Ecologists have been active in their response to the publishing of a new study in the June issue of the Ecological Society of America’s journal Ecology. In it, Arthur Middleton and fellow researchers from the University of Wyoming, the Wyoming Game and Fish Department and the US Geological Survey discussed the Clarks Fork herd residing on crowded winter grounds outside of Cody, Wyoming. Their findings contradict those of a 1988 study that has long been regarded as a seminal work in the understanding of the benefit of migrations for animals that routinely undertake foraging treks.
In John Fryxell‘s 1988 paper, it was explained why migratory hoofed beasts, like the Clarks Fork elk, have outnumbered their non-migratory cousins by as much as an order of magnitude. When used for approximating purposes, an order of magnitude typically refers to one set outnumbering another by ten times. His reasoning for the migrants’ dominant survivability rates over the more sedentary herds focused on three key points. First, the migrant herd uses a much larger area. Second, they are able to make a more efficient use of resources. And finally, they have a diminished vulnerability to predators. He explains how, on their own, each point could not necessarily lead to a diminished mortality rate, but when present in conjunction, they each contribute to such an outcome.
Much more to the point, and as an example, Fryxell contends migration allows for animals to take advantage of seasonal vegetation while, at the same time, shelter themselves from predators and the elements. The migrants of the Clarks Fork herd, in seeking the new vegetation located on higher terrain, are able to leave their predators behind. In the spring, predators are less mobile due to the necessity of attending to the needs of their newborns.
The Clarks Fork herd is comprised of approximately 4000 elk that, each spring, leave their winter grounds for the snowmelt-fed greening grass located in the highlands of the Absaroka Mountains. Their journey is becoming something of a rarity for modern migratory animals as their route is unimpeded by roads, fences, metropolitan areas and other human-built barriers. Middleton’s findings, that the benefits of migration are less than the costs should afford, focuses on the end-point of the migration which happens to lie inside the border of Yellowstone National Park.
The team contends the elk herd has, over the last few decades, been returning to the winter grounds outside Cody with fewer and fewer calves. This is in stark contrast to the herds that stay year round in the Cody area. According to Middleton’s study, the decline in the number of surviving calves of the migratory elk must be attributed to both climate change and a marked increase in the number of predators that prey on newborn calves.
Marco Festa-Bianchet of the University of Sherbrooke recently edited a forum of five working groups of ecologists. These groups, while praising the work of Middleton and his team, commented on the data, at times challenging the interpretation. The above mentioned Fryxell was a contributing member to the forum.
Middleton and his team are not the first ecologists to report on new challenges facing migratory creatures and how those challenges are tied directly to changes in habitat resulting from human development and climate change. However, they do believe theirs to be a novel case study due to a conflagration of habitat and public policy.
Middleton’s study addresses how drought, in combination with the return of predators to Yellowstone, is directly responsible for the low pregnancy and calf survival rates of these elk. One predator, the grizzly bear, is cited specifically.
In response to criticism offered by the forum, Middleton stated, “Many of the forum commentaries discuss the implications of our work for management and conservation of large carnivores and their prey in Yellowstone, especially wolves. However, a persistent focus on the impact of re-introduced wolves among scientists, wildlife managers, and the public misses key roles of grizzly bears and severe drought in limiting elk populations.”
The study claims the summer months have been hotter and dryer in the summer range of the migratory Clarks Fork elk. They state satellite imagery has shown the length of time spring vegetation requires to “green-up” has been shortened by almost a full month over the last 21 years. The spring feeding is a critical time for the female elk to gain the amount of fat needed for reproduction. Over the same 21 years, wolves were reintroduced into Yellowstone National Park. The team claims wolf and bear populations have been growing. A US Fish and Wildlife survey has shown an increase in the number of bears in Yellowstone. However, in a recent articlepublished on redOrbit, the methodology of the survey was called into question. Dan Doak and Kerry Cutler claim the flawed methodology may have led to an overestimation of the growth of the grizzly bear population.
Allowing the premise the predatory populations have been increasing, the study claims this increased ecological pressure is a direct result of human choices. This addresses the human manipulation of predators and the harsh drought conditions linked in study after study to human-caused climate change.
Forum members Chris Wilmers and Taal Levi, in addressing the field irrigation within the Sunlight Basin Wildlife Habitat Management Area, state the non-migratory elk avail themselves of the grazing land more heavily during spans of pronounced drought. Due to their migratory behavior, the other elk herds are unable to take advantage of those hydrated lands.
“I think Middleton has an intriguing idea, and it might be what’s happening. We offer another hypothesis that also fits the data that they have. He says it’s climate change on the summer range and more predators on the summer range. I think it’s because there is irrigation that provides the sedentary elk with food. And I think it’s also that there is predator control outside the park,” said Wilmers.
Wilmers states the population boon within Yellowstone Park has coincided with an intense increase in predator control measures outside the park. “My hypothesis is that in that crucial winter period, the migrants are coming down to range that the resident elk have already been feeding on all summer, and now they are competing for in the winter,” Wilmers says. Of course, to discern between the subtleties of these competing hypotheses would require real-world testing.
According to Wilmers, this could be achieved through direct experimentation. He offers, as an example, stopping irrigation in the resident rangelands or irrigating the mountain highlands where the migratory elk travel to. He also claims the introduction of predator control within Yellowstone National Park or the ceasing of control measures outside the park would be important variable studies. The concept, however, of ‘world as laboratory’ can run into opposition on both the public policy and public opinion fronts. Wilmer’s ecologically invasive manipulations would likely draw ire from legislators, wildlife advocacy organizations and special interest groups.
John Fryxell and Robert Holt, in their forum submission, addressed the difficulty faced by ecologists in isolating underlying causes for the observable pattern changes. In their model, they employed an expression of the elements affecting the Yellowstone elk. As they state, the innate instinct to migrate is a delicate balance of environment, population saturation, predation and genetic predisposition. As they claim, if food scarcity is behind the decline in new population, it may soon be impossible for the elk to live in the park. However, if increased predation during migration is to blame, the elk may eventually split into two resident populations, both inside and outside the park. While Fryxell and Holt reserve judgment on the causes in the pattern change, they claim, as a result of rapid worldwide changes in the next few decades, changing migration patterns will become more and more common.
Another forum contributor, Atle Mysterud of the University of Oslo, chose to look at how a changing climate soon falls out of sync with both seasonal events and animal life cycles. He states the migratory behavior of the elk is malleable in relation to the timing of a spring green-up. However, the time period necessary for calf gestation and birthing is relatively fixed. Mysterud also commented on the potential non-lethal effects of predators on the elk’s behavior along with the “human shield” against predators for the resident elk that grazed on lands near agricultural and urban areas outside Yellowstone National Park.
Another forum contribution by ecologists from Oxford University was more critical of Middleton and his team. Their paper, “Will central Wyoming elk stop migrating to Yellowstone, and should we care?” might well be regarded as contentious, if only for the title alone. In their paper, they claim the trends in both vegetation and predator differential across the park boundary are compelling. However, they go on to say Middleton’s data cannot confirm that it was these two factors that led to the change in the demographic of the elk population.
“We don’t wish to sound critical of the huge effort they have put in. Nonetheless, despite their hard work, their data on elk condition and pregnancy rates come from a relatively small number of animals collected over only a relatively short time period. Given this, they are restricted to conducting a few piecemeal analyses and telling some compelling stories. But the problem with this approach is that it is easy to construct very many compelling stories. When that happens, the scientific literature can become opinionated and sometimes adversarial.”
To this point, the Oxford ecologists cited studies of the effects of predators on other elk herds in Yellowstone National Park. They found the existing data, sensitive to minimal environmental variation, are complicated. Additionally, they state wolves play little role in the survival of calves. Ultimately, they advocated for increased investment in broader, long-term, large-scale data collection initiatives.
The call for a longer-term data collection period was echoed by Jean-Michel Gaillard of the University of Lyon. He believes deriving data of consistent timescale and quality is necessary to garner a better understanding of the migratory tactics of female elk. Gaillard called for lifetime monitoring of individuals within the elk population.
Jack Massey of the Oxford group ended their forum submission cautioning that the Middleton paper, like many before that have mentioned both wolves and elk, might be co-opted by groups aiming to forward their own political agenda. For instance, those that advocate on behalf of large elk herds might claim this paper is evidence the elk population is being needlessly destroyed by wolves. Massey and his peers claim the causes of elk population decline are not so clear or so simple.
“As ever with such debate, whether we should care all depends on one’s view on what our wilderness should look like.” concluded Massey and his colleagues. Therefore, the answer to the Oxford group’s title question is not one that can be arrived at through science. Rather it will require an examination of an individual community’s values.