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Climate change
will alter the ecosystems that humanity depends upon in the coming
century. But given the complexity of the living world, how can you learn
what may happen?
A
team of Australian scientists has an answer: miniature ecosystems
designed to simulate the impact of climate change. The experiments are
already revealing dangers that would have been missed had researchers
tried to study individual species in isolation.
“If
you just take one fish and put it in a tank and see how it responds to
temperature, you can imagine that’s a huge simplification of reality,”
said Ivan Nagelkerken, an ecologist at the University of Adelaide who is
leading the research effort.
Yet
studying an entire ecosystem in nature, made up of thousands of
species, has its own drawbacks. “In nature you have all this complexity,
and you never know which factor is really causing the outcome you’re
observing,” Dr. Nagelkerken said.
Between
these two extremes, Dr. Nagelkerken and his colleagues have tried to
create a happy medium. They filled 12 pools with 475 gallons of seawater
apiece and built simple ocean ecosystems in each one.
They
put sand and rocks on the bottom of the pools, along with artificial
sea grass on which algae could grow. They stocked their small-scale
ecosystems, called mesocosms, with local species of crustaceans and
other invertebrates, which grazed on the algae.
For predators, they added a small fish known as the Southern longfin goby, which feeds on invertebrates.
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To
test the effects of climate change, Dr. Nagelkerken and his colleagues
manipulated the water in the pools. In three of them, the researchers
raised the temperature 5 degrees — a conservative projection of how warm
water off the coast of South Australia will get.
The scientists also studied the effect of the carbon dioxide that is raising the planet’s temperature.
The
gas is dissolving into the oceans, making them more acidic and
potentially causing harm to marine animals and plants. Yet the extra
carbon dioxide can be used by algae to carry out more photosynthesis.
To
measure the overall impact, Dr. Nagelkerken and his colleagues pumped
the gas into three of the pools, keeping them at today’s ocean
temperatures.
In
three others, the researchers made both changes, heating up the water
and pumping in carbon dioxide. The scientists left the remaining three
pools unaltered, to serve as a baseline for measuring changes in the
other nine pools.
On
its own, Dr. Nagelkerken and his colleagues found, carbon dioxide
benefited all three layers of the food web. Algae grew faster, providing
more food for the invertebrates. The invertebrates, in turn, provided
more food to the gobies.
But the combination of extra carbon dioxide with warmer water wiped out that benefit.
Even
with extra algae to eat, the invertebrates failed to grow faster,
perhaps because the algae provide less nutrition when they grow at
higher temperatures. It is also possible that the invertebrates are
under too much stress in warmer water to grow more.
The
invertebrates also faced more pressure from their predators. The warm
water sped up the metabolism of the gobies, making them hungrier. They
devoured more invertebrates. Hammered from above and below, the
invertebrate populations collapsed.
Mary
I. O’Connor, an ecologist at the University of British Columbia who was
not involved in the Australian research, praised it as an ambitious
advance on earlier studies. “It’s showed us something we haven’t seen
before,” she said.
Dr. Nagelkerken and his colleagues published initial results from these mesocosm studies last month in the journal Global Change Biology.
In a separate report published in the February issue of the journal
Oikos, Dr. Nagelkerken and his colleagues reported evidence that
acidification can interfere with the ability of fish to hunt.
In
that study, the researchers raised a species of sharks in warm,
acidified seawater. They found that the sharks hunted more for sea
urchins, one of the species they eat because of higher temperatures.
But
they were less successful at detecting prey, most likely because the
altered chemistry of the seawater interfered with their nervous systems.
Dr. Nagelkerken said these experiments had ominous implications for ocean ecosystems — as well as for the 3.1 billion people worldwide who depend on fish for 20 percent or more of their protein.
“As
you go further higher up the food web, you get more of a mismatch
between the need for food and the availability of food,” Dr. Nagelkerken
said. And it’s the species high in the ocean’s food webs that we fish
for.
Just
how vulnerable fish will be depends on their individual ecosystems. Dr.
Nagelkerken said he hoped the studies he and his colleagues are
carrying out will prompt other researchers to replicate them with
species and conditions from other parts of the world.
“These
kinds of experiments are essential tools for understanding change in
nature,” Dr. O’Connor, the University of British Columbia ecologist,
said.
Dr.
Nagelkerken’s research, she said, “is not a prediction of the future,
but it is nice proof that we can expect food web reorganization with
continued ocean warming and acidification.”
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