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Although debate continues to rage over the wisdom of reintroducing wolves into Yellowstone National Park, the 16-year-old program has been a boon to science, enabling biologists to better understand the interplay between predator and prey species and how that dance affects an entire ecosystem.
Now data from the Yellowstone Wolf Project has helped scientists improve models for predicting how plant and animal life will respond to warming climates. In a study published this month in the journal Science, a team that includes Utah State University wildlife ecologist Daniel MacNulty developed the "integral projection model" which fuses evolutionary and ecological perspectives to better understand species' vulnerability to environment changes.
The wolves' coat color, a genetic trait that has nothing to do with the environment, yielded valuable help in designing the new model.
"There is a presumption that evolution operates on long scales on the order of hundreds of thousands of years, while population size can change day-to-day. They are much different traditions," said MacNulty, an assistant professor of wildland resources. "We developed a quantitative tool that links those two disciplines and allows us to examine the impact of environmental change on the ecological and evolutionary characteristics in a population."
Among the study's key findings is that gradual, sustained environmental change has a greater effect on an ecosystem's species than fluctuating changes.
"If we change the total environment, such as temperature, we change a whole suite of characteristics for a species, including viability, fertility, population size, body size and generation length," said a news release quoting co-author Robert Wayne, a UCLA professor of ecology and evolutionary biology. "Traditionally, we have studied just a few ecological parameters at a time, like how much food there is or how the environment will change over time, and how that relates to population size. Here, we are analyzing everything at once."
The team applied the model to Yellowstone wolves, possibly the most closely monitored wild mammal population in the world.
Since wolves' reintroduction, the park's elk numbers have declined. In response to stalking wolves, the stately ungulates now avoid open riparian areas, allowing willow to return to the streams and beaver numbers to rebound. Meanwhile bison, a prey animal that is more difficult and dangerous for wolves to take down, have increased.
Will a relative abundance of bison spur wolf evolution toward larger body size? asked MacNulty, whose past research explored group hunting behavior.
For the new study, researchers were interested in how the park's wolf population has changed since this keystone predator was re-introduced in 1995, paying particular attention to the fur color of individual wolves.
The idea was to examine the wolves' ecological responses, such as population size, in conjunction with genetic adaptations, such as coat color. They examined existing data gathered from tracking 280 radio-collar-equipped wolves in the park between 1998 and 2009, analyzing it for survival and reproductive success. This data was matched against the wolves' genetic make-up and their phenotypes — observable traits such as body size and coat color.
MacNulty's model helped make sense of the persistence of gray-coated wolves despite the fact that the gene for black coats was dominant, which means a wolf needs just one copy of the black-coat gene to have black fur. Researchers discovered that black wolves with two copies of the black-coat gene have much lower survival rates and sire fewer offspring than those with just one copy.
"It's having one copy that somehow confers an advantage. The fur color is collateral to something else," MacNulty said. "The beauty of it is, if you ask, 'Why do you have black fur?' it opens the door to a very interesting scientific question."
Scientists stressed that the new model isn't specific to wolves, but is extremely general.
"It can be applied to any population, from a polar bear to a wolf to a beetle, even plants," Wayne said. "We want to use this model to make predictions about populations that are in dire situations, as their environments will be changing quickly."
Lead author Tim Coulson said the new model also yields "a bit of a conundrum," illustrating the urgent need to gather more data if this new tool is to be useful.
"We now have a way to predict with unprecedented detail how populations will respond to environmental change," said Coulson, a biologist with the United Kingdom's Imperial College London. "But for most systems, we simply don't have sufficient information to confidently make predictions."
Co-authors include the National Park Service's Daniel Stahler and Doug Smith, who oversee the Yellowstone Wolf Project, and Bridgett vonHoldt of the University of California, Irvine. Partial funding came from the National Science Foundation.
Yellowstone Wolf Project
Since their reintroduction in 1995, Yellowstone National Park's wolves have helped scientists better understand how ecosystems work. A new study uses data from the Yellowstone Wolf Project to design a new comprehensive model for predicting how particular species, from big predators to bugs to plants, will respond to environmental change.