Status and Trends of the Nation's Biological Resources - Land Use

L and-use change in the United States represents an enormous uncontrolled experiment in the ways habitat changes influence plants and animals. When cities were built, land was plowed, or forests were cut, the effects on our native biota were not considered. Of course, humans have influenced the flora and fauna of North America over the ages. Native Americans established settlements, practiced agriculture, hunted, and used fire to induce vegetation changes (Denevan 1992). Land-use changes, however, have been particularly profound since Europeans settled North America three centuries ago. Landscapes have become mosaics of natural and human-influenced patches, and once-continuous natural habitats are becoming increasingly fragmented (for example, Burgess and Sharpe 1981; Harris 1984). Our nation's lands experienced tremendous changes in response to human activities, but our understanding of how land-use history affected the plants and animals in our modern landscapes is incomplete. The term land-use change has several meanings; we use it to include changes in both land cover and land use. Land cover refers to the habitat or vegetation type present, such as forest, agriculture, and grassland. Land-cover change describes differences in the area occupied by cover types through time. Both losses and gains are included. For example, changes in how much forest occurs across a landscape may reflect additions as croplands or rangelands are abandoned and forests regrow. Forest cover may also be lost to harvest or to development. In addition to tracking the amount of cover types, land-cover change also describes shifts in the spacing of cover types across the landscape over time. For example, forest may occur in a large block, or it may exist as several smaller parcels. Land use is usually defined more strictly and refers to the way in which, and the purposes for which, humans employ the land and its resources (Meyer 1995). For example, a place that is in forest cover may be used for low-density housing, logging, or recreation. Land-use change encompasses all those ways in which human uses of the land have varied through time. Land-use patterns have important influences on biological diversity— that is, the abundance, variety, and genetic constitution of native animals and plants—for several reasons. First, land-use activities may alter the relative abundances of natural habitats and result in the establishment of new land-cover types. The introduction of new cover types can increase the variety of species by providing a greater diversity of habitats. Natural habitats, though, …