How Deer Influence Forest Diversity
It’s late fall, and once again Odocoileus virginianus, the American white-tailed deer, is on the minds of many people.
Farmers, who have completed their harvests, have a fresh appreciation of how much corn deer can eat. Hunters, eagerly awaiting the opening of the firearms hunting season, are daydreaming of trophy eight-point bucks. And motorists are remembering how careless deer can be at this time of year when they get near roads and highways.
Biology professor Tim Nuttle led a study on how increases in deer populations change forest composition. (Keith Boyer)
Deer have also been on the mind of IUP Biology professor Tim Nuttle, not so much because he is interested in deer, but because he’s interested in trees and birds and caterpillars. And they’re all connected, he said.
A few years ago, as Nuttle searched for local ecology research projects for his students, he talked with Scott Stoleson, a research wildlife biologist at the Forest Sciences Lab of the Northern Research Station of the U.S. Department of Agriculture’s Forest Service in Irvine, Pa. Nuttle commented that the density of birds in a particular section of the Allegheny National Forest seemed low.
“I think it’s because there’s so much black cherry,” Stoleson replied.
Nuttle, a Michigan native who’s been teaching at IUP since 2007, found that possibility interesting. “The back story of black cherry is that it’s becoming increasingly dominant in Allegheny hardwood forests,” he said. “There are a lot of factors, but one of them is that it’s avoided by deer. Deer much prefer other species to browse on.”
When there is a disturbance in a forest, whether a natural one, like a tornado, or a man-made one, like a logging operation, overstory trees are removed and deer have their choice of what to eat in the understory.
“They come into the young forest and they browse on plants and saplings they find most palatable,” Nuttle said. “What they leave behind is what is available to grow up into a new forest. So, the more deer you have, the more effect they have on creating a filter for what can grow up into the new forest. And because deer don’t like black cherry, when deer densities are very high, it’s the only thing that can grow.”
“The more deer you have, the more effect they have on creating a filter for what can grow up into the new forest.”
Nuttle formed a hypothesis for a new research project: Man, by choices and actions that increase deer populations, changes forest composition to trees that are less palatable to deer and to the insects that feed on trees. And that results in a smaller food supply for the birds that feed on insects.
A suitable research site was ready and waiting for Nuttle and his student assistants only a few hours from the IUP campus in the Allegheny National Forest.
In 1979, the Forest Sciences Lab started a ten-year experiment there in which enclosures ranging from thirty-two to sixty-four acres were built. Each enclosure was populated by one to four deer for target densities of ten, twenty, forty, and eighty deer per square mile. (One deer in sixty-four acres is the equivalent of ten deer per square mile.)
The deer populations in each enclosure were kept constant during the decade-long research period. “They [the researchers] were trying to look at densities of deer that were similar to what had occurred in the past and what occurs today,” Nuttle said.
Taking a closer look at the forest canopy were student Ethan Cullen, left, and Tim Nuttle. (Keith Boyer)
The researchers who initiated that study, which ended in 1990, were mainly interested in what the effects were on regenerating forests.
“But the neat thing is that these deer densities established a certain composition of forests that differed depending on how many deer there were,” Nuttle said. “And so I went back to these forests starting in 2008 to see if the changes the deer initiated were still evident in these roughly thirty-year-old forests, and what kinds of changes you might see.
“And what we found was, you have these lasting effects from deer on what’s going on in the canopy. There was something about deer in the past that is affecting the canopies today.”
The composition of the trees living there, certainly, has changed. “For example, the highest deer densities have pretty much 90 percent black cherries; lower deer densities have considerably more kinds of trees,” Nuttle said.
The research Nuttle and his team of students conducted is based on some concepts—like trophic cascade—that are likely unfamiliar to most non-biology majors.
“Trophic means nutrition. When one organism eats another, it gets energy from that,” Nuttle explained. “The bottom level is usually the plants. The things that eat plants, like herbivores, would be on the second trophic level, and things that eat herbivores, like carnivores, are on the third trophic level.
“The idea of a trophic cascade, and bottom-up and top-down effects, is that you have a food chain, and if you change something at the top—say you eliminate carnivores [such as wolves and cougars], as we have all over the eastern United States—that has a top-down effect on the food chain. If you eliminate wolves, that affects deer—mainly, deer would increase. And when deer increase, they exert increasing effects on plants.”
Reducing from the top of the chain results in flip-flopping effects going down, Nuttle explained. A negative effect on the carnivores has a positive effect on the herbivores, which has a negative effect on the plants.
“Humans are a trophic level above deer,” he continued. “We are affecting, directly and indirectly, the deer, in that people kill deer. And we make decisions about how many deer should be in the forests. And we’ve had the indirect effect in that we’ve eliminated the carnivores that eat deer. That’s a decision we’re making as a society, and this is having increased effects on plants. It’s good for the plants that aren’t eaten by deer and bad for the plants that are eaten by deer.”
An example of a bottom-up effect would be if something about the plants was changed, Nuttle said. “Then that affects what eats the plants and what eats the things that eat the plants.”
Unlike top-down effects, bottom-up effects are all in the same direction. “If you decrease the density of plants, you should decrease the density of herbivores, and then decrease the density of predators.”
Nuttle coined a new term, “trophic ricochet,” to capture the idea of the two interconnected food chains. “And they’re connected by the plants at the bottom,” he said. “So you’ve got the top-down effect that humans have had on deer and that deer have had on plants, and those then initiate bottom-up effects on the rest of the forest community that relies on those trees as the base of its food chain.
“So we’re investigating the changes in trees and the changes that has on insects, in particular, caterpillars, and the birds that are the main predators of caterpillars.”
“If you eliminate wolves, that affects deer—mainly, deer would increase. And when deer increase, they exert increasing effects on plants.”
What he has found is that the species of trees most preferred by deer seem to have the most caterpillars, and the most palatable caterpillars.
“Pin cherries, for example, are very negatively impacted by deer. It’s like deer candy,” Nuttle said. “When you have high deer densities, you basically have no pin cherries,” and pin cherries have exceptionally high densities of caterpillars.
“There are a lot of caterpillars on black cherries, too, but it seems they’re mostly species that the birds don’t like to eat. It’s also possible that black cherries have fewer leaves per tree for caterpillars to eat.” Fewer leaves for caterpillars in turn means less food for birds.
Determining which tree species various caterpillars eat was part of Nuttle’s study. (Keith Boyer)
Nuttle said the summer of 2011 was a very productive and successful field season for his research students. And in August he received notification of a grant of $197,354 from the National Science Foundation to continue the study for another two years.
“What we’re trying to do is figure out what about these plant species is affecting caterpillars and how those affect birds,” he said.
Ellen Yerger, an entomologist in IUP’s Biology Department; Todd Ristau, a research ecologist at the Forest Sciences Lab; and Stoleson have been collaborating with Nuttle in the research. “The scientists and the staff at the Forest Sciences Lab have been immensely supportive of the project,” Nuttle said.
So what’s the purpose of this research? Who will use the findings?
“When we make decisions about how many of any kind of species of wildlife to have, in this case, deer, we have to understand what the effects of those are on the rest of the environment that we care about,” Nuttle said. “What is the best number of deer to have out there? Is it forty per square mile? Is it twenty per square mile? And what does best mean? Best for whom? If you’re interested in birds, then you probably want fewer deer. If you’re interested in seeing a lot of deer, you’re probably interested in more deer. If you’re interested in hunting deer and getting a big buck, then probably what you want is some intermediate level of deer, so the deer are healthy and have enough to eat to grow into healthy animals.
“We’ve had the indirect effect in that we’ve eliminated the carnivores that eat deer. That’s a decision we’re making as a society, and this is having increased effects on plants.”
“The research contributes to our understanding of the effects of our decisions on a broader picture of our environment,” he said.
The new information may also help forest managers who make decisions about what trees are grown based on the value of those species as timber.
“Forest managers want to manage the forest for the best values they identify. But maybe they don’t have complete knowledge of the full value of some of these species,” Nuttle said. “From a forester’s standpoint, pin cherry is a trash tree. It takes up space and takes nutrients away from something else that is more valuable.”
But, as noted, pin cherries have high concentrations of caterpillars. “And so what we’re showing is that this is a previously unrecognized value for this trash tree,” he said. “It provides food for birds because it’s food for caterpillars.”
According to Nuttle, about twenty deer per square mile seems to be the optimal density of deer in terms of having a diverse forest. More than that seems to decrease growth and diversity of trees. “Probably at least 30 percent of eastern U.S. counties where deer live are above that density. Some states are more effective in keeping deer numbers down than others,” he said.
The findings of the research conducted by Nuttle and his students were published in the January 2011 edition of Ecosphere, an online-only journal of the Ecological Society of America that is open to public access.