Ch. 5— Maintaining Biological Diversity Onsite • 107 



of habitats. Also, the occurrence and persist- 

 ence of a species on any particular site may 

 be governed not only by the populations on that 

 site but also by whether groups of loosely con- 

 nected populations can survive in the region 

 (46,71). This sort of scientific question requires 

 long-term study, which is only beginning to be 

 conducted. 



As the Earth increasingly becomes a patch- 

 work of natural and developed areas, the ef- 

 fects of activities on or near the boundaries of 

 protected areas are becoming more important. 

 Small areas and those with angular boundaries 

 have a higher proportion of boundary-to-inter- 

 ior than larger or more circular areas. Nature 

 reserves seldom have sharply defined natural 

 boundaries like oceanic islands. Instead they 

 have political boundaries that can do only so 

 much to prevent movements in and out. Many 

 species can migrate across nature reserve 

 boundaries, and the results of human activities 

 (e.g., pollution] may enter by air, water, or land. 

 Consequently, another theory on the optimum 

 design of protected areas, "the boundary mod- 

 el," has been proposed. It accounts for the 

 boundary effects, including the effects of hu- 

 man activities (69). 



Designated protected areas include both po- 

 litical and biological boundaries. Some of the 

 biological boundaries are the natural edges be- 

 tween ecosystems; others result from human 

 activities, most of which originate outside po- 

 litical boundaries. Those biological boundaries 

 that fit the ecological definition of an "edge" 

 (box 5-B) may increase local diversity as edge- 

 adapted species prosper. Over time, however, 

 survival of species in the interior may be re- 

 duced if edges are enlarged, because the habi- 

 tat for species adapted to less-disturbed condi- 

 tions is reduced. Poor protection at the political 

 boundaries generally shifts the biological 

 boundaries toward an area's interior. 



Zones where resource-conserving develop- 

 ment activities are encouraged have been tested 

 to buffer the boundary effect (e.g., the United 

 Nations Educational, Scientific and Cultural 

 Organization's (UNESCO) Man and the Bio- 

 sphere Program). Such buffer zones can help 

 reserves by increasing the habitat area and min- 



Box 5-B.— The Edge Effect 



Natural boundaries between ecosystems, 

 or edges, are considered to be ecologically 

 diverse areas. Edges can be created by 

 human manipulation of vegetation in an at- 

 tempt to encourage maximum local diver- 

 sity (14). Along an edge, animals from each 

 of the abutting vegetation types may be 

 found, together with animals that make fre- 

 quent uso of more than one vegetation type 

 and those that specialize on the edge itself 

 (41). Game animals commonly are edge- 

 adapted, as are animals of many urban, sub- 

 urban, and agricultural areas (e.g., birds) (8). 



imizing the potential exposure to harm. The 

 idea of buffer zones is not new, but implemen- 

 tation has been slow; few evaluations have been 

 done yet to develop guidelines about the nec- 

 essary character and width of the zones or the 

 shifting nature of boundaries. 



Corridors of habitat to connect nature re- 

 serves have been proposed for sites where re- 

 serve sizes are below-optimum. These corridors 

 should facilitate gene flow and the dispersal 

 of individuals between protected areas, which 

 should, in turn, increase the effective size of 

 populations and thus raise the chance of sur- 

 vival for semi-isolated groups (6). Also, cor- 

 ridors could increase the recolonization rate 

 if species are eliminated locally (78). Corridors, 

 however, are another theoretical concept, and 

 they may not be appropriate for all sites. As 

 noted earlier, geographic isolation is a cause 

 of genetic diversity. Thus, corridors where none 

 previously existed might cause locally adapted 

 genotypes to be lost due to gene flow. The ap- 

 plicability of corridors is another aspect of de- 

 sign theory now being actively researched. 



The use of corridors and boundary zones has 

 been proposed for protected areas in the West- 

 ern Cascades region of the United States. This 

 area contains the largest tract of uncut forest 

 in the conterminous United States as well as 

 natural riparian habitats (32). The proposal sug- 

 gests surrounding islands of old-growth forest 

 with zones of low-intensity, long-rotation tim- 



