MEXICO CITY - A new study shows that small-scale logging is more extensive than previously thought in the Monarch butterflies' winter nesting grounds in central Mexico. And that may be contributing to the threats facing the Monarch's singular migration pattern.

Earlier studies found that tree-cutting peaked in 2005, when it depleted 461 hectares (1,140 acres). The new study finds that the losses over the years have been even greater. Differences due to small-scale woodcutting that were too minor to spot in year-to-year comparisons became apparent when researchers compared photographs from the first systematic aerial survey in 2001, to ones from 2011.

The government has cracked down on commercial logging. But it's more difficult to deal with impoverished people who use or occasionally sell wood from their own land.