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 tissues. Eisenberg (1979) and Emmons (1984), based on an analysis of variations in mammal densities 

 among various Neotropical sites, supported this hypothesis that areas with poor soils have fewer 

 individuals than areas with rich soils, but did not necessarily agree with the mechanism proposed by 

 Janzen (1974). 



Historical factors can also affect animal species abundance and diversity. Due to its location 

 on the land corridor that joins North and South America, the flora and fauna of the Yucatan Peninsula 

 strongly reflect the mingling of species that resulted after the closing of the Panamanian land bridge 

 about three million years ago ((jraham, 1993; Haffer, 1974; Stelhi and Webb, 1985). The impact of 

 the land bridge was especially notable on terrestrial mammals as nearly half of the families and genera 

 that now occur in South America belong to groups that emigrated from North America during the last 

 three million years (Marshall, 1988). A substantial number of South American mammalian taxa 

 likewise migrated to North America, but were less successful in establishing themselves than their 

 North American counterparts (Webb and Marshall, 1982). These movements continue today as the 

 distribution of the North American coyote (Canis latrans) approaches Colombia and the distribution of 

 the South American armadillo (Dasypus novemcinctus) moves northward into southern portions of the 

 United States (Hall, 1981). 



A second historical factor affecting the distribution and abundance of plants and animals is the 

 arrival of people to the New World. Crossing the Bering land bridge from Asia to North America 

 about 11,000-12,000 y ago, humans occupied most or all of the New World within possibly 1,000 y 

 (Martin, 1973). The human impact on the fauna was immediate and widespread as most of the large- 

 bodied mammals in North and South America went extinct (Marshall et al., 1984). Although Markgraf 

 (1985) proposed that climate change was responsible for these extinctions, Marshall (1988) attributes 

 the loss of these species to human hunters. 



The third historical factor important here is the development of the Maya civilization. While 

 there is some debate about when people first arrived in Mesoamerica (cf., Adams, 1991; Martin, 1973; 

 Nesbitt, 1980; Pipemo et al., 1990), a tangible Maya civilization dates from about 1,500 B.C. 



