As 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 of 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 trophic 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.