Humble lichens have long been silent witnesses to humankind's impact on the environment. Now for the first time scientists have coaxed these organisms into revealing the extent to which climate change and airborne pollution are redrawing the biogeographical map, writes Steve Farrar.
Different species of lichen - composite organisms made up of fungi and algae or bacteria - respond to environmental influences in different ways.
A Dutch team led by Kok van Herk, a research scientist at the Lichenologisch Onderzoekbureau Nederland, has monitored the evolving pattern of lichen species in selected sites across Holland since 1979.
Its results, published in the journal The Lichenologist , constitute the most definitive picture of global warming's influence on the distribution of wildlife.
The scientists noted the levels of every species of lichen growing on trees at 600 sites. Each was revisited every five years and changes noted.
Signs of human pollution were immediately apparent. Some species, such as green-branched lichen Evernia , known as oak moss, were observed to be in decline as levels of sulphur dioxide produced by industry rose. When pollution fell in recent years, those species recovered.
Concentrations of ammonia gas have risen as a result of cattle farming, and this has prompted the rise of once-rare species such as flat yellow Xanthoria .
When the latest data from 1995-2001 were analysed, a new factor emerged as a major influence on the mix of species - rising temperatures.
This has meant arctic-alpine species have been retreating while more tropical species such as grey Parmelia soredians have flourished.
Andre Aptroot, a research scientist at the Centraalbureau voor Schimmelcultures in Utrecht, said lichen were very sensitive to the environment and the distribution of species could change rapidly. Furthermore, according to Dr van Herk, they were not influenced by other factors such as soil composition.
Mark Seaward, professor of environmental science at Bradford University, said each species had a different story to tell.
"This is the first time we can see unambiguously global warming as one of several factors on species distribution," he said.