High in the Rocky Mountains of Colorado, mustard plants slumber through the winter in snow-covered meadows. When spring finally reveals these hardy perennials, the plants reach for the sky, unveiling tiny pink or white flowers.
This annual rebirth is timed to the snowmelt, and as warmer temperatures have moved melting earlier and earlier in the year, this particular mustard, known as Drummond’s rock cress (Boechera stricta), has followed. The herb’s trumpetlike flowers now bloom about 13 days earlier than they did four decades ago. For a plant that flowers for at most 30 days a year, says University of South Carolina plant ecologist Jill Anderson, “that’s an enormous change.”
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Drummond’s rock cress also responded to warming temperatures around 20,000 years ago, pushing its way north and up the sides of mountains after glaciers began retreating. In a relatively short time, geologically speaking, this little flower spread and adapted to a wide variety of landscapes and elevations.
But with climate expected to change 10 to 100 times faster than in the recent geological past, Anderson wonders if the little plant can adjust again. Last September and October, she began an experiment at the Rocky Mountain Biological Lab in Colorado, collecting thousands of seeds and small mustard plants from up and down the mountain and replanting them at different elevations. As spring approached in April, Anderson’s assistant began shoveling snow from the surface of half the gardens to simulate the earlier snowmelts expected with climate change; the team will repeat the process over the next several years. Anderson wants to know if the plants have the genetic capacity to deal with the environmental conditions of the future.
Concerns about the impact of a changing climate go beyond this single flowering plant, of course. Scientists want to know if the world’s organisms will be able to move to new habitats or adapt to new conditions fast enough to cope with the warmer temperatures, earlier springs, altered precipitation patterns and acidifying oceans that will come with climate change.
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Most ecologists and evolutionary biologists say that they’re not ready to make sweeping generalizations about what might survive. They will say, however, that the survivors will probably use multiple tactics — such as migration and adaptation — to persist in a rapidly changing world.
Hundreds of species are already moving toward the poles, up the sides of mountains and down to deeper ocean depths (SN: 6/30/12, p. 16). But often there’s no suitable habitat for species to relocate, or there’s no room, says evolutionary biologist Ary Hoffmann of the University of Melbourne in Australia. Migration takes time and requires open corridors, which are scarce in nations jammed with cities, agricultural fields and roads. And then there are physical limits to how fast species can migrate.
For species that can’t move, adapting to new conditions will have to be part of their survival. Even those that can move will have to adapt, since relocating will require a change of relationships. In a new location, a plant could encounter a fungal disease to which it has no prior resistance. Some will adapt fast enough, while others will buy time, for example, with flexible behaviors — adjusting when to give birth or what food to eat.
Identifying how species might change in response to shifts in the environment is more than just an intellectual enterprise. Scientists and conservationists won’t be able to save every species threatened by climate change. But they may be able to use the knowledge of how organisms adapt to help some important ones — such as plants that feed many animals — as well as crops and other species that humans depend on for survival.
Evolution on overdrive
The conventional view of evolution was that it progresses slowly, over the course of thousands to millions of years. “Natural selection will always act with extreme slowness,” Charles Darwin wrote in his 1859 book On the Origin of Species.
But Darwin was studying a world that moved at a much more leisurely pace than today’s. Perhaps more important, he didn’t have the tools that allow scientists to see minute changes in DNA. And humans are now driving organisms to adapt to new surroundings and new lifestyles — to evolve — faster than Darwin ever suspected possible.
Climate change is creating even more of these opportunities. The first concrete evidence of global warming altering natural selection and resulting in an evolutionary response came from tawny owls (Strix aluco) in Finland. In a study published in Nature Communications in 2011, scientists tied climate change to a genetic change in plumage color, which may help the birds survive.
The owls come in two colors, pale gray and reddish-brown. Until recently, selection favored the pale gray shade; those owls were more likely to survive in the snowy Finland landscape. The reddish-brown color persisted, though, because the responsible gene is dominant — when a brown owl mates with a gray bird, about half of their offspring will be brown.
But ornithologists tracking Finland’s owl population have noticed a change over the last few decades: a steady increase in the proportion of reddish-brown owls coinciding with a decrease in snow depth during the country’s milder winters. With less snow and warmer temperatures, more of the reddish-brown birds have been able to survive, mate and pass on that dominant gene, ultimately resulting in more brown owls.
Adaptation in nature is rarely so straightforward, however. To adapt to climate change, a species needs two characteristics. Some members must carry the genetic capacity for dealing with the new conditions. And they must be able to pass those genes to future generations. This makes species that have shorter life spans and lots of offspring more likely to evolve quickly.
Morgan Kelly, a marine ecologist at Louisiana State University in Baton Rouge, finds lessons in two marine invertebrates: tide pool copepods and sea urchins. The copepod Tigriopus californicus would appear to be well adapted to a wide range of water temperatures since it is found along the western coast of North America from Alaska to Mexico. Its various populations should therefore have a lot of genetic variability for natural selection to act upon, which would help the shrimplike copepods adapt in response to warming.
When Kelly was a graduate student at the University of California, Davis, she and her colleagues attempted to force the tiny invertebrates to evolve. The researchers gathered copepods from eight coastal locations and brought them into the laboratory, where they exposed the creatures to warmer water temperatures through up to 10 generations.
For the first half degree Celsius above what they were used to, the copepods tolerated the heat. Higher than that, the copepods died.
Although the species as a whole has a lot of genetic variation, each local population is only well adapted to the narrow range of temperatures it experiences, Kelly and her colleagues concluded in 2012 in Proceedings of the Royal Society B. For the species to adapt to warmer waters, there would need to be intermingling between populations, so that copepods from warmer regions could pass on the genes that let them live in warmer waters to copepods from cooler places. That doesn’t happen, however, because the copepods don’t live well outside of their tide pool homes; even the young don’t travel very far.
The floating larvae of purple sea urchins (Strongylocentrotus purpuratus), however, can travel for hundreds of kilometers. That may help the spiky animals adapt to another aspect of climate change, ocean acidification, Kelly says.
Rising acidity from increasing atmospheric carbon dioxide undermines the chemical reactions that sea urchins and many other marine organisms rely on to build their skeletons and other structures with calcium carbonate. Sea urchin larvae tend to be smaller and fewer reach adulthood when water pH is lower (acidity is stronger).
But some parts of the ocean are more acidic than others, and sea urchins can be found in waters that span a wide range of pH. Kelly and Jacqueline Padilla-Gamiño, while working in Gretchen Hofmann’s lab at the University of California, Santa Barbara, collected sea urchins from two sites — one with higher acidity and one lower — and bred them in the lab. They found that the urchins’ ability to live in more acidic waters was a genetic traitthat could be inherited. Like the copepods, they have a genetic capacity to deal with the changes expected in the future. But unlike the copepods, urchin populations intermingle, so acidity-tolerant genes could move from one location to another. That should help the species adapt to acidifying waters in the future, Kelly says.
“We know that species are going to evolve. That’s a given, with both temperature and pH,” says Jennifer Sunday, a climate change ecologist at Simon Fraser University in Barnaby, British Columbia, who has found similar results for another sea urchin species. “We know that there’s variation out there. There [are] going to be changes in the genetic makeup of populations. And that’s evolution.”
But evolution isn’t the only way that species can change in the face of global warming. Some display an innate resilience to environmental changes that on the surface can look similar to genetic adaptation.
Across Eurasia, from Scandinavia to North Africa, and from Portugal to China, small yellow-and-black birds about the size of a house sparrow nest in the cavities of forest trees. These birds, called great tits (Parus major), have short life spans, about three years typically. They reproduce quickly and have lots of offspring. Each of these conditions allow for faster evolution.
In Wytham Woods, near Oxford, England, however, the great tits are handling environmental changes by changing their behavior.
The birds in these woods have been monitored since 1947, when Oxford researchers erected nesting boxes for the great tits to track the population from year to year. Eventually, researchers began recording the “half-fall date,” the date by which about half of the winter moth caterpillars — which the great tits feed to nestlings — had fallen to the ground to begin their next stage of life.The timing of when the birds lay their eggs is tied to thatcaterpillar half-fall date and to climate change, reported Oxford ornithologist Ben Sheldon and his colleagues in Science in 2008. The availability of caterpillars depends on spring temperatures. In warmer springs, there are more caterpillars earlier in the year. The birds, the researchers found, can shift the date they lay eggs so there will be lots of food for their nestlings. In warmer springs, they lay their eggs earlier.
This behavioral change was not due to genetics, because individual birds changed their behavior from year to year. If this were a genetic response, Sheldon says, the average laying date would be decreasing smoothly and the birds would be laying eggs earlier and earlier.
Instead, the birds rely on a phenomenon called phenotypic plasticity, the ability of an organism to change its behaviors or features in response to the environmental factors it experiences during its lifetime. This is how identical twins end up looking different as adults. It’s also how something like a bird or a mustard plant can appear to adapt to climate change without going through much, if any, underlying genetic change.
Phenotypic plasticity can actually be more useful than genetic change in some cases, Sheldon says. If the great tits, for example, were to lay their eggs slightly earlier every year, there would inevitably be mismatches between when nestlings are born and when food is available, since local weather varies and the region will certainly experience above- and below-average spring temperatures.
The ability to adjust behavior may provide some organisms with time to allow natural selection to act. “If you’re sensitive to the right parts of the environment,” Sheldon says, “then even long-lived, slow-reproducing species can survive.”
The path to survival for any species will likely be a complex mix of strategies. “The most successful species that are capable of surviving through climate change,” Anderson says, “migrate, adapt to new conditions and use phenotypic plasticity.”
Even if some species or populations appear to be coping with climate change for now, that doesn’t mean that they’re in the clear for the future. “Climate change is a moving target,” Anderson says. “Populations are going to be running behind it, trying to catch up.”
Many species just won’t be able to manage it. John Wiens, an evolutionary ecologist and herpetologist at the University of Arizona in Tucson, and Ignacio Quintero, a graduate student at Yale University, recently looked at past evolution in 540 vertebrate species, including amphibians, birds, mammals and reptiles. The researchers compared sister species — two species that diverged from a recent common ancestor — and calculated how long it took them to evolve to fit into their current environmental habitats.
Based on past rates of evolution, most vertebrate species won’t be able to evolve fast enough to adapt to the climatic changes expected over the next century, Wiens and Quintero concluded in Ecology Letters in 2013. To keep up with the changing climate, most species would have to evolve 10,000 to 100,000 times faster than they have in the past.
Many will be able to use a combination of adaptation and migration to persist, Wiens says, but “we may have lots of extinctions and declines before we ever get to adapting to climate change.”
Assisted migrationSome scientists, though, are beginning to think about how they can actually help species along. “If you’re trying to conserve species, you may have to think about intervention in a radical way,” says Melbourne’s Hoffmann.
Researchers may be able to identify not only the most threatened organisms but also the ones most important for conserving, such as the keystone species that keep ecosystems working. These are species like sea otters that keep sea urchins in check and prevent them from destroying kelp forests, and prairie dogs, whose burrows provide homes for many other critters. What’s more, Hoffmann says, scientists may even be able to assist in the process of adaptive evolution, by transplanting individuals with desired traits from one part of a species range into another, providing diversity that natural selection can act upon.
Researchers from the British Columbia Ministry of Forests are testing the idea. They’ve taken seeds from trees in the United States and Canada and planted them in new sites, hoping their actions will better prepare forests for a warmer future. Others have succeeded with similar efforts that are unrelated to climate. After the Florida panther population dropped to fewer than 30 cats in the 1990s, eight female pumas were shipped in from Texas. The cats were from a different subspecies, but they provided enough genetic variation to boost the Florida population, which was suffering from inbreeding. Today, there are some 100 to 160 adult Florida panthers.
This idea of shuffling genes between populations and even species in the name of conservation is becoming more acceptable. It “might seem a little bit out there, but certainly there are plenty of people thinking about it,” Hoffmann says. “To secure the future, when we know there’s this massive change in climate happening … we know we have to do something fairly dramatic.”