Forming Groups Stabilizes Populations of Predators and Prey
May have prevented prehistoric human hunters from being too successful
Breaking with 80 years of ecological theory, scientists at the University of Minnesota and the Universities of Guelph and British Columbia have found that the best way to spot a sustainable relationship between social predators and prey is to count not the animals, but the groups they form.
The study may help explain the rise of humans -- the most social predator -- and suggests the need to curb activities that break up animals' social structure. The work appears in the October 25 issue of Nature.
"The juncture between two fields of study--in this case, social behavior and predator-prey theory--often yields the most interesting findings," said Robert Sterner, director of the National Science Foundation (NSF)'s division of environmental biology, which funded the research. "A realistic appraisal of long-term dynamics and sustainability often requires in-depth biological knowledge, as this study shows."
Ecologists have been puzzled for decades over the stability of predator-prey relations, said ecologist Craig Packer of the University of Minnesota, a co-author of the paper. "Traditional ecological models have erroneously predicted that predators would inevitably over-exploit their prey, leading to frequent population crashes. But most highly vulnerable prey species form herds, swarms, schools or flocks, and group living reduces predators' efficiency to the point where co-existence is likely to be the rule rather than the exception."
"This result is remarkable in light of the intense scrutiny that predator-prey relationships have received in ecology," said Saran Twombly, program director in NSF's division of environmental biology. "The finding is likely to have broad implications for diverse types of interactions and their effects on community or ecosystem stability."
Ecologists have long modeled interactions between predators and prey by taking head counts of each species and assuming that individuals are evenly dispersed over a featureless landscape, ignoring the fact that many predators and their prey both form social groups.
But the number and distribution of groups, rather than individuals, is most important in determining how often -- and for how much longer -- the two species will interact, the researchers found.
The finding implies that even if an ecosystem has lots of carnivores and their herbivores, the two populations may be in trouble if the animals are social but cannot readily form groups.
Fryxell said that, for example, managers of South African national parks sometimes bore holes to create watering spots for animals, an activity that spreads wildlife out and may keep them from grouping. Road-building or disruptions by tourists may do the same.
The research was also funded by the Natural Sciences and Engineering Research Council of Canada.
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