Gardeners of the Forest

Small seedling grows out of elephant dung. © Elephant Listening Project

As the most frugivorous (fruit eating) of the elephants, the forest elephant plays a key gardening role by dispersing seeds it consumes.

Many tropical rainforest plants produce toxins to defend against being eaten. Elephants have simple, single-chambered stomachs (monogastric) and are ill-equipped to handle these diverse toxins. They can compensate for this weakness by increasing food diversity, which reduces intake of each particular plant toxin. This means elephants consume a very wide range of plants.

Steve Blake’s dissertation on forest elephants near Nouabalé-Ndoki National Park¹, not too far from where ELP’s Andrea Turkalo has been working, showed that those elephants consumed 351 plant species. This diet is far more diverse than the diets of other mammals, including other elephants. Of course, the elephants aren’t dispersing seeds for all 351 species. Some species are only eaten for their leaves or bark, and some species’ seeds do not survive digestion. However, many species do benefit by passage through the elephant’s gut.

Blake and Ahimsa Campos-Arceiza² studied forest elephant seed dispersal and found that elephants dispersed seeds farther from the parent tree than any other vertebrate, which can strongly affect seed survival rates. According to the widely accepted Janzen-Connell hypothesis³ ⁴, density-dependent factors (e.g. disease and herbivory) decrease survival in an area already populated by the same species. However, when a seed is brought far from its parent plant, it is separated from the diseases and herbivorous predators that lurk near the parent. The distance directly increases the young plant’s survival chance.

A more recent 2013 study⁵ measured a decline in diversity in Salonga NP where poaching has driven elephants to near extirpation. David Beaune’s team revealed that healthy adult trees deceptively portray a lively forest, while many species severely lack recruitment of new seedlings into the community. This situation, empty forest syndrome, quietly develops as elephant populations dwindle.

Rainforests need free-roaming elephants in order to carry out this important role of seed dispersal, helping to retain vegetative diversity. A forest with abundant elephants is a healthy forest!


References:

¹ Blake, Stephen. “The ecology of forest elephant distribution and its implications for conservation.” PhD diss., University of Edinburgh, 2003.

² Campos-Arceiz, Ahimsa, and Steve Blake. “Megagardeners of the forest–the role of elephants in seed dispersal.” Acta Oecologica 37, no. 6 (2011): 542–553.

³ Janzen, Daniel H. “Herbivores and the number of tree species in tropical forests.” The American Naturalist 104, no. 940 (1970): 501–528.

⁴ Connell, J. H. “On the role of natural enemies in preventing competitive exclusion in some marine animals and in rain forest trees, Dynamics of Population.” Ed. PJ Den Boer and GR Gradwell. Wageningen: Pudoc (1970).

⁵ Beaune, David, Barbara Fruth, Loïc Bollache, Gottfried Hohmann, and François Bretagnolle. “Doom of the elephant-dependent trees in a Congo tropical forest.” Forest Ecology and Management 295 (2013): 109–117.


Find out more about the Elephant Listening Project
To conserve the tropical forests of Africa through acoustic monitoring, sound science, and education, focusing on forest elephants