As I mentioned in my last post, researchers are still learning about the ways in which elephants, and other large herbivores, impact the environment around them. In many ways, the goal is to figure out how many elephants can fit into an area before their resource needs lead to environmental degradation. One way to do that is to exclude elephants from specific spots and then compare those spots with others that elephants can access. This is exactly what Lagendijk et al. (2012) did with sites in South Africa, although they added a little twist since nyalas, large antelopes, were also present in the area- they used one type of fence to exclude elephants and a separate type to exclude both elephants and nyalas. As it turned out, elephants weren’t the only animals altering vegetation- while plant communities stayed largely the same in areas that only smaller herbivores could reach, plant communities changed in the same ways when elephants and/or nyalas were present- when elephants were excluded, nyalas increased their browsing pressure.
“Elephant browses the trees at Inyati, Sabi Sands Game Preserve, Limpopo, South Africa”
Sometimes you’re trying to assess impact over a very large area or sometimes the animals you are working with don’t like hanging out with people, so you look for other options. Simms (2009) used satellite imagery to investigate vegetation changes in a private game reserve in South Africa (if any of you have heard of archaeologists finding Mayan cities by looking for patches of younger forest, it’s the same principle- although the green may all look the same to our eyes, at certain wavelengths, the light reflected by older forests is different from that reflected by younger forests). Once again, although elephants did damage and alter vegetation, fire and rain patterns were also very important, and Simms even suggested that the impact of the grazing community (zebra, wildebeest, etc.) within the reserve was of bigger concern.
Are there any surprises about elephant roles? There were quite a few moments when I thought, “Wow, that’s amazing and/or scary”- here’s a brief summary:
- In Kruger National Park, Kohi et al. (2011) found that elephant-damaged mopane trees had many more (30x!) green leaves and more leaves closer to the ground (easier access for smaller animals) than untouched trees
- Forest elephants in the Rep. of Congo moved 14% of the seeds they ingested more than 10 km from the parent plant (Blake et al. 2009)
- Although not a study directly on elephants, when researchers looked at which animals were dispersing which seeds in a park in Rwanda, they found that no one was really visiting Parinari excelsa trees (Gross-Camp et al. 2009)- in other places, elephants disperse those seeds, but there were no elephants in the studied park. Without dispersal agents, the researchers were concerned about the future of P. excelsa in the park. (for more info on similar studies, check out the Center for Tropical Ecology and Conservation)
- And (saving the best for last!), observations from a research in Sri Lanka (Campos-Arceiz 2009)suggest that, in addition to providing food and other resources for invertebrates, elephant dung provides habitat for amphibians (specifically frogs)- really?!
Photo by David Govtaski for USFWS Northeast Region
Do any other herbivores play similar roles in other places? Yes, and no. There are a variety of large herbivores that disperse seeds, impact plant communities through browsing pressure, create special habitats for other species, and so on, but no one else does it quite like elephants. Cristoffer & Peres (2003) feel that one of the reasons rain forests in the Americas differ so much from those in Africa and Asia is the lack of elephants- tapirs in the Americas are important for seed dispersal and they are big enough to be dangerous, but they are still much smaller than elephants. Outside of the tropics, moose, reindeer, and other browsers alter vegetation patterns and influence habitat. Moose in particular have received a lot of attention in terms of their environmental impact: their browsing tends to open up the canopy, creating more room for shrubs and herbs; because they are selective eaters, areas heavily influenced by moose tend to become more dominated by conifers; and their dung facilitates nutrient cycling (Persson et al. 2000). Like elephants, moose have an impact on their world simply through the steps they walk- based on Persson et al.(2000)’s formula and the estimate of 4500-5000 moose in New Hampshire, I calculated that each year moose here trample an area equivalent to Squam and Newfound Lakes, combined- not bad for animals that wander around at their own pace.
Disturbing the landscape or disappearing from it? This is the big issue with all of these large herbivores. As I have mentioned before, elephants are increasingly confined to parks with fences that limit seasonal movements and tend to concentrate their impact within specific areas- too many animals, and the environment starts to degrade; too few and grassland areas can start to close up again. In forested locations, many large-seeded plants depend upon large herbivores to disperse their seeds so that seedlings don’t compete with each other or cluster together where a fungus or plant-eater can take them all out at once. Without dispersers, forests begin to lose their biodiversity, as in the Central African Republic where a study found that sites with high levels of hunting had fewer plant species (Vanthomme et al. 2010). Moose in large numbers in Fennoscandia damage timber trees and change plant communities, but their actions also increase the number of dead and dying trees which other species need for survival (Edenius et al. 2002)- in NH moose numbers are down and white-tailed deer don’t interact with the environment in the same manner, so a declining moose population could be a big deal.
When you are a big animal, you generally need more food and tend to have a larger home range. You also tend to have a lot of influence on what happens in the ecosystem around you. As human use of the landscape expands around the globe, it’s becoming harder for populations of some large herbivores to find some level of stability, and loss of these animals would, like a rock thrown in a pond, have consequences that expand through the ecosystem. But there are ways to balance the needs of different species and the needs of human society and wildlife; there are also ways that each of us can contribute toward that balance. In my next post, I’ll look at why large herbivores are important to us for more than aesthetic or intrinsic reasons and what we can do to support both them and the people who live with them around the world.
Works cited:
Blake, S., Deem, S.L., Mossimbo, E., Maisels, F., and P. Walsh. 2009. Forest elephants: tree planters of the Congo. Biotropica 41(4): 459-468.
Campos-Arceiz, A. Shit happens (to be useful)! Use of elephant dung as habitat by amphibians. Biotropica 41(4): 406-407.
Cristoffer, C. and C.A. Peres. 2003. Elephants versus butterflies: the ecological role of large herbivores in the evolutionary history of two tropical worlds. Journal of Biogeography 30: 1357-1380.
Edenius, L., Bergman, M., Ericsson, G., and K. Danell. 2002. The role of moose as a disturbance factor in managed boreal forests. Silva Fennica 36(1): 57-67.
Gross-Camp, N.D., Mulindahabi, F., and B.A. Kaplin. 2009. Comparing the dispersal of large-seeded tree species by frugivore assemblages in tropical montane forest in Africa. Biotropica 41(4): 442-451.
Kohi, E.M., de Boer, W.F., Peel, M.J.S., Slotow, R., van der Waal, C., Heitkonig, I.M.A., Skidmore, A., and H.H.T. Prins. 2011. African elephants Loxodonta Africana amplify browse heterogeneity in African savanna. Biotropica 43(6): 711-721.
Lagendijk, G., Page, B.R., and R. Slotow. 2012. Short-term effects of single-species browsing release by different-sized herbivores on Sand Forest vegetation community, South Africa. Biotropica 44(1): 63-72.
Persson, I., Danell, K., and R. Bergstrom. 2000. Disturabce by large herbivores in boreal forests with special reference to moose. Annales Zoologici Fennici 37: 251-263.
Simms, C. 2009. The utilisation of satellite images for the detection of elephant induced vegetation change patterns. Masters thesis, University of South Africa.
Vanthomme, H., Belle, B., and P. Forget. 2010. Bushmeat hunting alters recruitment of large-seeded plant species in Central Africa. Biotropica 42(6): 672-679.