| The practice and policy of putting out fires.
In the 20th century, government agencies such as the U.S. Forest Service, the Bureau of Land Management, and the National Park Service became so adept at putting out fires that until 1970 or so, annual acreage lost to fire dropped to 15% of what it was before fire suppression.
However, since then, and paradoxically due to the very policies of these land management agencies, wildfires have become more frequent and larger, sometimes burning tens of thousands of acres. The Western United States has this problem-- if you suppress fires in temperate forests, the shade tolerant understory builds up, increasing the fuel load and creating a homogeneous landscape. Fires can then do more damage when they occur, as the fuel on the ground both spreads the fire uniformly through the understory and brings it up to the canopy. In a forest left on its own, the natural fire cycle's burns would create irregular patches of habitat, with gaps in the canopy- letting in sun, creating a patchwork of differing vegetation types, not only increasing biodiversity but keeping fires from spreading uniformly.
Source: USGS, Status and Trends of the Nation's Biological Resoures, http://biology.usgs.gov/s+t/ | 
