Climate Change Offers Opportunities for “Transformative” Restoration

In the Southwest, a plant’s citizenship status often determines its fate. Conservationists work to keep invasive species at bay with chainsaws and chemicals even as climate change begins to turn ecosystems into jigsaw puzzles. As global temperatures rise and precipitation patterns shift, native and exotic species alike head for new locations.

Bethany Bradley, a biogeographer at the University of Massachusetts, has a novel suggestion: Make climate change an ally in our efforts to restore the damage caused by invasive species.

In a 2009 study in Global Change Biology, Bradley examined the distribution of five problematic invasive plants in the West. Her research predicts how some unwanted exotics will shift their ranges during the next century, opening up unique opportunities for restoration.

The study predicts that yellow starthistle and salt cedar trees, two invasive species often blamed for monopolizing water resources, are likely to expand their ranges. Cheatgrass will shift northward into Idaho, Montana and Wyoming, relinquishing its hold on southern Nevada and Utah. Similarly, spotted knapweed will march to higher elevations. Leafy spurge may contract its range, although Bradley notes that the study focused on the U.S. and did not examine possible expansion into Canada.  

That’s both good and bad news for conservationists. In regions where invasive species will shrink their ranges, a unique opportunity for restoration appears. But the native species that historically existed in those regions will likely pull up roots, too. In the Great Basin, for example, native sagebrush will head north along with exotic cheatgrass because it thrives in a similar climate.

Bradley points out that the word restoration implies recreating conditions that historically existed on a landscape. That may no longer be possible, as climate change has already begun reshuffling ecosystems. A study published in Science this August calculated that plant and animal species worldwide are moving to higher latitudes at a rate of 11 meters per decade, and up mountain slopes at a slightly slower rate, responding much quicker to shifts in climate than previously expected.

“If you take that into account, you’re embarking on a fool’s errand, putting something back that’s not going to survive,” Bradley said. Instead she suggests sneaking new cards into the deck by replacing the retreating invasive species with entirely new plants—preferably ones indigenous to the broader region and capable of supporting wildlife habitat—to fill the gap that fleeing invasives leave behind. 

 How would this type of "transformative restoration" work?

Scientists can map the distribution of certain invasive species with remote sensing data. In visible light, plants appear dark-colored. In near-infrared wavelengths, they shine brightly against the background. Light alone, however, doesn't offer enough information to separate different species. For that, Bradley employs a trick that relies on phenology, the science of how climate guides plants through seasonal and multiyear rhythms, like repeating patterns in a complex ballroom dance.

Invasive species often stand out from natives with distinctly different footwork; for example, by greening up earlier or dropping their leaves a few weeks later. Bradley found that cheatgrass, a Eurasia exotic that dominates rangelands, reacted more strongly than native species to El Niño events, the warm surge of water in the southern Pacific that brings an extra dose of rainfall to the Southwest about every five years. Cheatgrass takes advantage of the additional moisture to grow rapidly and produce more seed.

Bradley compared wet and dry years to create a distribution map of cheatgrass and estimate the range of temperature and precipitation that allows it to thrive. She compared this data with climate change scenarios, a technique called bioclimatic envelope modeling. The science involves some uncertainty. Most climate models agree on rising temperatures in the West, but predictions for precipitation are much less clear—and precipitation is the major indicator for where certain species can survive.

“People who do restoration and management are constantly dealing with certain levels of uncertainty, so it’s not an insurmountable problem,” Bradley said. “It’s just that there’s a range of possible futures.” 

Creating accurate maps, however, might turn out to be the easy part. Once ecologists identify regions ideal for transformative restoration, they need to identify a suitable replacement species. In the southern Great Basin, sagebrush isn’t likely to return to its old stomping grounds after cheatgrass disappears. Bradley suggests considering creosote bush or white bursage as substitutes, both natives of the nearby Mojave Desert. 

Bradley's proposal for transformative restoration, outlined in a 2009 Restoration Ecology article, is both intriguing and worrisome. Choosing the wrong replacement species could make the cure worse than the disease—at best, waste the time and money of environmental managers, and at worst, unleash another harmful invader on the landscape. And the window for this type of restoration is small. Red brome, an exotic spreading in the Mojave Desert, is already marching north as the climate warms and might fill the open niche. ­

“This all assumes a lot of human control and knowledge about ecosystems,” Bradley admits. “But if you’re restoring an ecosystem from a previously invaded state you don’t have as much to lose.”  

Bradley’s proposal recognizes a fact that ecologists have just begun to grapple with: Whether we wish it or not, Southwestern landscapes are already transforming. Limited budgets and the risks involved with this kind of tinkering may mean that we leave the transformation entirely in nature’s hands. Some ecologists suggest that exotics like the salt cedar tree can even have a beneficial role to play in the landscape, such as providing wildlife habitat and erosion control, if the conditions that supported natives no longer exist (see Mark Davis’ commentary in Nature, June 2011).

All this argues for adaptive management, a technique in which resource managers return year after year to monitor, experiment and alter their plans based on the best available science. Restoration for the sake of history may no longer be possible, but managers and scientists can still work together for the health of the new ecosystems now emerging.