As go the elephants, so go the trees. That's the message of a new study published in the May 2013 issue of Forest Ecology and Management that found more than a dozen elephant-dependent tree species suffered catastrophic population declines in new plant growths after forest elephants were nearly extirpated from their ecosystems. The fruit-bearing trees all rely on forest elephants as their primary means of seed distribution, a process known as megafaunal dispersal syndrome.
The study was conducted in Salonga National Park in the Democratic Republic of the Congo, a region where more than 98 percent of the forest elephants (Loxodonta cyclotis) have been killed by poachers over the past few decades. "As an ecologist studying plantanimal interactions, I was really worried about elephant survival and the plants that had coexisted with them for a million years," says the paper's lead author, David Beaune, a research associate with the Biogeoscience Laboratory at the University of Burgundy in France and the Max Planck Institute for Evolutionary Anthropology in Germany. "I started to assess the trees that required elephants for seed dispersal, and I became increasingly worried with time when I did not find any evidence of new growths. We decided then to study if those plant species were able to survive without their seed dispersal partner, the elephant."
Elephants, with their massive appetites, play two key roles for many of these trees. For one thing, forest elephants roam over large swaths of land, spreading seeds far and wide via their dung. For another, previous research has shown that seeds that have been softened by an elephant's stomach acids tend to germinate at a much faster rate than seeds that have not. This dual relationship allows fruit trees with no other dispersal partners to ensure reproduction.
The researchers conducted their study at the LuiKotale Research Site, where data on medicinal plants has been actively collected since 2002. They found that 18 different elephant-dependent tree species at that location had dropped in productivity; 14 of those species were not producing enough new trees for to replace the old ones as they die off, a process known as self-replacement.
The loss of these trees could have wide-ranging ecological effects. "These trees also feed many herbivores and frugivores such as bats, birds, insects and other mammals," Beaune says. "Consequently, the loss of these tree species can affect them." For example, chimpanzees and bonobos in certain forest areas rely on the fruit of one of the elephant-dependent trees, Irvingia gabonensis, for months at a time. (Bonobos also play an important seed-dispersal role, which Beaune and his fellow researchers are studying.)
Human populations could also suffer, Beaune says, as many of these trees have long been used for traditional medicine. "Local people have barely any access to Western medicine, he says. They rely on traditional use of plants such as these trees that have not yet revealed all their secret molecules to modern science."
At least one of the tree species mentioned in the study, Autranella congolensis, is already listed as critically endangered due to habitat loss. Efforts could be taken to preserve and protect it and the other tree species, but would such efforts be effective if elephants are not part of the equation? "This is a big issue in conservation biology," Beaune says. "The protection of an area is useless if the ecological functionality is not maintained. In the case of Autranella congolensis, imagine a special reserve dedicated to their conservation which contains big, healthy adult trees and no elephants! The conservation plan could be severely compromised."
In their paper Beaune and his co-authors recommend that artificial nurseries be established to preserve some of these tree species as well as call for increased law enforcement to protect forest elephants and reintroduction programs to bring them back.
Elephants are about as charismatic as endangered megafauna get and therefore are an emotional issue for a lot of people. Plants rarely engender as much passion. Even if they did, it might not be enough. "Unfortunately the sole emotional value of a species hardly helps them to survive our strange era," Beaune says.
Photo: A forest elephant family by Richard Ruggiero, U.S. Fish and Wildlife Service