Ch. 5— Maintaining Biological Diversity Onsite • 103 



• are located where social and economic 

 conditions are conducive to conservation, 

 and 



• are not adequately represented in existing 

 protected areas. 



The major patterns of nature can be described 

 for most terrestrial areas with existing data. Sev- 

 eral biogeographic systems have been devel- 

 oped that relate data on distribution of plant 

 and animal species to factors such as climate 

 and natural barriers like oceans, deserts, and 

 mountain ranges. These systems classify the 

 Earth into zones, with each zone containing 

 distinctive ecosystems and life forms. 



Much less information is available on aquatic 

 sites, such as lakes and streams. Aquatic eco- 

 systems are difficult to map on a large scale, 

 and the way to integrate them into land clas- 

 sifications is poorly understood. The same is 

 true of riparian vegetation, mountain meadows, 

 and other azonal ecosystems. 



Classification systems take two broad ap- 

 proaches. "Taxonomic" methods establish land 

 units by grouping resources or sites with simi- 

 lar properties. "Regionalization" methods sub- 

 divide land into natural units on the basis of 

 spatial patterns that affect natural processes 

 and the use of resources (1). The two approaches 

 can be integrated to identify ecosystem diver- 

 sity in considerable detail. 



The taxonomic approach is typified by the 

 Society of American Foresters (SAF) Cover 

 Type Classification system, which aggregates 

 similar stands of forest trees on the basis of the 

 kind, number, and distribution of plant species 

 and the dominance by tree species (19). The 

 basic taxonomic units— forest cover types— are 

 named after the predominant tree species. The 

 Renewable Resources Evaluation of the U.S. 

 Forest Service further aggregates many of the 

 SAF categories into 20 "major forest types," 

 which are the basis for the only map of forest 

 cover types available for the United States as 

 a whole. 



The regionalization approach, on the other 

 hand, begins with a nation or continent and 

 subdivides it into progressively smaller, more 



closely related units. An example of this is the 

 ecoregions classification system, used exten- 

 sively by U.S. Federal land-managing agencies 

 (1). Ecosystem regions for North America are 

 defined as domains on the basis of climate. The 

 domains are subdivided into divisions, which 

 are subdivided into provinces on the basis of 

 what plant communities can be expected to de- 

 velop if the natural succession of species is not 

 interrupted by human activity. Provinces are 

 subdivided into sections on the basis of the com- 

 position of the vegetation types that eventually 

 would prevail. Extending this ecoregion clas- 

 sification system to cover the world on a scale 

 of 1:25 million is being considered. 



A recently developed system for classifica- 

 tion of the world's marine and coastal environ- 

 ments combines physical processes with biotic 

 characteristics (34). This classification system 

 will be used as a basis for selecting U.S. coastal 

 biosphere reserves (13). 



Each classification system has advantages 

 and disadvantages for programs to maintain 

 biological diversity. The taxonomic approach 

 identifies and classifies each component. For 

 example, separate taxonomies are used to iden- 

 tify flora, fauna, and soils. This separation 

 facilitates location of natural areas that will 

 conserve concentrations of high-priority com- 

 ponents, such as a vegetation type or animal 

 species. The regionalization approach allows 

 scientists to determine whether the same type 

 of ecosystem in distinct biogeographic regions 

 actually represents two different ecosystems (2). 



Biogeographic classification maps indicate 

 what ecosystems would be found under natu- 

 ral conditions, but the discrepancy between ex- 

 pected and actual features is often great because 

 of human intervention. Sparse grasslands may 

 occur where climate, physical features, and spe- 

 cies distribution records suggest tropical moist 

 forest should grow. Furthermore, the major 

 classification systems cover only broad zonal 

 features of the environment. Azonal features — 

 e.g., wetlands, riparian areas, and coral reefs — 

 cannot be included. So conservation strategies 

 must take a different approach to identify pri- 

 orities for these ecosystems. Typically, plans 



