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Common Cannibals -- New Study Examines Role That Intraspecific Predation Plays In Determining Dynamics Of Population

While humans may find the thought of cannibalism unsettling, other species seem to eat their own kind quite readily. Once viewed as a laboratory artifact, species specific predation is now being reevaluated. A recent study conducted by scientists at the University of Calgary, and published in the December issue of Ecology, has shown a definitive link between cannibalism and population stabilization among flour beetles.

Researchers Hugues P. Benoit, Edward McCauley and John R. Post studied twenty-one populations of the flour beetle, Tribolium confusum, in the laboratory. Their research built on the theory that cannibalism within this species is most likely to occur between mobile and immobile life stages; adult beetles mainly consume eggs and pupae, while larvae consume eggs.

When the beetles were separated according to life stage and a fixed proportion of the various stages were removed (temporarily) from the population, the incidence of cannibalism was dramatically reduced and in some instances almost entirely eliminated. In the absence of cannibalism, the growth of the adult population approached exponential rates. When the densities of larger larvae were relatively high, cannibalism of eggs was also very high, causing a large decrease in egg numbers. This resulted in a smaller population of larvae as these eggs hatched. This smaller population did not have a large cannibalistic impact on the subsequent egg population, resulting in little mortality, and consequently a large population of larvae some time later.

"As a result the population densities of eggs and larvae will cycle up and down, out of phase with one another, through time," explained McCauley.

Conversely, when the beetles were left to their own devices and allowed to eat flour and one another freely, population numbers tended to stabilize.

Similar studies have been conducted on populations of other animals. Past research has indicated that size structured cannibalism among certain species of crabs, fish and copepods can result in skewed age distributions among their populations.

Other studies have also drawn conclusions about flour beetle cannibalism. But this most recent study is the first to empirically examine the cannibalistic interactions of the species throughout all the stages of its life cycle.

The flour beetle, in particular, presents an intriguing example for scientists because populations of this species tend to be demographically stable under natural conditions.

"The really interesting finding to us was that relatively simple mathematical equations could be used to predict, qualitatively, the dynamic effects of cannibalism for this flour beetle species," said Benoit. "These experiments provide a direct test of the role of cannibalism in a long-term study of population dynamics."

Ecological Society Of America. December 1998.

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