A web page review by Mark Walker
This web page reviews the recent scientific paper by Sarah Durant, Competition refuges and coexistance: an example from Serengeti carnivores. in the Journal of animal ecology (1998) 67:370-386. This paper investigates whether cheetahs (Acinonyx jubatus ) use 'competition refuges' where there competitors are in low densities. Cheetahs are out competed by lions and hyaenas and as cheetahs are mobile it is expected that they will move away from these predators if possible.
Introduction
The cheetah is the least numerous 'big cat' of the Serengeti (Caro and Durant 1995). It hunts its prey in a characteristic stalk then sprint manner and is well adapted to running. Its favourite prey are Thompson's gazelle, though it will take other prey species (Caro 1994). Its method of hunting favours small groups which have less vigilance and where individuals are easier to pick out (Fitzgibbon 1990). The lion (Panthera leo) and Spotted Hyaena (Crocuta crocuta) are the cheetahs main competitors. They compete for prey, but the main way in which they compete is by killing Cheetah cubs (Laurenson 1994). As the cheetah is a weaker competitor you would expect it to struggle to survive.
It is thought competing species can survive because of spatial heterogenity (Chesson 1985, Hanski 1994). This is where there are varying physical features over an area. This means resources and competitors are not evenly spread and tend to be clumped. This allows weaker competitors such as the cheetah to find areas with less competition to live in (Shorrocks 1991).
It has been shown that prey can have a restbite from predation by using 'Prey niches'(Hassell and May 1973). Durant wanted to find out if there were also 'competition niches' on the Serengetti plains which allowed cheetahs to keep away from lions and hyaenas and allow them to coexist.
To discover if 'competition niches' existed for the cheetah Durant needed to research cheetah densities, lion and Hyaena densities, and the cheetah prey densities to see how the different species interacted. If the idea of 'competition niches' is correct then as the cheetah is highly mobile you would expect to find them in areas with a low density of competitors. Durant also wanted to discover if lactating cheetahs were more likely to be in areas with competitors. Lactating cheetahs have a limited mobility as they have to remain close to their lair where their cubs hide. Durant thought that this reduced mobility would mean lactating cheetahs would come into contact with competitors more often.
Method
The experiment was conducted in the Serengeti national park in Tanzania. There is a variable rainfall throughout the year which affects the vegetation, there are distinct rainy and dry seasons (Sinclair 1979). This causes the seasonal 'migration' of cheetah prey such as wildebeest (Connochaetes taurinus), Thompson's gazelle (Gazella thomsoni), and Zebra (Equus burchelli), which move into the area in November with the rainy season (Maddock 1979, Mc Naugton 1976). The experiment was conducted from 1991 to 1995. Cheetahs use certain vantage points for scanning the surrounding area, and these can be used as scanning points for cheetahs. Scans were conducted from these and additional vantage points, and the vegetation cover, prey densities and predator distribution was estimated by Durant.
Each scan point used was located by using maps and geographical positioning systems. The weather was recorded by using a scale system. To measure the prey densities the number of gazelle within a kilometre of each scan were counted. The mean group size was calculated for some scans to see how the group size affected the distribution of cheetahs. When predators were spotted their distance away was estimated- although the numbers seen at greater distances was less due to reduced visibility and because the predators were well camouflaged. When a cheetah was spotted another scan was conducted from close to its position. Lactating cheetahs were noted and were classified as those that had enlarged mammary glands. The scans took place when cheetahs are most active in the early morning and late afternoon. The results were analysed using varying statistical techniques.
Discussion
The results showed a number of trends. When cheetahs were present there were fewer gazelles, compared to when lions were present. When there were few gazelles they tended to be in smaller scattered groups. Lions and hyaenas are attracted to higher densities of prey, while smaller densities suit a cheetahs mode of hunting. By staying with lower densities, not only does the cheetah hunt more effectively but it also avoids its competitors.
Cheetahs avoid lions in the wet season but in the dry season they are forced into areas with lions as the prey tend to aggregate around water supplies. However because the lions are territorial (Schaller 1972) it is probable that the cheetahs can avoid the lions locally even at this time. The hyaenas population fluctuates during the year. In the dry season there densities are low so they don't cause a problem to cheetahs and they don't need to actively avoid them. But in the wet season there is an influx and the density of hyaenas increases because of the prey migrations. The cheetah is forced to actively avoid hyaenas at this time.
Lactating cheetahs tended to be in areas with higher lion densities as opposed to the more mobile none lactating cheetahs. This helps to show that cheetahs rely on their mobility to survive and avoid competitors.
Durants study shows that cheetahs seek out areas with low densities of prey and low densities of competitors which allow it to persist. Durant has also played tapes of lion and Hyaena calls which the cheetahs responded to by avoiding. This is more evidence that avoidance is active. It has been suggested that cheetahs are in low densities because of a poor rate of reproduction, however they are probably in low densities because of competition from other predators (Caro and Laurenson 1994). The cheetah is a low ability competitor but survives by living in low densities but by being widespread.
References
Caro T.M. and Laurenson M.K. (1994).Ecological And Genetic Factors In Conservation:A Cautionary Tail.Science 263:485
Caro T.M. (1994) Cheetahs of the Serengeti Plains: Group living in an asocial species. University of Chicago press.
Caro T.M. and Durant S.M. (1995) The importance of behavioural ecology for conservation biology: Examples from studies of Serengeti carnivores. Serengeti II: Dynamics, management and conservation of an ecosystem (edited by A.R.E.Sinclair) pp451-472. University of Chicago press.
Chesson P. (1985) Coexistance of competitors in spatially and temporary varying environments: a look at the combined effects of different sorts of variability.Theoretical Population Biology 28:263-287.
Fitzgibbon C.D. (1990) Why do hunting cheetahs prefer male gazelles? Anim. behav. 40:837
Hanski T. (1994) Spatial scale, patchiness and population dynamics on land. Philosophical Transactions Of The Royal Society. London B343,19-25
Hassell M.P. and May R.M. (1973) Stability in insect host-parasite models. Journal of animal ecology 42,693-736
Laurenson M.K. (1994). High juvenile mortality in Cheetahs (Acinonyx jubatus) and its consequences for maternal care.journal of zoology. London234,387-408.
Maddock L. (1979). The 'migration' and grazing succession. Serengeti: Dynamics of an ecosystem (edited A.R.E. Sinclair)pp 104-129. University of Chicago.
Mc Naughton S.J. (1976) Serengeti migratory wildebeest: faciliatation of energy flow by grazing. Science 191,92-94
Shorrocks B. (1991) A need for niches? Trends in ecology and evolution 6 262-263
Sinclair A.R.E. (1979) The Serengeti Environment : Serengeti: Dynamics of an ecosystem (Edited by A.R.E. Sinclair) pp 31-45. University of Chicago press.
