Forest ecosystems of Caribbean islands have proven to be more resilient than one would assume on the basis of the relationships used in Table 6–2 or the idea of the fragility of island biota (Carlquist, 1974; Soulé, 1983). The Caribbean islands are densely populated (100 to 500 people per square kilometer, or about 10 times more densely populated than surrounding continental tropical lands (Lugo et al., 1981), and their lands have been intensively used and degraded for centuries. All the ills that Carlquist (1974) and Soulé (1983) described for islands (e.g., the introduction of exotic species, intensive predation, and habitat destruction) are present in this region. There are many examples of catastrophic waste of natural resources in the Caribbean islands [see, for example, Ambio 10(6) 1981, which was dedicated to environmental problems of the Caribbean], but there are also examples to give us some hope; these are the ones I am emphasizing.
In Puerto Rico, human activity reduced the area of primary forest by 99%, but because of extensive use of coffee shade trees in the coffee region and secondary forests, forest cover was never less than 10 to 15%. This massive forest conversion did not lead to a correspondingly massive species extinction, certainly nowhere near the 50% alluded to by Myers (1983). In an analysis of the bird fauna, Brash (1984) concluded that seven bird species (four of them endemic) became extinct after 500 years of human pressure (this is equivalent to an 11.6% loss of the bird fauna) and that exotic species enlarged the species pool. In the 1980s more birds are present on the island (97 species) than were present in pre-Columbian times (60 species). The resiliency of the bird fauna was attributed to its generalist survival strategy (a characteristic of island fauna) and to the location of secondary forests and coffee plantations on mountaintops along the east-west axis of the island, which acted as refugia.
Secondary forests in Puerto Rico have served as refugia for primary forest tree species as well (Wadsworth and Birdsey, 1982; R.O.Woodbury, University of Puerto Rico, personal communication, 1986). After 20 to 30 years of growth, the understory of these ecosystems is supporting species characteristic of mature forests. A random survey of 4,500 trees in secondary forests of two life zones (moist and wet forests) resulted in a tally of 189 tree species (Birdsey and Weaver, 1982). This survey excluded four of the six forested life zones in the island and the species-rich mature publicly owned forests. Yet it is important that 25% of the tree species identified on the island were recorded in this survey of secondary forests. (Puerto Rico has 750 tree species, 203 of which are naturalized; Little et al., 1974.) Dominant species in these secondary forests owe their dominance to human activity, and many of the native species that are typical of mature forests are rare in the forest canopy (142 tree species accounted for 16% of the total basal area of secondary forests) but are now beginning to appear as pole-size individual trees in these forest sites. Secondary forests in high-impact regions obviously require time to fulfill their role as foster ecosystem for endangered species, but in due time, a wide variety of tree species appear to return to forest lands.
An extreme example of the importance of species conservation and of human-dominated habitats acting as foster ecosystems for endangered species is that of the Chinese maiden hair tree (Ginkgo biloba). No one has ever seen a wild individual