BRUSSELS — Europe's bat population is vulnerable, but conservation policies have boosted it by more than 40 percent after years of decline, the European Environment Agency (EEA) said on Thursday.

European bat populations shrank, particularly during the second half of the 20th century, because of intensive agriculture, disappearing habitats and toxic chemicals used in treating roof timbers where they roost.

The new report found conservation policies had helped to reverse the decline, but concluded bats should "still be considered vulnerable".

Bats have a slow rate of reproduction so their numbers can decline very rapidly.

They are also extremely sensitive to environmental change, which means they serve as an early indicator of climate change. Differences in temperature, for instance, can affect their ability to forage, reproduce and hibernate.

"Monitoring bats also helps understand changes in wider ecosystems, including climate change, as they are highly sensitive to environmental change," EEA Executive Director Hans Bruyninckx said in a statement.

"Many bat species are still endangered, so preserving their habitats is still an important priority."

Surveyors counted and catalogued bats hibernating at 6,000 sites and found numbers had increased by 43 percent between 1993 and 2011, with a relatively stable trend since 2003.

The European Environment Agency, which provides scientific data to guide EU policy-makers, said its study was based on the most comprehensive research yet.

It analyzed data from 10 bat monitoring schemes in nine EU countries: Austria, Germany, Hungary, Latvia, the Netherlands, Portugal, Slovakia, Slovenia and the United Kingdom.