Ravenous Deer Might Not Destroy Biodiversity After All

Frequently cast as plant-gobbling, biodiversity-destroying villains, deer may actually play a vital role in making their forests more lush and vibrant.
Image Vicki SokolowskiFlickr
Image: Vicki Sokolowski/Flickr

Frequently cast as plant-gobbling, biodiversity-destroying villains, deer may actually play a vital role in making their forests more lush and vibrant.

Researchers who allowed white-tailed deer to feed on plots of saplings while keeping other plots off-limits found that the healthiest communities, where young trees were diverse and vigorous, grew where deer foraged.

It's a counterintuitive result, given the voracious reputation of deer — but perhaps it shouldn't be, given the interdependent elegance of ecology.

"In a world where there were no deer at all, we might lose some our plant diversity," said ecologist Susan Cook-Patton of the Smithsonian Environmental Research Center, co-author of an April 8 Proceedings of the Royal Society B study describing the findings.

In the experiment, conducted in a Maryland forest by Cook-Patton and fellow Smithsonian ecologist John Parker, the researchers planted 140 3'x3' plots of year-old saplings. Some contained one sapling apiece from 15 different tree species. Others contained 15 saplings, but of the same species.

The setup was designed to investigate a phenomenon previously observed in grasslands, where the presence of deer created a cascade of effects resulting in more-diverse plant communities.

More diversity — plants blooming or fruiting at different times, rooted deeply or in shallow ground, requiring more of one nutrient rather than another, and generally filling every available niche — tends to result in communities of life that are especially resilient and productive.

>'The whole is greater than the sum of the parts.'

"They interact with each other synergistically," said Cook-Patton. "The whole is greater than the sum of the parts."

In forests, however, deer tend to be viewed as enemies of diversity, obliterating young plants before they can grow. It's not an entirely unfair reputation: in some places, an absence of predators and proliferation of brushy habitat have produced population densities unprecedented in recent history.

Deer populations in the Smithsonian forest, however, were in tune with historical norms. And when the researchers counted and weighed their trees after three years, they found that the high-diversity plots actually did much better when deer had grazed in them.

According to Cook-Patton, that's because deer appetites prevented hardy, fast-growing species from proliferating. "Deer help maintain diversity by keeping competitive tree seedlings from becoming too dominant," said Cook-Patton.

Conversely, monoculture plots benefited most from being left undisturbed — hinting, perhaps, at the sort of winner-take-all dynamic that may eventually prevail if deer are completely excluded from an area.

"Seeing a positive effect was really surprising," said Parker. It also has a potentially thorny implication: often people try to protect habitat from browsing, but that may backfire. "If we protect plants from browsing, maybe we're eliminating the very factor that makes diversity work," he said.

Whether the findings hold in real-world settings remains to be tested, as do the impacts of deer across larger areas and longer time periods than the current study investigated.

"One of the big questions is about scale," said Parker, and one he hopes to answer. Recently his group finished planting an experimental forest containing 24,000 trees. It will produce results for decades to come.

In the meantime, the new findings exemplify nature's sometimes unpredictable complexity. "In almost all ecosystems, there are a small number of species that have a big effect, but it's often difficult to predict in advance what they are," said Emmett Duffy, a Virginia Institute of Marine Science ecologist.

"We have a general sense of the way things work," Parker said, "but in terms of nuance and complexity, we're still in the infancy of ecological study."