Riparian willow dynamics in Yellowstone – Associate Editor commentary

Large carnivores have succumbed to human pressure worldwide.  They have been hunted to near or complete local extinction or their food sources have been reduced drastically.  A recent review1 shows their continuing decline throughout the world.  The review also highlighted the direct and indirect roles that large carnivores play in structuring trophic cascades, and the sometimes unexpected consequences of reducing numbers of these apex predators.  For example, the large reduction in numbers of lions and leopards in many parts of sub-Saharan Africa has resulted in increasing numbers of olive baboons – a mesopredator – that not only prey upon ungulates, but also affect human welfare because they raid crops, which also forces families to take children out of school to help guard fields2.

Coexistence between humans and large carnivores was never an easy accommodation, even when the human population was much lower.  Restoring large carnivores as a means of reinstating trophic cascades meets resistance because of perceived threats to human livelihood.  A clear example of this is in the western United States where grey wolves were hunted to extinction by 1960 because of perceived threats to livestock.  Their reintroduction to Yellowstone National Park in the mid-1990s was controversial and remains so: there is growing pressure to hunt wolves, which occupy only a small fraction of their former range in the United States3.

Studies in Yellowstone National Park since wolves were reintroduced have highlighted their role in trophic cascades1.  Wolves have direct effects by competing with another predator (coyote) and by preying upon elk, which are abundant in the Park.  This in turn has indirect effects; for example reductions in the number of elk has resulted in reduced browsing pressure on aspen trees1,4. However, attribution of changes in vegetation in Yellowstone National Park solely to the reintroduced wolves is controversial4.

A new study5 adds to a view of complex interactions that drive the regeneration of forests in Yellowstone.  Kristin Marshall and co-authors examined the regeneration of riparian willow forests in the northern portion of the Park during a 30-year period, about half of which was before wolves were reintroduced to the Park.  Herbivory by elk, with their numbers affected by wolves, was one of the predictors of willow regeneration.  However, climatic and landscape factors were also important.  Willows had greatest height growth rates, hence a greater ability to escape the “browse trap”6, if they were in parts of the landscape where moisture was least likely to be limiting, and there were episodes of willow recruitment that resulted from a series of years with above-average precipitation.  A nuanced view seems to be emerging from this well-studied system that the trophic cascades associated with wolves, both in terms of their direct and indirect effects, need to take account of the the interactive effects of other predators1, other mammalian browsers5, climatic, landscape, disturbances such as fire, and historic influences.

Peter Bellingham
Associate Editor, Journal of Ecology

References

1Ripple WJ, Estes JA, Beschta RL, Wilmers CC, Ritchie EG, Hebbelwhite M, Berger J, Elmhagen B. Letnic M, Nelson MP, Schmitz OJ, Smith DW, Wallach AD, Wirsing AJ 2014 Status and ecological effects of the world’s largest carnivores. Science 343, 1241484

2Prugh LR, Stoner CJ, Epps CW, Bean WT, Ripple WJ, Laliberté AS, Brashares JS 2009 The rise of the mesopredator. BioScience 59, 779–791.

3Morell V 2014 Science behind plan to ease wolf protection is flawed, panel says. Science 343, 719.

4Marris E 2014 Legend of the wolf. Nature 507, 158–160.

5Marshall KN, Cooper DJ, Hobbs NT 2014 Interactions among herbivory, climate, topography and plant age shape riparian willow dynamics in northern Yellowstone National Park, USA. Journal of Ecology, doi: 10.1111/1365-2745.12225

6Staver AC, Bond WJ 2014 Is there a ‘browse trap’? Dynamics of herbivore impacts on trees and grasses in an African savannah. Journal of Ecology, doi: 10.1111/1365-2745.12230

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