Poo's clues: Moose droppings indicate Isle Royale ecosystem health

Michigan Technological University

IMAGE: An adult moose must eat approximately 40 pounds of vegetation per day just to keep itself going. Yet despite their need to consume large volumes of food every day, moose... view more 

Credit: Sarah Hoy/Michigan Tech

Given the choice between ice cream and vegetables, for many people it'll be the ice cream. But sometimes it depends on the situation. If you'd eaten ice cream every day for a week, you might prefer the salad. Human preferences for different foods often depend on what's common fare and what's rare.
For non-human animals, like moose, the situation is equally complicated. An adult moose must eat approximately 40 pounds of vegetation per day just to keep itself going. Yet despite their need to consume large volumes of food every day, moose do not eat everything they come across. Instead, moose are considerably more selective than is obvious when deciding which plant species to eat.
Sarah Hoy, assistant research professor, and John Vucetich, distinguished professor, in the School of Forest Resources and Environmental Science at Michigan Technological University, in collaboration with scientists from the United States Geological Survey (USGS) and the University of Wyoming, have developed a method to analyze why moose choose to eat what they do, how their choices change in the presence of predation and how moose diets actually affect the stability of entire ecosystems.
The results appear in "Negative frequency-dependent foraging behaviour in a generalist herbivore (Alces alces) and its stabilizing influence on food web dynamics" published in the Journal of Animal Ecology.
"The research shows how what you would think is a simple decision -- what to eat -- is a complex process that depends on many environmental factors, such as how common food types are, how likely a moose is to be killed by a predator and how difficult deep snow makes it to move around and find food," Hoy said. "The moose eat upwards of 40 pounds each day. You'd think if you had such dietary requirements you'd stuff your face with anything you can find, but that doesn't appear to be the case."
"Something one might consider small, even trivial -- what a moose chooses to eat -- appears to have a stabilizing effect on the whole food web." -- Sarah Hoy, assistant research professor
The advantage to moose of taking the time to seek out and eat plant species that are relatively rare is a well-balanced diet, which requires nutrients that might be found only in those rarer plants. Many plants also contain chemicals that are toxic to moose in large quantities, which means that moose can ingest them only in limited amounts. However, a moose whose palate is too discerning pays a price; a cost of focusing too much on the rare plants is the time spent on the search. Additionally, a moose in search of a delicacy might be a more likely target for a wolf.
"Moose have a choice: eat the rare stuff at risk of not eating enough food overall, or eat what is most common in the forest at risk of missing out on a well-balanced diet," Hoy said. "We hadn't really known how moose manage that choice until now."
Polarized Poop and Mathematics By analyzing a decade's worth of moose droppings under a polarized light microscope -- a technique known as microhistology, which is further explored on Michigan Tech's Unscripted science and research blog -- to determine what exactly moose are eating on Isle Royale, the researchers concluded that moose preferred to eat what was relatively rare in their home range. If balsam fir is rare, they prefer it; if balsam fir is common, they show less preference -- even passing it up in many cases to find a less common plant. However, moose appeared to become less fussy eaters in years when the risk of being killed by wolves was high and in years when deep snow likely made it more difficult for moose to move around and find food.
By combining the evidence of years of meticulous fieldwork with a mathematical model representing the Isle Royale system, Hoy and her fellow scientists were able to draw conclusions about why it's important that moose are choosy eaters in the context of the ecosystem.
Enter Rongsong Liu, associate professor of mathematics at the University of Wyoming. Liu built a mathematical food chain model that she said, "demonstrates that the selective foraging strategies of moose can have an important stabilizing effect on community dynamics and provide a useful framework for assessing the influence of the other aspects of foraging behavior on community stability."
The model further illuminates the strength of the connections across three trophic levels of the Isle Royale landscape: vegetation, herbivore, carnivore.
"The mathematical model is a way to test how important the patterns in moose behavior we observed are for the community as a whole," Hoy said. "Moose may change their diet in response to a harsh winter or a high risk of being killed by wolves, but how important is that to the ecosystem?"
Don DeAngelis, a research ecologist for the USGS, has worked with Liu to develop and analyze models of herbivores of the boreal forest, including moose. One factor influencing what a moose prefers to eat is the aforementioned toxins in certain plants and how those toxins can effectively skew moose diets toward better overall balance.
"The data implied the moose were deliberately limiting their intake of coniferous vegetation, and also that this effect was related to the level of other environmental conditions, probably the level of predation by wolves," DeAngelis said. "My role was to work with Liu to translate the way that we think wolves, moose and forest vegetation all interact with each other into mathematical equations, and then use these equations to build a model that reflects the way that the Isle Royale ecosystem works."
Ecological theory indicates that simple food chains, such as that of Isle Royale National Park, are prone to extinction. Where there is a single predator -- wolves -- and a single herbivore -- moose, which eats two basic kinds of plants: deciduous and conifer trees -- there can be erratic population fluxes. However, Hoy, Vucetich and colleagues discovered that the foraging behavior of the moose might be one factor that favors the persistence of wolves, moose and the different tree species in the food chain.
This distinctive combination of theoretical models and field observations from the predator-prey study on Isle Royale provides ecologists with more insight about how and why populations tend to persist where basic theories of ecology otherwise suggest that they should not.

Fearing cougars more than wolves, Yellowstone elk manage threats from both predators

IMAGE: Wolves are often implicated as the top predator affecting prey populations. New research from Utah State University indicates that cougars are actually the main predator influencing the movement of elk... view more 

Credit: National Park Service

Wolves are charismatic, conspicuous, and easy to single out as the top predator affecting populations of elk, deer, and other prey animals. However, a new study has found that the secretive cougar is actually the main predator influencing the movement of elk across the winter range of northern Yellowstone National Park.
The study highlights that where prey live with more than one predator species, attention to one predator that ignores the role of another may lead to misunderstandings about the impact of predators on prey populations and ecosystems. It also offers new insight into how prey can use differences in hunting behavior among predators to maintain safety from all predators simultaneously.
Utah State University researchers Michel Kohl and Dan MacNulty co-led the study, published in Ecology Letters, with Toni Ruth (Hornocker Wildlife Institute and Wildlife Conservation Society), Matt Metz (University of Montana), Dan Stahler, Doug Smith, and P.J. White (Yellowstone National Park). Their work was supported, in part, by the National Science Foundation, Ford Foundation, and Utah State University as part of Kohl's doctoral research. The study was based on long-term data from the Park's wolf and elk monitoring programs and Ruth's cougar research, which is detailed in a forthcoming book from the University Press of Colorado.
The team revisited global positioning system (GPS) data from 27 radio-collared elk that had been collected in 2001-2004 when numbers of wolves and cougars were highest. Kohl and MacNulty combined the elk GPS data with information on the daily activity patterns of GPS-collared cougars and wolves and the locations of cougar- and wolf-killed elk to test if elk avoided these predators by selecting for 'vacant hunting domains', places and times where and when neither predator was likely to kill elk.
"Cougars hunted mainly in forested, rugged areas at night, whereas wolves hunted mainly in grassy, flat areas during morning and at dusk" said Kohl, lead author of the paper and now an assistant professor at the Warnell School of Forestry and Natural Resources at the University of Georgia in Athens. "Elk sidestepped both cougars and wolves by selecting for areas outside these high-risk domains, namely forested, rugged areas during daylight when cougars were resting, and grassy, flat areas at night when wolves were snoozing".
Recognizing that cougars and wolves hunted in different places and at different times allowed the researchers to see how elk could simultaneously minimize threats from both predators. "Had we ignored the fact that these predators were on different schedules, we would have concluded, incorrectly, that avoiding one predator necessarily increased exposure to the other," said MacNulty, who is an associate professor in USU's Department of Wildland Resources and Ecology Center. "Movement out of the grassy, flat areas and into the forested, rugged areas to avoid wolves did not result in greater risk from cougars and vice versa because these predators were active at different times of the day".
Despite the compatibility of elk spatial responses to cougars and wolves, Ruth, who is now executive director of the Salmon Valley Stewardship in Salmon, Idaho, cautioned that "some adult elk still end up on the cougar and wolf menu, with those in poor condition during winter being most at risk".
Nevertheless, "the findings help explain why we observe wolves, cougars, and elk all coexisting and thriving on the Yellowstone landscape" said Stahler, who leads the current study of cougars in the Park. He notes that the ability of elk to coexist with wolves and cougars is consistent with their "long, shared evolutionary history".
More surprising, however, was that cougars, not wolves, exerted the most pressure on elk habitat selection. "Wolves are often the presumed or blamed predator for any change in a prey population, numerical or behavioral," said Smith, who leads the Park's wolf program. "Our research shows that this is not necessarily true, and that other large predators in addition to wolves need to be considered."
"Despite the fact that most prey species live in habitats with multiple predators, the majority of research on predator-prey interactions focuses on a single predator species," added Betsy von Holle, program director for the National Science Foundation's Division of Environmental Biology. "The novelty of this research is the simultaneous study of multiple predator species, revealing the complexity of predator avoidance behavior by the prey."