The lime dimension Tropica! forests illustrate the importance of the time dimension in considering the roles of species in ecosystem functioning. Through selective harvesting of plants and animals we have performed many "experiments'" to test the role of various species and functional groups in the functioning of total ecosystems. However, interpreting the results of these experiments may take a long time, since many organisms live for centuries and many ecosystem processes have very long time constants, e.g. soil formation. Tropical forests provide a good example of this. As pointed out in the tropical forest chapter, only 50 tree generations have elapsed since the last glacial retreat, when temperatures were considerably cooler. In much more recent time there has been a massive, and selective, harvesting of the large mammals of the tropical forests by humans. Because of the long lifespan of the dominant trees, we may not readily see the dramatic and long-term impact of the shifting balance of herbivores and carnivores in this system on plant reproductive biology, and hence the structure and function of the forest.
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