Spatio-temporal dynamics in the response of woodland caribou and moose to the passage of grey wolf

  • Predators impact prey populations not only by consuming individuals, but also by altering their behaviours. These nonlethal effects can influence food web properties as much as lethal effects. The mechanisms of nonlethal effects include chronic and temporary anti-predator behaviours, the nature of which depends on the spatial dynamics of predators and the range over which prey perceive risk.
  • The relation between chronic and ephemeral responses to risk determines predator–prey interactions, with consequences that can ripple across the food web. Nonetheless, few studies have quantified the spatio-temporal scales over which prey respond to predation threat, and how this response varies with habitat features.
  • We evaluated the reaction of radio-collared caribou and moose to the passage of radio-collared wolves, by considering changes in movement characteristics during winter and summer. We used an optimization algorithm to identify the rate at which the impact of prior passage of wolves decreases over time and with the predator’s distance.
  • The spatial and temporal scales of anti-predator responses varied with prey species and season. Caribou and moose displayed four types of behaviour following the passage of wolves: lack of response, increased selection of safe land cover types, decreased selection of risky cover types and increased selection of food-rich forest stands. For example, moose increased their avoidance of open conifer stands with lichen in summer, which are selected by wolves in this season. Also in winter, caribou increased their selection of conifer stands with lichen for nearly 10 days following a wolf’s passage. This stronger selection for food-rich patches could indicate that the recent passage of wolves informs caribou on the current predator distribution and reveals the rate at which this information become less reliable over time.
  • Caribou and moose used anti-predator responses that combine both long- and short-term behavioural adjustments. The spatial game between wolves and their prey involves complex and nonlinear mechanisms that vary between species and seasons. A comprehensive assessment of risk effects on ecosystem dynamics thus requires the characterization of chronic and temporary anti-predator behaviours.

Latombe, G., Fortin, D., Parrott, L. (2013), Spatio-temporal dynamics in the response of woodland caribou and moose to the passage of grey wolf. Journal of Animal Ecology. doi: 10.1111/1365-2656.12108

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How the relationship between grasshoppers and spiders affect the carbon cycle

A new study shows that the predator-prey relationship can affect the flow of carbon through an ecosystem. This previously unmeasured influence on the environment may offer a new way of looking at biodiversity management and carbon storage for climate change mitigation.

The study, conducted by researchers at the Yale School of Forestry & Environmental Studies, comes out the week of June 17 in the Proceedings of the National Academy of Sciences. It looks at how the relationship between grasshoppers and spiders — herbivores and predators in the study’s food chain — affects the movement of carbon through a wild grassland. The study shows that predators can cause increased carbon retention in plants in the ecosystem.

“We’re discovering that predators are having important effects on shaping the make-up of ecosystems,” says Oswald Schmitz, professor of ecology and one of the co-authors of the study. “But we’ve not really spent a lot of time measuring how that translates into other functions like nutrient cycling and recycling.”

Carbon, the basic building block of all organic tissue, moves through the food chain at varying speeds depending on whether it’s being consumed or being stored in the bodies of plants. However, this pathway is seldom looked at in terms of specific animal responses like fear from predation.

The researchers manipulated the food chains of grassland ecosystem to see how the levels of carbon would change over time. Schmitz and his team created several controlled ecosystems: Some that contained only native grasses and herbs, others that had plants and an herbivore grasshopper, and some others that had plants and herbivores along with a carnivore spider species — all three tiers of the food chain. In addition, a form of traceable carbon dioxide was injected into sample cages covered with Plexiglas, which allowed the team to track the carbon levels by periodically taking leaf, root, and dead animal samples.

The study found that the presence of spiders drove up the rate of carbon uptake by the plants by about 1.4 times more than when just grasshoppers were present and by 1.2 more times than when no animals were present. It also found that the pattern of carbon storage in the plants changed when both herbivores and carnivores were present. When predators were around, the grasshoppers damaged grasses less because they were afraid of being eaten by the spiders, which caused them to be alert and eat less frequently and eat more herbs instead of grass as a preference. Herbivore consumption can cause plants to reduce their photosynthesis and increase respiration, which lets out carbon in the form of carbon dioxide. The predators reverse this physiological process by reducing herbivore damage to plants, leading to greater carbon uptake and storage.
Moreover, the grasses stored more carbon in their roots after the predators alleviated herbivore impacts on plants. These stress responses, then, caused both the plants and the herbivores to change their behaviors and change the composition of their local environment.

The findings have significance for biodiversity conservation and ecosystem management, according to Schmitz. Although the study was carried out on a small scale, the principles that were learned could inform practices done in much larger areas. Places such as the Alaskan wilderness, for example, are home to animals that have the same predator-and-prey dynamics that drive the carbon cycle, and so protecting lands and storing carbon could be linked at the same time. Appreciating the role of predators is also important now because top predators are declining at rates faster than that of many other species in global trends of biodiversity loss, as the study points out.

“It’s going to force some thinking about the vital roles of animals in regulating carbon,” said Schmitz, noting that the UN’s body of scientific experts who study climate change don’t consider these multiplier effects in their models. “People are arguing for a paradigm change.”


CONTACT: Oswald Schmitz 203-432-5110 or

CITATION: Michael S. Strickland, Dror Hawlena, Aspen Reese, Mark A. Bradford, and Oswald. J. Schmitz. Trophic cascade alters ecosystem carbon exchange. PNAS (2013)

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University Of Wyoming Study: Wolf Harassment Has Little Impact On Elk

The mere presence of wolves, previously shown to affect the behavior of elk in the greater Yellowstone ecosystem, is not potent enough to reduce the body condition and reproductive rates of female elk, according to new research published Tuesday.

The research, led by recent University of Wyoming Ph.D. graduate Arthur Middleton, provides the most comprehensive evidence to date refuting the idea that wolves are capable of reducing elk calf recruitment indirectly through predation risk. The findings were published in the scientific journalEcology Letters.

“Elk respond to wolves, but less strongly and less frequently than we thought,” says Middleton, who for three years closely followed the Clarks Fork elk herd west of Cody, Wyoming, along with the wolf packs that prey on it. “We found that wolves influence elk behavior, but the responses were subtle and — over the course of winter — did not reduce body fat or pregnancy. Our work indicates that the effect of wolves on elk populations is limited to direct predation and doesn’t include so-called harassment, stress and fear, which have been proposed as additional indirect effects on prey populations.”

Working as part of the Wyoming Cooperative Fish and Wildlife Research Unit — a U.S. Geological Survey program housed at UW in cooperation with the Wyoming Game and Fish Department — Middleton and colleagues used state-of-the-art GPS collars and firsthand observation to track the interactions of the Clarks Fork herd with wolves from the Sunlight, Hoodoo, Beartooth and Absaroka wolf packs in 2008, 2009 and 2010.

The detailed movement data on both wolves and elk allowed the researchers to identify each time one of the collared elk encountered a collared wolf. The elk herd, one of several migratory herds in the greater Yellowstone area, spends summers in Yellowstone National Park and moves into the Sunlight Basin during winter. The researchers also recaptured GPS-collared elk at the end of winter and the end of summer each year to assess their annual fat dynamics and pregnancy rates using ultrasonography.

The research found that when wolves approached within 1 kilometer (a little over a half-mile), elk increased their rates of movement, displacement and vigilance. However, the behaviors only lasted about 24 hours and didn’t significantly reduce elk foraging or force elk into poor habitats. And such encounters with wolves took place at a rate of only one in nine days on average for the migratory elk in the Clarks Fork herd — the maximum was once every four days.

The key finding of the study is that even though elk varied widely in their encounters with wolves, those that encountered wolves frequently were not less fat — or any less likely to be pregnant — than those that rarely bumped into the predators. This finding differs from some previous studies that indicated wolves influence elk behavior strongly enough to contribute to regionwide declines in calf production.

“Our research was unique in that we tracked wolves while also monitoring the movements, foraging behavior, body fat and pregnancy of the elk they hunted,” says Middleton, who worked under Matt Kauffman, head of the Wyoming Cooperative Fish and Wildlife Research Unit and assistant professor in UW’s Department of Zoology and Physiology. “This approach, essentially connecting the dots from wolf movements all the way to elk behavior and nutrition, revealed that elk respond to wolves too weakly and too infrequently for those behaviors to carry nutritional costs.”

The study casts additional doubt on the idea that wolf reintroduction has caused what scientists call a “behaviorally mediated trophic cascade” in the greater Yellowstone ecosystem — prompting elk to alter their foraging behavior or avoid risky areas, thereby allowing aspen and willows to recover from overbrowsing. These new findings are consistent with studies by Kauffman and others showing little or no evidence for cascading effects caused by purported broad-scale shifts in elk habitat use or foraging behavior in response to wolves.

Both Middleton and Kauffman have pointed out in their studies that the wide-ranging hunting strategy of wolves, which differs from the tactics of a stalking predator, might be the reason that elk responses are too weak and inconsistent to alter their foraging patterns or nutritional gain.

“A key factor in the ability of predators to cause these sorts of cascading effects is the ways in which they hunt and kill their prey,” says Kauffman, who initiated the study in 2006 along with Game and Fish Department wildlife biologist Doug McWhirter and U.S. Fish and Wildlife Service wolf manager Mike Jimenez. “Wolves are coursing predators that chase down their prey, as opposed to stalking predators that lurk in concealed areas. We are learning that coursing predators are less likely to induce strong behavioral responses in their prey, and this new work suggests that the coursing hunting mode of wolves may constrain both their ability to influence prey condition and cause cascading ecological effects on plants.”

Kauffman adds that the concept of non-consumptive effects of predators on prey has been well tested in small, well-controlled environments — often involving insect predators and prey — but that researchers are still sorting out how it all works in the large landscapes occupied by species such as wolves and elk.

Calf production has been declining among migratory elk herds in the greater Yellowstone area, but wolves may not be the primary culprit, Middleton says. For the Clarks Fork herd at least, other recent research findings point to high rates of bear predation and reduced habitat quality due to drought — both on summer ranges largely inside Yellowstone — as being the more likely cause of declines in elk calf numbers.

“The recovery of large carnivores, particularly grizzly bears, has brought major increases in predation on newborn elk during early summer,” wrote Middleton, who added that the region has experienced severe drought and warmer temperatures in recent years. “These effects of drought and predation could largely explain both low pregnancy and declining calf production among elk of the Yellowstone region.”

Kauffman, a USGS scientist, is one of the co-authors of the report published today — along with a collaborative team of researchers from the Game and Fish Department, the Fish and Wildlife Service, the National Council for Air and Stream Improvement, Yellowstone National Park and UW.

To read the article summarizing the research in Ecology Letters, visit this site.

For more information, Middleton, now a post-doctoral fellow at the Yale School of Forestry and Environmental Studies, may be reached at 307-460-0880 or