The Conservation Science Blog

In a new study published in the journal Conservation Biology (link), researchers have found that long-term prospects for recovery of gray wolves in the western US may hinge on wolves being able to successfully disperse between widely-separated populations. While previous recovery efforts for wolves and other endangered species have acknowledged the importance of such connectivity, this study is perhaps the first times that detailed genetic and habitat data for any species have been used to project exactly how many dispersers are needed to sustain genetic health, and what areas offer the best prospects for dispersing wolves to move through. Although wolf recovery has achieved notable successes in areas such as Yellowstone National Park, the study’s findings suggest that long-term recovery may depend on overcoming barriers to dispersal in areas where wolf habitat is intermixed with ranchland and human settlements. “This study is the first time that scientists have taken a detailed look at what biology suggests is needed for long-term wolf recovery. Our findings imply that we can’t restore formerly widely-distributed species like the wolf to isolated populations in a few parks and expect them to remain genetically healthy”, said Dr. Carlos Carroll of the Klamath Center for Conservation Research.

The research group led by Dr. Carroll included two scientists from the Mexican Wolf Recovery Team. The Team was convened in 2010 to draft a recovery plan for the subspecies, which at only 75 individuals in the wild is among the most endangered mammals in North America. To support the Recovery Team effort, the scientists analyzed information on the pedigrees of wild and captive Mexican wolves, and forecast the potential future for Mexican wolves if their population remained isolated or was connected to other populations. They found that as isolated populations became increasingly inbred, the number of wolves in each litter declined, leading many populations to go extinct. Projections were made using a computer simulation model called Vortex developed by Dr. Robert Lacy of the Chicago Zoological Society (link). The researchers then used methods first developed to design electrical circuits (link) to map which areas of habitat held the best prospects for allowing dispersing wolves to survive and reach another population.

The study’s findings have implications for the current US Fish and Wildlife proposal to delist (remove federal protections for) wolves across the western US, outside of a single population of Mexican wolves in Arizona and New Mexico. Although the study is now completed, the Mexican Wolf Recovery Plan which spurred the research effort is on hold, after the last scheduled recovery team meeting in June 2012 was cancelled at short notice, a development that has been attributed to qualms about the political implications of the scientists’ findings (link).

A lot of media attention continues to be focused on the question of whether and how wolves trigger trophic cascades in ecosystems, by suppression of herbivory by ungulates and consequent release of vegetation and species such as birds that are dependent on the vegetation for their habitat needs. Trophic cascades can be caused by numeric effects (declines in ungulate populations), behavioral effects (prey foraging differently and avoiding areas of high predation risk), or a combination of the two. Behavioral or non-consumptive effects can be linked to numeric effects when altered behavior leads to poorer nutrition and lower pregnancy rates.

This last effect has been proposed as a cause of elk declines in the Greater Yellowstone Ecosystem, but a new paper by Arthur Middleton and coauthors in Ecology Letters challenges that hypothesis, and finds that changes in elk behavior due to encounters with wolves have little effect on elk body fat or pregnancy rate, probably because elk encounter wolves infrequently (about every 9 days) in this area. This leaves open the possibility that wolves are contributing to elk population declines directly via predation. This last point has at times been missed by the media, resulting in headlines such as “wolves not to blame for elk decline”.

Another paper by the same research team found that a factor contributing to elk population declines in Yellowstone National Park was the decline in native cutthroat trout in Yellowstone Lake after nonnative lake trout were introduced. Because spawning cutthroat trout, but not lake trout, form an important food source for grizzly bears, the invasive speceis indirectly caused grizzly bears to shift their diet towards increased predation on elk calves, contributing to a decline in elk that had previously been attributed primarily to wolf predation (figure below).

While most research of wolf-induced trophic cascades has taken place in Yellowstone, two new papers test the trophic cascade hypothesis in the northcentral US and Poland. Both take a correlative approach that compares vegetation in areas with vs. without wolves. In Wisconsin, Ramana Callan and coauthors found that species richness of both forbs and shrubs was significantly higher in areas with high wolf use. This supports the hypothesis that wolves, by reducing the intensity of browsing by white-tailed deer, are reversing the biotic impoverishment of understory plant communities caused by decades of overabundant deer populations. Similar contrasts between areas of high and low wolf use were found by DPJ Kuijper and coauthors in Poland, where browsing intensity of tree saplings was lower inside wolf core areas. At a finer scale within wolf core areas, sites with more coarse-woody debris, which is an impediment to escape from wolf predation, had even lower browsing rates, supporting the conclusion that at least a portion of the effects on vegetation are behaviorally-mediated rather than solely due to lower numbers of ungulates.

From Middleton et al. 2013

An interesting new paper by Brad McRae and colleagues has been published in the journal PLoS One. Entitled “Where to Restore Ecological Connectivity? Detecting Barriers and Quantifying Restoration Benefits” (download link), the research uses techniques to identify connectivity barriers that are similar to those used in microchip design, where simulated voltage levels reveal areas with strong voltage gradients where electrical connectivity must be enhanced. Software to implement the method is freely available (download link).