Predators & Plants
Studies reveal how meat eaters impact vegetation
A coyote pounces gracefully and gulps down a montane vole.
A red-tailed hawk swoops, snatching its next meal — a ground squirrel — in its talons.
A stealthy weasel summarily dispatches and devours a Columbian ground squirrel.
Each of these carnivorous acts represents a relatively new way in which scientists are looking at the ecology of plants.
And these examples of predator-prey relationships also are elements of a groundbreaking study directed by UM plant ecologist John Maron in collaboration with research ecologist Dean Pearson of the U.S. Forest Service’s Rocky Mountain Research Station at UM.
The goal of the study, initiated in 2002 in the Blackfoot Valley of Western Montana, is to determine the indirect effects of top predators on plant communities.
Historically, says Maron, traditional plant ecology research focused on the effects of the physical environment (i.e. soil nutrients, moisture and climate), as well as competition between plants, on the diversity, distribution and productivity of plant communities.
“Those factors,” he says, “are clearly important, no question.”
However, he adds, it’s only been fairly recently that scientists have explored the idea that top predators could affect plant communities in ways that rivaled those traditional concepts.
“It’s a different way of thinking about plant ecology,” says Maron. “That’s why plant ecologists are so interested in it.”
Some of the best large-scale studies to demonstrate the effects of predators on plant communities examined aquatic systems, Maron says.
By adding or removing the primary predatory fish, he says, researchers observed dramatic changes in aquatic plant productivity in lakes. The experiments demonstrated a kind of chain reaction, called a trophic cascade, in which predator fish ate smaller fish, which fed on plankton, which fed on other organisms that ate algae.
Indirect effects of predators on plant communities, Maron explains, are much more difficult to study in a terrestrial environment. Most of the previous land-based research in the field has been done on a very small scale, examining the indirect impacts of predators such as spiders on plant communities.
“What we don’t know much about,” Maron says, “is the role of large predators on vertebrate prey and what effect that has on plant communities and, therefore, the strength of those indirect effects.”
The significance of the role predators play in plant ecology was driven home to him during his participation in a four-year study of the introduction of foxes on the Aleutian Islands of Alaska.
Historically, says Maron, the chain of islands was exploited by fur traders, who hunted the population of sea otters to near extinction. To replace the resource, Russian traders introduced foxes to most — but not all — of the Aleutian Islands around 100 years ago.
It’s well known by scientists that introductions of predators, especially on islands, often have devastating effects on the native prey of those closed environments.
“But,” Maron says, “we know less about the indirect effects of these introductions on biological systems, especially plants.”
Historically, the Aleutians supported rich and diverse breeding populations of seabirds, with as many as 29 different species. The birds fed on fish in the ocean and returned to the islands to rest, nest and feed young. The huge quantity of guano deposited by the birds on the islands provided nutrients to subsidize the plant community.
As in the case of most predator introductions around the world, says Maron, the arrival of foxes on the Aleutians wiped out the islands’ seabird population. In the last 15 or 20 years, the U.S. Fish and Wildlife Service has responded by removing foxes from the Aleutian Islands to restore the birds.
During three weeks each summer for four years, researchers — including Maron — studied the plant communities of islands with and without foxes.
“It was a big natural experiment,” he says. “One thing we found is that on the islands with foxes, you don’t have a lot of seabirds, and the plant community looks a lot different” than on the islands without foxes.
Islands without foxes had very productive plant life, he adds, dominated by extremely dense and tall grasses. Those with foxes had much lower plant productivity, though more diverse, dominated by low-lying forbs.
“But fundamentally,” says Maron, “they were very different plant communities. It was a pretty strong example of the indirect, cascading effects you get from the introduction of predators.”
Soon after his arrival at UM as a professor and researcher in the Division of Biological Sciences, Maron embarked on a similar study involving the indirect effects of predators on plant ecology.
And in the nearby Blackfoot Valley, he found an ideal natural laboratory.
“Two things about it are unique,” he says. “The first thing is that all the predators that have historically been there are still there. That’s not common in the continental U.S.
“The second thing that’s unique is that the plant community is very rich, and it’s not been heavily invaded. Exotics (non-native plants) are there, but in very low density. And the native grasslands are spectacular.”
In one of his test locations in the Blackfoot Valley, Maron adds, he has found 23 species of native bunchgrass.
Part of the motivation for his ongoing study there, he explains, is that “in the conservation world, there’s a lot of interest in putting predators back into the system. The rationale is to restore their ecological function.
“We want to know how strong their role is, especially in controlling small mammals and how that affects plant communities.”
To find the answers, Maron has created his own “islands” in the form of a series of one-hectare (100-by-100-meter square) enclosures at four widely separated test sites. Each site features three enclosures.
One enclosure at each site excludes predators — including coyotes, foxes, badgers, weasels, mountain lions and more, as well as a variety of raptors — by using specialized fencing with attached flashings, as well as overhead wires to repel birds of prey.
A second enclosure at each site excludes native ungulate herbivores — primarily deer and elk — but does not keep out predators. The idea is to separate and compare the effects of ungulates and predators on the vegetation.
A third enclosure — as a control — is open to predators, ungulates and rodents.
Embedded within each enclosure is a smaller plot that excludes rodents.
The study focuses on three of the area’s most common rodent herbivore species: Columbian ground squirrels, montane voles and deer mice.
The purpose of the small, embedded enclosures is to compare the vegetation inside and outside the plots to determine the effect of the rodents on the vegetation, and compare it to the strength of the effects of the predators.
“If the predators strongly control the rodents, you’d expect the rodents’ effect on the vegetation to be much smaller when predators have access to them,” explains Maron.
As part of the study, researchers visit the sites in winter to observe tracks to see what predators are visiting the test plots. In addition, motion cameras record the presence of predators. During the summer, researchers use traps to catch, weigh and measure rodents. The captured rodents are tagged, and, when recaptured, help the scientists calculate their population abundance.
As far as Maron knows, his Blackfoot Valley study is one of only a handful of large-scale experiments to manipulate large vertebrate predators and small mammal herbivores to examine long-term impacts on the plant community in a native terrestrial ecosystem.
While researchers have made some intriguing observations involving the study during the past six years, Maron says, definitive results will require more time.
“One of the challenges in this type of work,” he explains, “is that it’s a slow-moving system. The plants are slow-growing and long-lived. It can take a long time to go from seeds to plants. So it can take a long time to see effects. It’s only been in the last year or so that we’ve started to see some of the effects.”
Grants from the U.S. Department of Agriculture have provided the primary funding for the research project, along with additional federal funding from the Bureau of Land Management and the Forest Service. Most of the grant money is used to hire four full-time technicians to work in the field from May through August each year.
Maron says the USDA’s financial support of the project is related to the agency’s interest in ecosystems that humans are affecting through activities such as grazing, farming and logging, which influence the ways those ecosystems are managed.
A better understanding of those effects, says Maron, could allow resource managers to tailor their activities to be more in tune with the ecosystem and to interact with it more effectively.
There are many potential practical applications to his research, according to Maron.
“But really,” he says, “it’s about trying to understand how nature works.”
— By Daryl Gadbow
|Above: UM doctoral student Mary Bricker (holding hat) and an assistant inventory the vegetation in a Blackfoot Valley enclosure designed to keep out rodents, such as ground squirrels (left). Studies are examining predator and rodent impacts on plants.
|UM plant ecologist John Maron was part of a team that showed the extent introduced foxes changed vegetation on Alaska’s Aleutian Islands.