Ecologists have necessarily had to simplify matters in looking at predator–prey dynamics. Study of a situation in which predator and prey live in groups reveals that a key process was previously overlooked.
Eating is a necessity of animal life. So you might expect that individuals would do everything possible to maximize their food intake. An example, often invoked, is that lions live in prides because group hunting increases food availability by allowing the lions to kill prey that would be too large for an individual to tackle. On page 1041 of this issue, however, Fryxell et al.1 argue that living in groups decreases the amount of food each lion eats. Moreover, that decrease is even more severe if the lions' prey is also gregarious. How can this be explained?
Many large predators live in groups, as is dramatically seen in the familiar TV programmes that show, for example, a pack of wild dogs running down an impala or a pod of killer whales attacking a shoal of herring. Many of the prey species of these top carnivores also live in groups (Fig. 1). But there has been surprisingly little research into how group living influences individual food-intake rates, and the dynamics of the populations of predators and their prey.
A concept called the functional response2 lies at the heart of behavioural and population ecology, and of Fryxell and colleagues' paper. This is the curve that describes the intake rate of a single predator as a function of prey density. The shape of the curve is the result of two processes — the rate at which predators encounter prey and the speed with which they consume it3. Much of the theory investigating the consequences of different shapes of functional response has been developed following observations and experiments from systems where both predator and prey are solitary. These systems often show dynamics in which the numbers of both predator and prey increase and decrease, sometimes leading to extinction of first prey and then predator. Altering the form of the functional response can, in some circumstances, prevent extinction4.
Fryxell et al.1 examine how group living in prey, in predators and in both kinds of species influences the shape of the functional response and the interaction between predator and prey populations. They show theoretically that gregarious living in either the prey or the predator species reduces the rate of prey consumption by each predator. Intake rates are lowest when both species live in groups. If prey lives singularly, an increase in the number of prey will lead to a linear increase in the likelihood that a predator will encounter a meal on its daily wanderings. However, if the prey forms clumps, there will be large holes in the landscape through which a predator can roam without finding something to eat. Group living in prey therefore decreases intake rate. When a predator does find a group of prey, it encounters an embarrassment of riches and quickly becomes satiated. The intake rate of predators is reduced if they live gregariously because each individual searches the same area and then has to share the prey that it kills.
Fryxell et al. primarily use data from surveys of lions preying on wildebeest in the Serengeti National Park, Tanzania, to reach their conclusions for a system where both predator and prey live gregariously. They show that the consequence of both species living in groups is a predicted reduction in the food-intake rate per lion of 90% compared with the rate when lions forage solitarily. This is an enormous amount, and is equivalent to the decrease in food availability that results if the migratory wildebeest is present in a lion-pride territory for only a fraction of each year. The authors also examine the consequence of such a large reduction in intake rate on the dynamics of the lion and wildebeest populations. They predict that, if both species shunned group living and wandered the plains singly, the dynamics of both populations would be highly unstable, with both predator and prey likely to become extinct. In contrast, if both predator and prey live in groups, it is much more likely that both their populations will persist.
But the question of why lions live in groups remains. Fryxell et al. argue that the benefits primarily accrue from territory defence and the communal protection of young against males that can kill cubs when they take over a pride5,6. However, an argument familiar to most ecologists is that lions live gregariously because group hunting is required to bring down large prey. Fryxell et al. accept that group hunting does allow lions to attack and kill the formidable Cape buffalo, but they also state that: “Most individual lions refrain from contributing to group hunts.” This carefully worded statement does not rule out the possibility that substantial benefits arise from group hunting, and it flags one of the problems of parametrizing functional responses. It is challenging to work out individual intake rates accurately across different sizes of predator and prey groups for a range of prey densities, especially for species with such complex social arrangements as those seen in lions. I would be interested to know whether functional responses derived entirely from observations on the feeding of individual lions would yield similar conclusions to those obtained by Fryxell et al. using survey data.
This work1 shows that extending the functional response to include some realistic natural history helps to explain why the inevitable extinction of predators and prey, predicted by some simple population models, is not observed in the wild. The paper will stimulate researchers who have obtained functional responses using detailed observational data on group-living predators. And it should encourage theoreticians to examine how other aspects of animal behaviour might affect the predictions derived from simple population models.