The cover article from this week’s PNAS has important implications for how we plan for, and deal with, climate change. Post and Pedersen report that the way an arctic plant community changes in response to warming depends heavily on the presence of large herbivores [$-a], like muskoxen and caribou.
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Previously, it was thought that one effect of global climate change would be for woody shrubs and dwarf trees to become more common in arctic and subarctic plant communities. This increase in woody plants could trap more atmospheric carbon and increase the albedo of the land – meaning more heat could be reflected back into space. Both of which effects might help slow a warming global climate.
However, Post and Pedersen show that large herbivores can reduce this shift in community composition. In huge five-year study, they set up experimental plots from which caribou and muskoxen were either excluded by fencing, or not excluded. Within each class of plot, they also placed 1.5-m “open-topped chambers” (OTCs) made of fiberglass – basically, cylinders that help trap solar heat, warming the ground inside. In the fenced plots, the plant communities inside the OTCs shifted toward more woody species; but in the unfenced plots, where large herbivores could reach in and graze inside the warmed cylinders, plant communities didn’t develop greater cover by woody species.
Now, it’s not surprising that large herbivores can have a profound effect on the plant species that grow in their grazing land. Where I come from, in the northeast U.S., large swaths of forest have been dramatically altered [$-a] by a population explosion of white-tailed deer freed from their natural predators. But Post and Pedersen have drawn a connection between this effect and the ways in which natural communities may respond to the most dramatic environmental change in human history. It just goes to show what a massively complex system we humans are tinkering with, and how little we know about what that tinkering may ultimately do.
E. Post, C. Pedersen (2008). Opposing plant community responses to warming with and without herbivores PNAS, 105 (34), 12353-8 DOI: 10.1073/pnas.0802421105
F.L. Russell, D.B. Zippin, N.L. Fowler (2001). Effects of white-tailed deer (Odocoileus virginianus) on plants, plant populations and communities: A review The American Midland Naturalist, 146 (1), 1-26 DOI: 10.1674/0003-0031(2001)146[0001:EOWTDO]2.0.CO;2