New insights into responses of Yellowstone wolves to environmental changes

The study, which was partially funded by the National Science Foundation, appears in the Dec. 2, 2011 issue of Science.

Part of the Yellowstone Wolf Project, researchers tracked changes in various characteristics of wolves living in the national park between 1998 and 2009. They found some tracked characteristics--such as population size--are related to population ecology, while other tracked characteristics--such as coat color--are genetically determined through evolution.

The project also involved using a new model to compare data collected on Yellowstone wolf characteristics to environmental conditions through the years covered by the study. Researchers defined conditions in the park during each year of the study along a continuum from "good years" to "bad years"--with good years more favorable to wolf survival than bad years.

Tim Coulson of Imperial College London, the study's lead author, explains, "The novelty of the new model is that it looks at how the frequencies of changes in environmental conditions along the 'good to bad' year continuum simultaneously impact many wolf characteristics."

Study results indicate:

• Environmental changes will inevitably generate simultaneous ecological and evolutionary responses in the Yellowstone wolves.

• Changes in mean environment conditions will impact the size of the Yellowstone wolf population more than will changes in the variability of environmental conditions.

• A single environmental change may impact various wolf characteristics differently, depending on which particular aspects of wolf biology it impacts.

Researchers say to understand their conclusions, suppose environmental conditions in a "good year" helped increase the population size of Yellowstone wolves by increasing their survival rates. Also, suppose that a grey coat color would confer a survival advantage to wolves. Then, under those particular "good" conditions, an increase in the size of the wolf population would be expected to produce an increase in the prevalence of grey coats among the wolves.

By contrast, suppose that certain environmental conditions in a "good year" helped increase the population size of Yellowstone wolves by increasing the availability of their prey. Because the availability of prey and coat color are not related to one another, under those particular "good" conditions, an increase in the size of the wolf population would not be expected to produce an increase in the prevalence of grey coats among the wolves.

Coulson says increasing the specificity of the model's predictions requires collecting more data on the ecological and evolutionary responses of Yellowstone's wolves to various environmental conditions and on the relationships of these responses to one another.

As part of this effort, the Yellowstone Wolf Project research team currently is studying the differential impacts of various environmental changes on ecological and evolutionary characteristics of Yellowstone wolves during various stages of their life cycles. The team also is working to identify the types of environmental conditions--such as the sizes of various populations of prey species and the amount and residence time of snow on the ground--that define good, bad and intermediary years for wolves.

The researchers hope once the methods developed through this study are refined, they may be applicable to other types of species, such as insects or crop pests, that live in other types of ecosystems. What's more, Coulson suggests that these methods may ultimately help answer questions about human populations. As just one example, the methods developed through this study might ultimately be used to help predict the impacts of the ongoing obesity epidemic on survival and fertility rates and the resulting influence of those variables on the growth rate of selected human populations.

The National Science Foundation provided funding to all of this paper's co-authors: Daniel R. MacNulty of the University of Minnesota at St Paul, Daniel Stahler of the National Park Service, Bridgett vonHoldt of the University of California at Irvine, Robert K. Wayne of the University of California at Los Angeles and Douglas Smith of the National Park Service.