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More is not always better: Too much high-quality habitat can cause as much trouble as too little

Apr. 12, 2016

Writer: Beth Gavrilles, bethgav@uga.edu

Contact: Daniel Becker, dbecker@uga.edu

Providing food for wildlife—whether as a management tool for a species in decline or a desire to attract birds for backyard viewing—can have unintended consequences, according to a recent study by ecologists at the University of Georgia. They found that putting out too much food across a landscape can promote the spread of disease and may end up reducing the very populations the food is intended to help.

Researchers Daniel Becker, a doctoral student, and associate research scientist Richard Hall, both in the Odum School of Ecology, developed a model to explore the effects of supplemental feeding on wildlife populations and infectious disease dynamics. Their study, published in Theoretical Ecology, has practical implications for the conservation of imperiled species, for wildlife enthusiasts, and for public health.

Local populations of wildlife species inhabit patches of suitable habitat within a larger landscape. If the quality of a habitat patch is poor, that local population can die out. If too many local populations die out, the entire metapopulation—a network of local populations connected by individuals moving from one to another—may become extinct.

If, on the other hand, the patch quality is high, the local population is more likely to persist longer and reproduce more. One way to improve patch quality is to provide additional food.

“In the case of provisioning for conservation, the thought is that it’s necessary to improve habitat quality in at least some of the patches in order to allow the species to persist,” said Hall.

This strategy has been used to help imperiled species such as the Iberian lynx in Spain, where conservation managers set up feeding stations during periods of food scarcity in an effort to improve survival and reproduction rates.

“The more patches you can improve, the more of those patches you expect to be occupied, so that metapopulation is going to be more robust and less prone to extinction,” Hall said. “But when you add a harmful pathogen, you get a plot twist.”

The model showed that in the presence of a virulent pathogen, having too many high quality patches allowed disease to build up in local populations and spread throughout the entire metapopulation. Once enough patches were improved to allow disease to spread, further increasing the number of high quality patches actually reduced the number of patches occupied by the wildlife species in question.

“It turns out that having some low-quality patches could be important, because when a pathogen comes into a low-quality patch with some colonizing animal from another patch, it can’t establish and maintain itself,” said Becker. “The pathogen winks out, and can’t continue to spread.”

Becker and Hall found that provisioning also affects disease spread in cases where animals capitalize on food put out recreationally, as with backyard bird feeders, or inadvertently, such as in agricultural fields or unsecured trash cans.  

“You can increase the number of occupied patches in those situations, but again that comes with this downside of allowing pathogen invasion and in some extreme circumstances you can end up with infection being everywhere,” said Hall.

“What our model shows is that in these cases you only have to improve a small proportion of the habitat to get this phenomenon where the pathogen can invade, persist, and sometimes spread throughout the landscape,” said Becker. “Especially for species in urban areas, improving patch quality can increase the chance that the pathogen is present everywhere and can spill over into human populations and other wildlife that we might care about.”

To prevent such outcomes, Becker and Hall’s research suggests that an intermediate number of high quality patches is ideal.

“It’s sort of a balance,” said Hall. “Maintaining a mixture of high- and low-quality patches can reduce the risk of unoccupied patches being colonized by infected hosts and prevent pathogen spread.”

Becker and Hall, along with a number of colleagues at UGA, are continuing to explore the effects of resource provisioning on disease dynamics. Hall is part of a team of UGA scientists funded by the National Science Foundation to investigate how recreational feeding of white ibis influences their exposure to pathogens of concern to human and wildlife health, such as Salmonella. Becker recently received funding from the NSF to study how the expansion of livestock into vampire bat habitat in Peru is affecting the spread of rabies virus. He has also organized a symposium on provisioning and wildlife disease for the upcoming annual meeting of the Ecological Society of America.

“The fact that provisioning in some cases can be detrimental to the systems by facilitating pathogen invasion is an important finding that needs to be looked into more,” said Hall.

The study, Heterogeneity in patch quality buffers metapopulations from pathogen impacts, was supported by the National Science Foundation, the ARCS Foundation, and the James S. McDonnell Foundation.

For more information about the Odum School of Ecology, see www.ecology.uga.edu.

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