Wild game is a major food source for many people in the tropics. Rural population growth and an increasingurban demand for meat have made hunting a major threat to biodiversity. Most researchers investigating this problem assume that the impact hunters have on wildlife is somehow proportional to the amount of time and effort they exert in hunting. But how do you calculate their effort? According to Janna Rist and her colleagues from London’s Zoological Society and the Imperial College of London, different researchers employ a myriad of methods. In order to gauge hunting effort and derive impacts on wildlife, different calculations may integrate days spent hunting, distance traveled while hunting, or distance traveled to a hunting location.
With so much variation in how, where, and what is hunted, Rist and her colleagues question whether there is a consistent relationship between the
time hunters spend in the field and the number of animals harvested. After evaluating numerous studies, they found that the significance of these
potential sources of variation has not been assessed. From the literature, the authors identified nine assumptions that commonly occur in the bushmeat literature. They then formulated these into hypotheses, that they tested with data from an intensive 15-month study of hunting in West Africa (Table 1).
The study was based in Midyobo Anvom, a remote jungle village in Equatorial Guinea. Most hunters work from camps located up to 13 km from the village; the rest hunt within a day’s hike from home. Wire leg snares and neck traps of various sizes are located in trap groups around the camp; a few hunters also used guns. Hunters leave the camp each week to sell game to traders from the regional capitol of Bata.
Rist or her assistants observed 225 hunting trips, recording every detail. They timed the durations of all aspects of the hunt, including travel to hunting camps, active hunting, and resting. They also recorded the time it took to remove animals from traps or retrieve animals that hunters had shot. Different types of traps catch different animals, so the number and proportion of each trap type was noted. The identity of every animal killed, whether it was useable for food or not, was also recorded.
Of the nine assumptions, Rist and her colleagues rejected four of them outright (Hypotheses 1, 4, 7, 8). Many researchers assume that the amount
of time spent actively hunting is proportional to the total time spent away on a hunting trip (Hypothesis 1). The number of days or hours spent out hunting can therefore be used to calculate the impact of the hunters. Rist’s data, however, indicates that the time spent actively hunting varied with the distance of the camp from the village. Similarly, it is often assumed that the time spent checking traps is always the same for all hunters (Hypothesis 2). In the field, Rist found that hunters based in the village spent 10% less time checking their traps because they spent more time travelling from home to their hunting sites than hunters based from camps. Another common assumption is that hunters always use the same number of traps, and the same portion of trap types (Hypothesis 7). In fact, Rist’s data indicates that hunters use different mixes of traps at different locations in the forest. Finally, researchers often assumed that traps are not species specific (Hypothesis 9). Rist, however, found that leg and neck traps target different species.
Only two common assumptions were found to be true in this study (Hypotheses 3 and 5). As many researchers assume, the handling of prey does not take up much of a hunter’s time. It is therefore not necessary to account for handling time when calculating hunting effort. It is also assumed that the number of traps set by hunters does not vary between hunting sites. Rist’s data supports this, making it acceptable to estimate hunting effort from the number of trap groups set without knowing the exact number of traps. The remaining three hypotheses had equivocal evidence (Hypotheses 2, 6, and 9). Through their literature review and their detailed study of this African hunting system, Rist and colleagues have been able to systematically identify sources of bias in bush meat studies. Information on the activities of hunters is relatively easy to collect and is therefore a popular tool for assessing the impact of bushmeat hunting. Rist’s detailed fieldwork will help future researchers collect data that accurately translates into impacts on prey populations.
Rist, J., M. Rowcliffe, G. Cowlishaw and E.J. Milner-Gulland. 2008. Evaluating measures of hunting effort in a bushmeat system. Biological Conservation 141: 2086-2099.
Nathan Brouwer (firstname.lastname@example.org) is a PhD student at the Department of Biological Sciences, University of Pittsburgh, USA.