Ch. 5— Maintaining Biological Diversity Onsite • 109 



Even if such models were developed, they could 

 prove very costly to use [72). 



The theoretical population models are yield- 

 ing other plausible hypotheses, some of which 

 have important implications for conservation. 

 Extinction probabilities depend critically on 

 population growth rates, on environmentally 

 induced variability in this rate, and on particu- 

 lar catastrophic scenarios to which the species 

 are subject. One recent analysis employs a sto- 

 chastic population model and the general rela- 

 tionships between body-size and population 

 growth rates and between body-size and pop- 

 ulation density to estimate the sizes of popula- 

 tions and habitats necessary for mammals to 

 have a 95 percent probability of persistence for 

 1,000 years. 



The preliminary results from this analysis are 

 startling. For larger animals, the viable popu- 



lation size is smaller, but the necessary habitat 

 must be larger to support the requisite popula- 

 tions. Thus, smaller mammals can have a via- 

 ble population size of a million individuals but 

 a habitat requiring only tens of square kilome- 

 ters. The largest mammals, on the other hand, 

 may have a viable population with only hun- 

 dreds of individuals but may need a million 

 square kilometers of habitat (3). 



These are preliminary analyses. But even if 

 subsequent work reduces the estimates by two 

 orders of magnitude, larger mammals may need 

 contiguous habitats of tens of thousands of 

 square kilometers to survive indefinitely. Few 

 protected natural areas are that large, imply- 

 ing that conservation strategies for certain spe- 

 cies should not depend as much on protected 

 reserves as on monitoring and managing larger 

 areas (24). 



ESTABLISHMENT OF PROTECTED AREAS 



Since the world's first two national parks 

 were established in the 1870s, some 3,500 pro- 

 tected areas have been set aside for conserva- 

 tion, covering some 4.25 million square kilom- 

 eters (1,050 million acres) (35). (See figure 5-1 

 for rate of growth.) 



Growth in the number and size of protected 

 areas was slow at first. It accelerated during 

 the 1920s and 1930s, halted during World War 

 II, and regained momentum by the early 1950s. 

 The number doubled during the 1970s, but 

 growth has slowed over the past few years (33). 

 Before 1970, most protected areas were located 

 in industrial countries. But for the past 15 years, 

 the Third World has led in both numbers added 

 and rates of establishment. 



Designation as a protected area does not nec- 

 essarily mean that protection is effective, of 

 course. The extent of actual protection in the 

 3,500 areas has not been determined, but anec- 

 dotal evidence indicates that illegal or un- 

 managed hunting, fishing, gathering, logging, 

 farming, and livestock grazing are common 

 problems (83). Thus, data on designated areas 



exaggerate the scope of conservation actually 

 being achieved. 



Acquisition and Designation 



Most protected areas are established by offi- 

 cial acts designating that uses of particular sites 

 will be restricted to those compatible with nat- 

 ural ecological conditions. At the Federal level 

 in the United States, designating a land area 

 or water body for conservation involves mak- 

 ing a formal declaration of intent to assign a 

 certain category of protection and then provid- 

 ing an opportunity for extensive public com- 

 ment on the proposed action. Other govern- 

 ments use similar processes, although the 

 extent of public participation varies. 



The degree of protection depends partly on 

 the objectives of the acquisition or designation. 

 There are many different types of designations. 

 Kenya, for example, has national parks, na- 

 tional reserves, nature reserves, and forest re- 

 serves. The wildlife sanctuaries in Kiribati in 

 the South Pacific are very different in conser- 



