The four-degree rise in temperature predicted by the end of this century could change the way ecosystems work even if it doesn't affect biodiversity.
Ecosystems perform important tasks – like nutrient cycling, breakdown of waste, and carbon storage – on which humans depend, so it's important we understand how climate change might affect them.
Researchers measured the effect of a four-degree rise in temperature on communities of benthic organisms in fresh-water bodies – creatures that live at the 'bottom' of the ecosystem.
They found that the number of species in the community didn't change, but the balance of small and large organisms shifted significantly, and this in turn affected how efficiently organic material broke down in the water.
Lots of studies have looked into the potential ecological effects of global warming, but the relationship between community structure and ecosystem functioning in the context of climate change is not so well understood.
A team of researchers, led by Dr. Gabriel Yvon-Durocher of Queen Mary, University of London, wanted to see how warming might affect the structure of benthic communities and what impact that would have on ecosystem functioning. They established such communities in a series of outdoor tanks – designed to mimic shallow lakes – then raised the temperature of half the tanks by four degrees.
After allowing the communities a year to develop, PhD student Matteo Dossena sampled the experiment, once in April and again in October. He identified and weighed nearly 20,000 organisms ranging from micro-organisms through to invertebrates like dragonflies, whose larvae live in the sediments at the bottom of the water.
The study revealed that the diversity of species in the community was unaffected by warming, but the relative number of small versus large organisms was strongly affected by the temperature change.
The researchers also observed a large a seasonal difference in the effect of warming on community structure. In the spring, there was a big decline in the number of larger organisms, while in the autumn there were relatively more of the larger organisms in the warmed tanks. In the tanks that hadn't been warmed up, the size structure of the community stayed the same year-round.
"The effect of warming on the seasonality of community size structure was unexpected," says Yvon-Durocher. "From similar studies on plankton we were expecting to see an increase in smaller organisms at higher temperatures, but the marked seasonal change in response to warming was a surprise."
Benthic organisms play an important role in the food chain by breaking down organic matter and releasing nutrients. So the researchers added leaf litter to the tanks, to see how changes to the organisms present might affect the way the whole ecosystem worked.
They found a strong relationship between the changes in community structure and rates of decomposition, which were lower in April when the biomass was lower and higher in October when there were relatively more big organisms.
"There is a direct link between community structure and ecosystem functioning that is due to the scaling of metabolic rate with body mass – in other words small organisms have a faster relative metabolic rate compared to large organisms – and this effect is independent of the diversity of species present," explains Yvon-Durocher.
These results don't mean that biodiversity isn't important for healthy ecosystems but they reveal another layer of complexity in the role communities of organisms play in them.
The research is published in Proceedings of the Royal Society B.
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More information: Matteo Dossena, et al. Warming alters community size structure and ecosystem functioning. Proc Roy Soc B: Biological Sciences. (2012) doi: 10.1098/rspb.2012.0394
