66 • Technologies To Maintain Biological Diversity 



plants. A global agricultural environment 

 that includes domestic varieties of a crop 

 (such as corn) and the crop's wild ances- 

 tors has more diversity than the same envi- 

 ronment after the wild ancestors are elim- 

 inated to make space for more domestic 

 crops. 



The quality of information used to assess the 

 loss of biological diversity varies greatly for 

 different ecosystems and different parts of the 

 world. In general, both data and theories are 

 better developed for temperate than for tropi- 

 cal biology; better for birds, mammals, and 

 flowering and coniferous plants than for other 

 classes of organisms; and better for the few ma- 

 jor crop and livestock species used in modern 

 agriculture than for the many species used in 

 traditional agriculture. 



Ecosystem Diversity 



Natural ecosystem diversity has declined in 

 the United States historically (26), and no evi- 

 dence suggests that this long-term trend has 

 been arrested. By comparing a nationwide eval- 

 uation of potential natural vegetation (PNV) 

 with data on existing land uses from the 1967 

 Conservation Needs Inventory, scientists esti- 

 mate what portion of land in the United States 

 is still occupied by natural vegetation (26). This 

 study estimates a percent change in area for 

 each ecosystem type (each PNV) since presettle- 

 ment times. 



The greatest area reduction was 89 percent 

 for the Tule Marsh PNV in California, Nevada, 

 and Utah, mainly due to agricultural develop- 

 ment. Twenty-three ecosystem types that once 

 covered about half the conterminous United 

 States now cover only about 7 percent. The agri- 

 cultural States of Iowa, Illinois, and Indiana 

 have lost the highest proportions of their natu- 

 ral terrestrial ecosystems (92, 89, and 82 per- 

 cent, respectively). 



States with the lowest losses were Nevada, 

 Arizona, and New Hampshire (4, 7, and 12 per- 

 cent, respectively). This assessment does not 

 imply that 96 percent of Nevada is in the same 

 condition that it was 400 years ago. The study 

 did not assess degradation of areas still oc- 



cupied by natural vegetation; rather, it indicated 

 the areas whose uses remain unchanged. Also, 

 the study was unable to consider some impor- 

 tant ecosystem types, such as riparian and wet- 

 land areas, which are not included in the PNV 

 categories (26). 



Wetlands inventories are conducted by the 

 U.S. Fish and Wildlife Service. The estimated 

 total wetland area in the conterminous States 

 in presettlement times was 87 million hectares. 

 This amount was reduced to 44 million by the 

 mid-1950s and to 40 million by the mid-1970s 

 (figure 3-2). Thus, half the Nation's wetland area 

 was lost in about 400 years, and another 5 per- 

 cent was lost in the following two decades. 

 Drainage for agricultural development has been 

 the main cause of wetland ecosystem loss (48). 



Riparian ecosystems are generally too small 

 to be included as PNV types in major analyses. 

 However, riparian areas contribute dispropor- 

 tionately to biological diversity, especially in 

 the Western States, where they provide luxuri- 

 ous habitats compared with the adjacent up- 

 lands (9). Further, their maintenance is impor- 

 tant to biological diversity in the streams and 

 lakes they border. Natural riparian (mostly 

 streamside) vegetation in the United States has 



Figure 3-2.— Changes in Wetlands Since the 1950s 

 (percentage of total) 



Wetlands lost 

 14% 



\ 



New wetlands 

 3.4% 

 \ 



SOURCE: Data from Fish and Wildlife Service's National Wetland Trends Study, 

 1982, 



