater and are often the only water sources that persist during times of drought. Many animals rely on these gator holes for drinking water during times of low rainfall (although they have to keep an eye out for the lurking, hunting alligator!).
Female American alligators also modify their wetland habitats via the construction of nesting mounds. These mounds can be as much as 3.5 feet high and up to 7 feet wide. These mounds serve not
Alligator mound. Photo by L. Oberhofner, Wikimedia Commons
only as incubation sites for the alligator eggs but can also can significantly add to the topographic complexity of the wetland habitat. A variety of plant species that require slightly drier soils can grow on these mounds thus increasing the vegetative diversity of the wetland. Also, a significant number of bird and mammal species can use these mounds for their own nests and dens.
So, forest elephants and American alligators both function in their ecosystems to increase biotic diversity and stability. They are keystone species whose influence and importance was not recognized until they were almost gone!
Next week, another keystone species that humans tried very hard to exterminate: the black tailed prairie dog!