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Predators increase the health of prey populations, but reduce their size

For many years, biologists argued that top predators such as lions, wolves, or cheetahs can benefit prey herds by targeting the weakest and slowest animals, and thus providing better opportunities for the healthy, strong animals to reproduce – an assumption formalized in 2003 as the “healthy herd hypothesis.” 

While this hypothesis has been often used to suggest that manipulating predator numbers to protect prey could be a useful conservation strategy, hard evidence supporting it has been scarce, and recently, many of its assumptions and predictions have been questioned. 

To test the healthy herd hypothesis, a team of researchers led by the University of Michigan (U-M) has conducted an experiment on a predator-prey-parasite system consisting of predatory fly larvae that feed on the water flea Daphnia dentifera, which hosts a virulent fungal parasite (Metschnikowia bicuspidate). 

According to the healthy herd hypothesis, increasing the density of the predator in this three-organism study would help eliminate the fungal pathogen and increase the overall health of the flea populations. 

However, the experiment revealed that, although at the highest levels, predation managed to completely eliminate the fungal pathogen, it also dramatically reduced Daphnia population sizes – an outcome not supporting the healthy herd hypothesis.

“The appeal of the healthy herds hypothesis lies in the alignment of multiple conservation goals – simultaneous conservation of predators, reduction of parasitism, and protection of vulnerable populations – as well as the potential to reduce spillover risk to other populations, including humans,” said senior author Meghan Duffy, an aquatic and disease ecologist at U-M. “But even when predators reduce disease in their prey populations, that does not necessarily lead to increased prey population size, as our study shows.”

Other well-known historical examples of attempts to apply the healthy herd hypothesis that have gone wrong include the culling of badgers in the United Kingdom in order to reduce the incidence of bovine tuberculosis in livestock or the culling of bats to reduce the spread of rabies in domestic dogs or wildlife (badgers and bats being the wildlife reservoirs of bovine tuberculosis and rabies, respectively). In both cases, these strategies failed to succeed, and the diseases continued to spread in other animal populations.

“Unless we develop a more comprehensive understanding of when and how predators influence disease, management strategies that propose to reintroduce or augment predator populations could backfire,” explained study lead author Laura Lopez, a former postdoctoral researcher in Professor Duffy’s laboratory, who now works as a Research Vaccine Safety Officer at the National Center for Immunization Research and Surveillance in Australia.

“If your primary concern is the overall population size of a vulnerable animal species, then adding high levels of predation that eliminate disease could be detrimental. Interestingly, intermediate predation levels reduced parasitism in our study without incurring a cost in terms of overall prey density. Any management decisions would need to weigh the potential costs and benefits associated with increasing predation,” Duffy concluded.

The study is published in the journal Ecology.

By Andrei Ionescu, Staff Writer

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