Concluding Remarks

Humanity needs to protect and nourish tropical forests for many reasons. Without implying that we think other reasons are less important than the ones we discuss here, we direct attention to those components that derive from the biological complexity of tropical forests. Because of their complexity, tropical forests have an extremely high information content. This information resides in the genomes of the individual species, the interactions among them, and the resulting ecosystem patterns and processes. Most of this information is not yet accessible to us because we have described only a modest fraction of the species living in tropical forests; we know almost nothing about ecological relationships among the species we have described, and we have only crude measures at just a few tropical sites of the rales and magnitudes of ecological processes.

Many benefits can be derived from preserving and studying the forests that are the repositories of that information. With improved knowledge of the players and the tropical forest theater we will gain a better under standing of how complex systems work. To live sustainably on Earth, humans need to understand the dynamics of many kinds of complex systems - physical, biological and social. Many degraded environments, both tropical and temperate, need to be restored. Knowledge of how tropical forests work is certain to be helpful in the design, development and execution of restoration efforts worldwide. Increasingly, humans are required to manage ecosystems more intensively in order to increase production of desired products, reduce losses of energy and materials through undesired channels, and establish integrated landscapes whose components interact in ways that improve the rates and stabilities of processes that maintain those systems. Management plans are more likely to achieve their objectives if they are based on solid understanding of the behavior of the systems being managed.

Throughout history, humankind has drawn upon tropical forests for products such as food, fiber, medicines, drugs and esthetic pleasure that enrich human life. All of these components are directly proportional to biological diversity. The woods and fibers of different species are useful for different purposes. The chemicals synthesized by living organisms that arc the bases of medicines and drugs tend to be highly species-specific, or at least are produced by a small number of species, usually closely related ones. Future options to find and use new products are sacrificed as forests are lost and biological diversity is minimized. Also, as a result of our poor understanding of how tropical forests work we inadvertently cause losses of many species living in the forests we do preserve, further rcduce options, and make the remaining forests vulnerable to perturbations they can currently withstand.

Perhaps the feature of tropical forests that most hinders our ability to understand their dynamics is the slow rate at which they change over time. The magnificent trees that dominate and give structure to tropical forests live, on average, more than a century. Some live much longer. Once a tree has gained its position in the forest canopy, it usually survives many years after the clues about the causes of its initial success have disappeared. Its current associates may be quite different from those it had when it was young, and the local climate may have changed as well. Only about 50 tree generations have elapsed since the final retreat of the last of the Pleistocene glaciers. During glacial advances, temperatures dropped on average about 6:C in tropica! lowlands (Bush and Colinvaux 1990}. Pollen profiles from tropical regions reveal that trees now restricted to middle elevations on mountainsides were intermingled with today's lowland trees close to sea level (Bush et al. 1990; Colinvaux et al. 1996). During glacial maxima, levels of atmospheric carbon dioxide were much lower than they were 100 years ago, and very much lower than today's levels, and the difference is steadily increasing (Intergovernmental Panel on Climate Change 1990). Tropical forests are probably still readjusting to post-glacial climatic changes. Some types of disturbances, such as fires and hurricanes, produce immediate and sometimes catastrophic effects on tropical forests and their functional properties (Weaver 1989; Boucher et al. 1990; Walker et al. 1991). However, the longevity of trees causes long lags between the imposition of some types of disturbances and the completion of the functional responses of the forests. For example, loss of certain frugivores may not affect the composition and functioning of a forest for more than century, even if that loss will eventually result in the extirpation of a suite of forest tree species.

For these reasons, much attention will need to be given to understanding the rates at which different perturbations are likely to affect tropical forest processes, which processes they affect, how they exert their influences, and the time-frames over which their effects are likely to be realized. The processes that operate slowly are the ones most likely to be ignored and unappreciated, yet they may ultimately be among the most important determinants of the long-term functioning of tropical forests.

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