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succession of native species or form stable communities themselves. There is no biological criterion on which to judge a priori the smaller or greater value of one species against that of another, and if exotic species are occupying environments that are unavailable to native species, it would probably be too costly or impossible to pursue their local extinction.
The paradox of exotic species invasions of islands with high levels of endemism is discussed by Vitousek in Chapter 20. He correctly points out that if the invasion of exotic species is at the expense of the extinction of local endemics, the total species richness of the biosphere decreases and the Earth’s biota is homogenized since most of the invading exotics are cosmopolitan.
NEED FOR BETTER LAND AND RESOURCE MANAGEMENT
In summary, strong evidence can be assembled to document the resiliency of the functional attributes of some types of tropical ecosystems (including their ability to maintain species richness) when they are subjected to intensive human use. Initial human intervention results in the loss of a few, highly vulnerable species. Massive forest destruction is probably required to remove more widely distributed species. Because massive species extinctions may be possible if human destruction of forests continues unabated, the evidence for ecosystem resiliency is not to be construed as an excuse for continued abuse of tropical environments. Rather, ecosystem resiliency is an additional tool available to managers if they choose to manage tropical resources prudently.
We cannot tell the needy of the tropical world that they must cease and desist in their struggle for survival to prevent a catastrophe whose dimensions, consequences, or mitigating conditions we cannot define with any certainty. It may turn out that the public call for conserving natural diversity is also an expression of frustration over the poor use of the natural resources of the tropics and our apparent inability to do something about it. Scientists have the responsibility of focusing the debate. Its fundamental essence, I believe, is the need for better land and resource management.
Experience in the Luquillo Experimental Forest Biosphere Reserve in Puerto Rico has demonstrated that species richness can be partially restored to lands previously used heavily for agriculture, that growing timber need not eliminate all natural species richness on site, and that tropical lands respond to sensible care through management. I know of no technical reason why sensible land management in tropical areas cannot lead to the success that is usually associated with temperate zones. The obstacles to progress are social and rooted in poor training and education programs, lack of facilities and infrastructure, weak institutions, misguided foreign aid programs, lack of commitment to forestry research and to enforcement of regulations, and the absence of a land conservation ethic. A strategy for forest and species conservation in tropical regions should focus on the restoration of forest production on former forest lands where food production is not sustainable. This, and sensible use of secondary forests and tree plantations, will reduce pressure on