CASE 8 BIODIVERSITY HOTSPOTS: RAIN FORESTS AND CORAL REEFS

A key feature of any community is its biodiversity, or biological diversity. Biodiversity is often taken to mean the number of species, but we can also think of it more broadly and at many levels, including the variety of different genetic sequences, cell types, metabolisms, species, life histories, phylogenetic groups, communities, and ecosystems. We can also consider biodiversity for a particular place, such as the island of Hispaniola, or a tropical rain forest on the island of Hispaniola, or the entire planet.

Biodiversity is usually measured by counting the number of species present in a particular area. Because of limitations in time and expertise, scientists often focus on a subset of the taxonomic groups present. In terrestrial ecosystems, a good predictor of species diversity is the number of plant species, which reflects local moisture and temperature conditions. From this number, scientists can then estimate the numbers of other species, including the arthropods and other small animals that live on or near the plants, the birds and other vertebrates that prey on the insects and plants, the larger predators that prey on these, and the fungi and other decomposers that return the nutrients in dead bodies to the soil.

As a result, the number of species of flowering plants is among the best known, while the number of species of their ecological associates, the insects and fungi, is among the least known. On the island of Hispaniola, nearly every flowering plant species, approximately 6000 in all, is known. Because scientists estimate that there are about 2–3 times as many kinds of insects as flowering plants at any given place, the number of insect species will likely total about 12,000–18,000 once all are known. Each year studies of different insect groups are published that report a doubling or tripling of the known species. For example, recent studies of fireflies on the island reported a new total of 69 species, up from the 17 previously known. Other insects show the same pattern, as do other animals and fungi.

Tropical islands such as Hispaniola, Hawaii, and the Galápagos are missing elements of biodiversity that have not colonized them from the nearest mainland. Only a few of the South American frog and lizard groups are found on Hispaniola, and none but large iguanas made it to the Galápagos, which are farther away from the mainland. Nevertheless, the presence of many still unknown species of most animals and fungi makes ecological studies of tropical regions difficult. For this reason, scientists have turned to simpler communities such as those in extreme environments, where the ecological interactions can often be more completely understood.