— The efficiency of wood utilization has improved substantially since 1900. Much 

 less material is left in the woods, many sawmills produce more than double the 

 usable lumber and other products per log input than they did in 1900, 

 engineering standards and designs have reduced the volume of wood used per 

 square foot of building space, and preservative treatments have substantially 

 extended the service life of wood. These efficiencies have reduced by millions of 

 acres the area of annual harvest that otherwise would have occurred. 



Although there have been favorable trends in forest resources over the past century, 

 increasing human demands will continue to affect natural resources. The ability of 

 forest and rangeland systems to maintain a balance of successional stages through 

 natural disturbance has been severely reduced by human development. For example, 

 the present forest area does not necessarily have the same balance of stand struc- 

 ture, and therefore cannot support the same groups of plants and animals as found in 

 large, natural forested areas. As a result, the distribution and abundance of biological 

 communities has been significantly altered. 



Issue Highlight— 

 Analysis of Status and 

 Trends in Biological 

 Diversity 



Biological diversity is a broad concept that encompasses natural variety within and 

 among ecological systems. Significant land use changes have occurred in the United 

 States since European settlement in the 1600's (Langner and Flather, 1994). About 

 30 percent of the forest land, over half of the wetlands, and most of the native prairies 

 of the Midwest have been converted to agriculture and other uses. 



— On a State-by-State comparison, the loss of natural vegetation has ranged from 

 4 percent in Nevada to 92 percent in Iowa. 



— Ten States have lost over 70 percent of their original wetland area. 



— Less than 1 percent of the original tallgrass prairie of the Midwest remains in 

 natural vegetation. 



— The distribution of forest land by successional stage has been altered by human 

 influence. For example, in the eastern forests of the Pacific Northwest the 

 proportion of forest in mid-successional stages has increased, while the 

 proportion in early and late stages has declined in the last 40 to 50 years. 



— At least 20 percent of the Nation's 1 million stream miles have been modified by 

 channelization, reservoir construction, or other conversions. 



— Remaining forest lands tend to be more highly fragmented by roads and other 

 types of human development. 



These land use shifts have affected the distribution and abundance of species. 

 Species tolerant of extensive land uses, such as agriculture and managed range- 

 lands, have replaced the inhabitants of the original communities. Intensive develop- 



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