Constant Gardeners: Primates Shape African Forest Structure

18 April 2013 by Anne-Marie Hodge, posted in Uncategorized

Charismatic animals are capable of stirring up strong emotional responses amongst the general public, and (hopefully) inspire people to take wildlife conservation seriously. These are your pandas, your tigers, your elephants, your gorillas, your baby seals—the “cover girls” of conservation. Fundraisers often laud these animals as “flagship species,” while advocates for less photogenic organisms lament that they distract people from more ecologically-based conservation issues. For example, far less media attention is granted to less glamorous organisms and processes that are critical for ecosystem integrity, despite being less ideal for glossy magazines and brochures.

Young chimpanzee (Pan troglodytes). Image via WikimediaCommons

Both camps make valid points. While there may be nothing wrong with valuing a species based on the fact of its right to exist as an entity alone, scientists often don’t fully drive home information about the ecosystem-wide effects that losses of these animals could have. Yes, it is a crying shame to lose a large, intelligent, beautiful animal, but what about all of the thousands of less charismatic organisms that will also be impacted if the elephants, tigers, and gorillas disappear?

Publicists crow about the importance of charismatic animals to protect species that share their habitats, but often they don’t actually give much attention or publicity to the details and mechanisms behind interactions on the metaphorical “flagship.” This overlooks many critical facts about species dynamics, and misses the opportunity to educate people about the complexity of ecological communities—teaching them why even non-photogenic species are key links in the chain of life, and that nothing can be valued or protected in isolation. Also, it is critical for non-scientists to understand that even these flagship species are dependent upon an enormous supporting cast, and just preserving them in zoos is not enough to ensure ecological viability in the long-term.

Fortunately, some ecologists are putting effort into investigating just how the disappearance of key species affects—and is affected by—entire ecological communities. A  study in the most recent issue of  Proceedings of the Royal Society B provides just such an example (Effiom et al. 2013). African primates consume vast quantities of fruit and vegetation, and often transport seeds in the process. These species—the largest seed dispersers for many trees in dense African forests—are also being decimated by the bushmeat trade. This is clearly devastating to primate populations. But what about all of those fruit trees that are suddenly losing critical seed dispersal services? This is precisely the issue that Effiom et al. sought to investigate.

Drill (Mandrillus sphinx). Photo by Matt Sabbath, via WikimediaCommons

Their study was conducted in southeastern Nigeria, home to a plethora of primate species, including gorillas, chimpanzees, the spectacular drills, and a number of smaller monkeys. The researchers surveyed plants and animals in two different types of sites: protected areas, and non-protected areas that experience significant hunting pressure. These surveys consisted of transects to document abundances of mammals, which were assigned to four categories:  “large primates” (gorillas, chimps, and drills), “other monkeys,” “large rodents” (which often act as seed predators), and ungulates. They also censused both mature trees and seedlings. Trees were classified by their method of seed dispersal: whether their seeds are transported by 1) large primates, 2) other mammals, or 3) “wind or ballistic ejection” (sounds exciting, right?).

Map of study area (Effiom et al. 2013)

The researchers compared the results of these surveys between the protected and hunted zones. Because primates are both heavily hunted for the bushmeat trade and critical seed dispersers, the researchers expected primates to be less abundant in the hunted zones, and hypothesized that if this was indeed the case, seedlings of primate-dispersed trees would also be scarcer in the hunted zones, while the abundance and species composition of mature trees and seedlings of non-primate dispersed trees would not differ between the two treatments.

The results strongly supported this hypothesis. First, there were stark and ominous differences in the species composition of protected and hunted areas. The hunted areas housed just one third of the number of primate groups as the protected areas, while rodent populations were fourteen times higher in the hunted areas. The number of ungulates was also twice as high in hunted areas as in protected zones, although this difference did not prove to be statistically significant. In other words, in zones that are open to hunting, it appears that primate populations decline dramatically, while rodent and ungulate populations manage to expand, possibly released from top-down pressures by loss of the larger animals impacted by hunting.

As predicted, the decimation of primate populations had a profound impact on the tree composition of hunted areas. Although mature trees of all dispersal types were comparably abundant between treatments, there were significantly fewer seedlings of primate-dispersed trees in the hunted zones. This altered seedling composition is a window into the forest's future. The upcoming generation of trees will have a much different population structure than their forebears, due to the precipitous loss of primates in recent years. Once this cohort of seedlings has matured, the forest itself will look profoundly different.

The problem doesn’t stop there. As we saw, seed predators (mostly rodents) increased dramatically in hunted zones, creating a double-whammy for trees that suddenly faced reduced seed dispersal in addition to increased consumption of seeds by animals. The researchers’ conclusions are not optimistic: “The predicted future state is a forest with few, if any, large-seeded species dispersed by primates.”

All of this creates a final, "big picture" issue to consider. Removing primates causes declines in recruitment of the trees that they use for food…meaning that in the future, these hunted zones are likely to be suboptimal habitat (having much sparser food resources). Thus, even in the highly unlikely event of a complete cessation of primate hunting, these habitats will be unlikely to support pre-hunting densities of primates again. Take away the animal, the plants it interacts with declines, and it becomes harder for the animal to move back into the neighborhood, furthering inhibiting seed dispersal of the fruit trees that are left…and the situation continues in an unfortunate feedback loop.

Effiom et al. (2013) briefly discuss the bushmeat trade as a primary driver of hunting in this part of Africa. They cite two primary forces that are increasing these hunting pressures: increasing human populations and the encroachment of road networks into dense forest, facilitated by the timber industry. Although the bushmeat crisis is often portrayed to be perpetrated by locals due to their own taste for wild animals (Effiom et al. do not make this claim, just for the record), the truth is that local people co-existed with wildlife for millennia before Europeans showed up. Locals did hunt many of wild animals, of course, but in a managed way that didn’t result in complete decimation of populations.

Image credit: National Geographic

The opening of Africa to international trade, however, has fueled much demand for wildlife products across the globe—from people desiring the novelty of a primate steak to the demand for primate parts as traditional medicinal remedies, and everything in between.  One investigation showed that between 4,000 and 29,000 tons of illegal bushmeat is imported into the UK alone every year (Kümpel 2005)—and that is only what was detected. It is estimated that the market for bushmeat generates several billion dollars every year (Brashares et al. 2011). So although westerners often blame Africans for the bushmeat crisis, that perception is far removed from reality.

The take-home message of Effiom et al. (2013)'s study is that hunting primates—which is a growing problem due to market demand from the bushmeat trade—is not just an issue for monkeys and apes, but for the ecological structure of the forest itself. As we’ve seen from some of the bushmeat investigations, this is not a local problem. This is a problem fueled by global market demand. Solutions will depend on wide public awareness about how eating key species can result in a complete overhaul of the landscape itself. Food for thought.

Gorilla (Gorilla gorilla). Image courtesy of WikimediaCommons




Brashares, J. S., Golden, C. D., Weinbaum, K. Z., Barrett, C. B., & Okello, G. V. (2011). Economic and geographic drivers of wildlife consumption in rural Africa. Proceedings of the National Academy of Sciences, 108(34), 13931-13936.

Effiom, E., Nunez-Iturri, G., Smith, H., Ottosson, U., & Olsson, O. (2013). Bushmeat hunting changes regeneration of African rainforests Proceedings of the Royal Society B: Biological Sciences, 280 (1759), 20130246-20130246 DOI: 10.1098/rspb.2013.0246

Kümpel, N. F. (2005). Bushmeat hunting and trade in Sendje, Equatorial Guinea. 228-229.

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