Rising temperatures and growing variability in rainfall are affecting the size of all species in the ecosystem from microscopic sea organisms to land-based predators, say researchers.
"Our study suggests that ectotherms (cold-blooded animals like toads, turtles, and snakes that rely on environmental heat sources) are already changing a lot," said David Bickford from the National University of Singapore and co-author of the study.
Both aquatic and terrestrial ectotherms have been shrinking, according to the study, with common toads' size and condition decreasing as temperatures rose 1.5 degrees Celsius over a 22-year period.
The study, published in the journal Nature Climate Change, draws on evidence from fossil records, experimental and comparative studies, as well as research implicating anthropogenic climate change over the last 100 years.
"What was most surprising to me was that it was such a uniform signal across all these different organisms," Bickford said.
What was most surprising to me was that it was such a uniform signal across all these different organisms
David Bickford, National University of Singapore
Fossils from a warming phase during the Palaeocene-Eocene Thermal Maximum (around 55 million years ago) reveal that burrowing invertebrates such as beetles, bees and ants shrank in size by up to 75%.
Other fossil records indicated that animals including pocket gophers, woodrats and California squirrels, also shrank during past warming periods, say researchers.
Experimental studies which increased water acidification -- an observed effect of rising atmospheric carbon dioxide levels -- revealed marine species like corals, scallops and oysters become smaller.
Each degree of warming has been shown to decrease the size of marine invertebrates by up to 4%, salamanders up to 14% and fish by up to a maximum of 22%, according to the study.
But perhaps most worrying for marine life was the reduced growth rates of phytoplankton in response to acidification which "could negatively affect all ocean life because (it) forms the basis of the marine food web."
Researchers also say plants, which were generally expected to get larger as CO2 levels rise, are not immune from reductions.
"Over the past century, various plant species have shown significant negative correlations between growth and temperature...resulting in smaller grasses, annual plants and trees in areas that are getting warmer and drier," according to the study.
The study cites experiments manipulating temperature showing biomass in some grass, grain and fruit plants was 3-17% smaller for every degree Celsius of warming.
But there are exceptions to the trend.
Recent studies have indicated that the common lizard, mallard and teal ducks, otters and some birds are increasing in body size, say researchers, but many of these inhabit high latitudes which have witnessed increased growing seasons associated with global warming making patterns of shrinking less common.
What we might see is that there are many ecological buffers out there. These systems are incredibly resilient
David Bickford, National University of Singapore
However, this might be short-lived if climate change increases in severity, according to the study.
Average global temperatures rose by nearly one degree Celsius over the past 100 years, according to the Intergovernmental Panel on Climate Change (IPCC), but worst-case scenarios predict warming of up to seven degrees Celsius by 2100.
The study's authors concede many factors will play their part in the size of organisms if warming trends continue, but highlight reduced availability of water and lower levels of soil nutrients as key factors for plants, and the animals that feed off them.
Bickford says more experiments need to be done to find out which mechanisms are important to different organisms in different places.
And scientists also need to discover how ecological balances are going to be disrupted in the future and what this will mean for different species up and down the food chain, he says.
"What we might see is that there are many ecological buffers out there. These systems are incredibly resilient. Nature has an amazing adaptive capacity," Bickford said.
But shrinking trends in the ecosystem are likely to impact heavily on humans, say the authors.
The study points out that nearly one billion people rely on fish as their main source of protein, and increasing variability in rain will make crop cultivation more difficult in many areas in the future.
"We've probably got such diverse food source system out there already that its going to a long time before it affects richer countries certainly. But it might not be so long for poorer countries unfortunately," Bickford said.
(cnn.com)
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