of this species. This primitive tree was preserved in courtyards of temples in China and is considered to be the first species saved by humans (Stebbins, 1979).

In the United States where extensive human-caused deforestation and subsequent forest recovery have occurred, remnant secondary forest islands account for a large portion of landscape species diversity (Burgess and Sharpe, 1981). As a group, these secondary forest islands constitute a landscape with greater species richness than found in a landscape dominated only by climax forests. Clearly, secondary forests require more scientific attention before their role and value in landscapes affected by human activity can be properly assessed.

Catastrophic natural events may also be deleterious to the maintenance of species diversity, particularly to those species already at the edge of extinction. However, these catastrophic events are natural phenomena with predictable rates of recurrence to which the biota as a whole is adapted. Evidence is mounting to show that tropical forest ecosystems have endured catastrophic events for millennia, e.g., periodic fires in the moist forests of the Amazon (Sanford et al., 1985) and in Borneo (Leighton, 1984). In the Caribbean, hurricanes appear to be important in the maintenance of species diversity. Long-term studies in areas of the Luquillo Experimental Forest Biosphere Reserve have shown that there are progressive reductions in tree species between hurricane events (Crow, 1980; Weaver, 1986). The effects of periodic hurricanes maintain a diverse mix of successional and climax species on a given site. Without hurricanes, successional species would be more restricted. Sanford et al. (1985) suggest that fire performs the same function in Amazonian moist forests; Sepkoski and Raup (1986) expanded this idea to the effects of global perturbations on the history of life on the planet.

Studies of regeneration strategies for mature forests have indicated that disturbance is usually associated with the early phases of seedling germination and establishment in most forest types, including tropical forests (Pickett and White, 1985). This has led Pickett and White to propose the concept of “patch dynamics” as a focus of scientific inquiry aimed at understanding ecosystem dynamics. The relevance of this to the maintenance of species diversity is that environmental change and disturbance may be required to maintain a species-rich tropical landscape.

Because humans have facilitated immigration and created new environments, exotic (nonnative) species have successfully become established in the Caribbean islands. This has resulted in a general increase in total species inventories of birds and trees. Some of these exotic species are pests and thus are called biological pollutants (CEQ, 1980). However, many exotic species have become so well integrated into the natural landscape that most islanders consider them native.

Although conservationists and biologists have an aversion to exotic species such as predatory mammals and pests (with good reason!), this may not be totally justified if the full inventory of exotic fauna and flora and certain ecological arguments are taken into consideration. For example, the growth of exotic plant species is usually an indication of disturbed environments, and under these conditions, exotic species compete successfully (Vermeij, 1986). They accumulate and process carbon and nutrients more efficiently than do the native organisms they replace. In so doing, many exotic species improve soil and site quality and either pave the way for the



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement