Life Altering

A new study shows humans cause the traits of other species to change at nearly twice the normal rate found in nature

March / April 2008

When faced with environmental change, countless numbers of species have adapted, allowing them to survive and, in some cases, thrive for millennia. Those without such resilience have not. But can species keep up with the modern changes wrought by humans?
For decades, scientists have studied threats to populations in an effort to advance their conservation. But only recently have they begun to consider how species might adapt to humans. Now for the first time, a team of biologists has quantified the rate and scale at which humans accelerate change in other species.

They have determined that humans are changing observable physical or behavioral traits in animals nearly two times faster than nature does.

As a result of their findings, the scientists — Michael Kinnison at the University of Maine and Andrew Hendry and Thomas Farrugia of McGill University — are calling for a reenvisioning of conservation biology, to include consideration of the changing nature of populations within the span of years rather than decades or centuries.

"Human influences are causing the features of animals to change much faster than what would happen in nature alone," Kinnison says. "In fact, we're changing the traits of animals almost twice as fast, and that gives us a lot to think about."

What is clear, Kinnison says, is that if we are concerned about these trait changes, we probably don't have the luxury of decades or centuries to deal with them.

"Our data suggest that changes seen in a few generations are often as large as those seen over hundreds," he says. "In some cases, we may be changing the face of life nearly as quickly as we are changing the environments on which life depends."

In an ever-growing database of trait studies, the researchers gathered more than 3,000 estimates of physical and behavioral changes occurring in recent times in wild species — from bugs to bighorn sheep — from around the world. They then compared the rates at which the traits of animal populations changed in one to 200 generations in response to either naturally occurring processes or human disturbances, such as harvesting (fishing, hunting), pollution and introduction of invasive species.

Their findings suggest that trait changes in animals can pick up the pace when exposed to human influences. When beneficial, these changes could help species persevere. However, the researchers caution that some of these changes may not be beneficial or sustainable over longer periods of human interference.

"The argument that observed changes in species are just isolated cases that can be brushed aside loses ground significantly when confronted by a pattern that emerges from the work of many scientists combined," says Kinnison, who, with his colleagues, published the findings in the journal Molecular Ecology. "It helps us to see the big picture."

Evolution is traditionally understood to be a life-altering process that is so glacially slow and gradual that only the ancient bones in the fossil record could prove that it even happens. But the science of evolution has undergone a dramatic evolution of its own in recent decades, providing ample evidence that it doesn't take millions or even thousands of years for animals to adapt to new environments. It's happening within our own lifetimes, in fact, at a pace swift enough that we're able to see life changing before our eyes.

Kinnison, an associate professor of biology, is at the forefront of the dynamic new discipline called contemporary evolution. He has witnessed evolution unfolding while researching guppy populations in the streams of Trinidad, chinook salmon introduced into the waters of New Zealand, and other fish species in Maine.

When Kinnison and Hendry first set out in the 1990s to build a database of rates of trait changes in animals through time and across generations, they discovered that evolution didn't play out exactly the way most people had always believed.

"We found that evolution is built upside down relative to most people's perceptions," Kinnison says. "Most people think it takes a long time for evolution to occur, but it's really buzzing along all around us, all the time. The fastest rates of change occur in the shortest time frames, but these changes often partly cancel out over longer periods. So while we can watch evolution in action, it will often appear slow when viewed over longer periods."

Take, for example, the Galapagos finches that inspired Darwin's early work on the origin of species. Modern research has found that in times of drought, when plant life is severely diminished and seeds become scarce, the little birds adapt by growing bigger beaks that can accommodate larger, harder and thornier seeds. In wetter periods, the finches' beaks get smaller as seeds become more abundant. Amazingly, these transformations can occur generation to generation.

"The beak size is going from big to small, back and forth, very quickly, following natural climate changes," Kinnison says.

Having amassed thousands of estimates of speedy and observable trait changes, Kinnison and Hendry then began to explore the twin influences that trigger this remarkable adaptive process: natural events, such as the weather patterns that alter beak size in Darwin's finches, and changes wrought by humans, such as the introduction of exotic species to native populations, hunting, fishing, pollution, urban sprawl and climate change.

"We looked at where nature was running the show and where humans were running the show," Kinnison says. "And where humans run the show, animal traits change almost twice as fast. Humans seem to be really stepping on the gas pedal."

The researchers also examined the biological mechanisms by which animals changed when human influences abruptly altered their worlds. Was classic evolution by natural selection — passing the most favorable, robust genes from one generation to the next — the primary adaptive method in most cases? Or was something called "phenotypic plasticity," the ability of organisms to change their physical and behavioral characteristics through existing physiological mechanisms, also important?

Their conclusion: phenotypic plasticity is an important component of these human induced changes.

"That says that animals might be having to use their whole bag of tricks to cope with human influence," says Kinnison, a New Hampshire native whose interest in aquatic ecology and cold-water fish like salmon, trout and Arctic charr led him to UMaine in 2002.

Snails living in Maine tidal pools provide a good example of this plasticity phenomenon. After humans accidentally introduced the European green crab into their midst, certain periwinkles soon began to grow thicker, harder shells to better withstand the alien predators' crusher claws.

To learn what prompted the defense mechanism, and how quickly it happened, a researcher at Northeastern University placed snails in a tank with green crabs, separating the creatures by a flow-through barrier. It turned out that the snails could detect the crabs' presence in the water — "smell" them, as it were — and so produced thicker shells in as little as three months in response to the risk.

"And when the snails grow in less crabby water, their shells are thinner," Kinnison says.

But not all changes are phenotypic plasticity. Classic evolution by natural selection is at work in many populations affected by humans. In the Rocky Mountains, researchers studying bighorn sheep have found that horn size in some populations has diminished over time as a result of selective hunting pressures. Because most jurisdictions specify that only rams with certain size horns can be shot, sheep with the biggest curled racks have been culled in some locations, leaving only smaller-horned animals to pass on their genes.

"So the changes we cause are not always to our advantage," says Kinnison. "Who wants to hunt small-horned bighorn sheep?"

Closer to home, as the fishing industry eventually depleted the cod populations, selection favored fish that started reproducing younger and smaller because those fish had better chances of reproducing before being netted. Unfortunately, the evolution of these smaller fish may not only have reduced their economic value, it may have also hastened the stock declines because those fish produce fewer offspring.

"Even though cod fishing has stopped, the fish are still smaller now and we may have to wait some time for them to get bigger," says Kinnison. "Natural selection favoring bigger fish might not be as strong as human selection was for smaller sizes."

Kinnison thinks his work provides compelling support for considering more than just outright extinction when assessing human effects on biodiversity. We need to consider how humans have changed many of the organisms that persist, and whether those changes will be sustainable, he says, which may be some of the toughest questions facing evolutionary and conservation biologists.

"On the positive side, many animals seem to show more ability to change in response to human disturbances than many people might have suspected," he says. "The downside is that we might not always like those changes and they might not be sufficient to keep up with humans in the long run. Phenotypic plasticity and evolution might only go so far."

And if certain animals are dying out because they're too slow to adapt, do researchers wind up measuring only the winners who have managed to keep up with the hurtling pace set by humans?

"We also have to wonder whether those winners can keep up much longer," he says.

Kinnison will be exploring those kinds of questions this spring, as part of a National Center for Ecological Analysis and Synthesis panel charged with predicting responses of salmon and other organisms to climate change.

He will also soon revisit the jungles of Trinidad to continue his work on guppies. This time, however, he will be part of an interdisciplinary scientific effort designed to explore the dynamic interactions of contemporary evolution and ecology in the wild.

The five-year, $5 million project, funded by the National Science Foundation's Frontiers in Integrative Biological Research, brings together experts from 12 universities worldwide to study how environmental changes can cause guppy populations to quickly evolve, and how that evolution can affect population growth, species interactions and even energy flow in an aquatic environment.

"In the past, evolution was largely ignored in ecological studies because it was thought to be too slow," says Kinnison. "We now know better, and this research team may uncover evidence leading to a new merger of these fields."

by Tom Weber

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