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
March-April, 2008
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