LiveScience.com – Mon Jul 20, 9:46 pm ET
s Earth's climate continues to warm, life might become the
province of the small, a new study suggests.
Climate change has already had
documented effects on species living across the globe, from polar
bears in the Arctic to coral reefs
in the tropical seas. While some changes are specific to certain
types of animals, others seem to be more universal.
Two such ecological changes that have been noted and predicted
are the shift of species' ranges
to higher altitudes and latitudes to keep within their temperature
comfort zones and the shift in the timing of key events in the life
cycle of organisms - for example, the flowering plants
or the migration of birds.
A third change can now be added to that list: As temperatures
rise, organisms get smaller, from the scale of whole communities
down to the individual.
This relationship was known to exist in nature, with warmer
environments tending to be dominated by smaller-sized species, said
study leader Martin Daufresne of Cemagref Aix-en-Provence (a French
governmental research institute). But what
Daufresne and his colleagues aimed to do with their study was "was
to test if this known relationship in ecology was working for
climate change," he said.
They found that it's working.
The results of their research are detailed in the July 20 issue
of the journal Proceedings of the National
Academy of Sciences.
Shift to the small
This relationship between warmer environments and smaller size
works at three different levels in a given biological community:
"Being small could mean that you are belonging to a small-sized
species; but it could also mean that you are younger, or that you
are small for your age," Daufresne explained.
He and his colleagues used long-term surveys to look at these
measures for several different aquatic communities that encompassed
a variety of organisms (including bacteria, phytoplankton and fish)
living in a variety of environments (large rivers, streams and salt
water).
The researchers found that, on average, these communities did in
fact get smaller as temperatures rose
in their environments over time.
For communities of fish in large French rivers, for example, the
team observed "a decrease on average of something like more than 60
percent of the mean size at the community scale," during the span
of about two decades, Daufresne told LiveScience. This decrease was due to a combination of an
increase in the proportion of small-sized species, an increase in
proportion of juveniles, and a decrease in size at a given age
among individuals within a species.
What exactly is behind this shift isn't clear yet and will
require further study, Daufresne said. "Since we observe this
decrease at the different scales, it's a bit difficult to find a
common trigger - maybe there are several triggers, or maybe just
one," he said.
What is clear though, "is that it's something which seems to
happen everywhere," which supports global warming's role, Daufresne
said. "The fact that we observe this common pattern among different
systems and among different kinds of organisms means that there is
a real effect of the global warming on size."
Sheep and consequences
More research will be needed to see if this same relationship
holds for other species and environments, particularly land-based
creatures and warm-blooded animals. One recent study in the journal
Science suggests that global warming might be causing the same
effect in Scottish sheep,
as the woolly beasts seem to have gotten smaller with milder
winters over the last few decades.
Whether or not these changes are adaptive to the animals
undergoing them is uncertain and something Daufresne hopes to
resolve with further research.
Daufresne and his colleagues also want to look into just what
the cascading effects of this shrinking might be to communities.
One potential way the size change could reverberate would be
through the food chain.
"The size of the species is related generally to the position
within the food chain, for example, the bigger you are, the [higher
up] you are in the food chain," Daufresne said. With sizes shifting
to the smaller end of the spectrum, "maybe the upper tropic level
could be more sensitive to climate
change," which could affect the relationships between
predator and prey, he added.
The effect could also be bigger at higher latitudes where
warming is expected to be starker, Daufresne said.
