Tropical Forests and the 

 Number of Plants and Animals 

 On Planet Earth 



by William Burger 

 Curator of Vascular Plants 



photos by the author 



'ur world is the home of a plentitude of different 

 animal and plant species. They vary in size and struc- 

 ture from microscopic bacteria to whales and giant red- 

 woods, from fleas to fish and flowers. Despite the rav- 

 ages of disease, predation, drought, flood, famine, and 

 fluctuating climates, planet earth supports millions of 

 different kinds of organisms. 



A little-appreciated aspect of this abundance is 

 how much of it is found on land. 



Despite their vastness (more than 70 percent of 

 the earth's surface) and depth, the world's oceans do 

 not support nearly as many species as does the land 

 surface. It seems likely that the number of named 

 plants and animals living in all the world's oceans 

 roughly equals the number of named beetles, most all of 

 which live on land. And if you argue that there are 

 many undiscovered little critters living deep in the 

 ocean bottoms, one can counter that there are enough 

 undescribed beetles in the high canopy of the world's 

 rain forests to match that number. Of all the different 

 habitats on land, tropical forest communities harbor 

 the vast majority of species. 



Why is it that tropical forests are the home of so 

 many different species? Part of the explanation is the 

 wide separation of different tropical forest regions from 

 one another on the surface of our planet. Latin Amer- 

 ica, Africa, southern Asia, Australia, and the oceanic 

 islands are isolated from each other, and all have their 

 own distinctive faunas and floras. Though they do 

 share a relatively small number of widespread cosmo- 



•C3 Broad openings in the forest canopy bring light to the lower parts of 

 this cloud forest. Tapanti, Costa Rica. 



politan or pantropical species, the large majority of 

 species in each area is found nowhere else. However, 

 even within a single tropical region the richness of 

 species is far greater than what we find in our north- 

 temperate areas. What is it about these tropical regions 

 that allows them to support so many species? 



Some people have the misconception that the 

 tropics are without seasons, and that their natural 

 vegetation is something called "jungles." The reality of 

 tropical nature is far more complex. In lands where 

 there are no cold seasons there can be severe dry sea- 

 sons; it is the length of the dry season that determines 

 whether the local vegetation is desert scrub, thorn- 

 bush, rain forest, or something in between. There are 

 some tropical regions without a stressful dry season, 

 and these support what we call rain forests or cloud 

 forests. The length of the dry season and overall rain- 

 fall are the primary factors determining local vegeta- 

 tion and forest distribution. 



Just as in our winter season, plants must become 

 dormant to survive a long dry season. Consistent pat- 

 terns of hot temperatures and low rainfall support de- 

 sert, grassland, and thorn-scrub communities. If the 

 wet season is short, there is not enough time to produce 

 new foliage, to flower, to fruit, and to build a large plant 

 body. Subdesert grasslands and thorn-scrub are char- 

 acterized by small tough plants. Like the tundra, 

 thornbush and grasslands are a reflection of a severe 

 environment. 



The maintenance of tropical forest vegetation re- 

 quires a reliable rainfall during the wet season. The 

 transition from a low thorn-scrub to a deciduous wood- 



