The Distribution Of Biodiversity

Life-forms of one kind or another are to be found in almost all parts of the surface of Earth, and insects are known to exist in most of these environments except the marine ecosystem. Clearly there is a strong latitudinal gradient in biodiversity, with few species occurring in higher latitudes and most species occurring in the tropics, peaking in tropical rain forests and coral reefs. Freshwater systems occupy a very small part of Earth's surface. Only 2.5% of all water on Earth is nonmarine, and most of this is unavailable to life; 69% of all fresh water exists as ice, principally in the polar regions, and another 30% is present underground. Just 0.3% of Earth's fresh water is freely available in rivers, streams, lakes, and freshwater wetlands, taking up only about 1% of the planet's surface! Although occupying a tiny percentage of Earth's surface, freshwater ecosystems support a rich and varied insect fauna. For some groups, the number of freshwater inhabitants is seemingly out of proportion to the representation of existing freshwater systems.

Of the world's open forest and shrubland, 75 and 42%, respectively, lie within tropical boundaries. At least two-thirds of all plant species are tropical, and thus, 6 to 7% of Earth's surface may contain 50 to 90% of all species of plants and animals. The high species richness of tropical forests is illustrated by La Selva forest of Costa Rica, 13.7 km2 of which harbors 1500 species of plants, more than the total in the 243,500 km2 of Great Britain. This Central American area also contains 388 species of birds, 63 of bats, and 42 of fish, as well as 122 reptile species and 143 butterfly species. A single site in southeastern Peru has yielded more than 1200 species of butterflies—almost a quarter of the 5000 species thought to be found in South America.

Two strata in forests are particularly noteworthy, both for their important roles in the functioning of animal and plant communities and for their high insect species richness: the canopy and the soil. The canopy of trees has been called by some the "last biotic frontier" because of the immense diversity of insects, plants, and fungi found there. Forest canopies came to the attention of biologists largely through the work of entomologists using knockdown insecticides to collect insects from the tops of trees. In 1982 Stork used knockdown insecticides released by a fogging machine hoisted in to the canopy of a 75-m-high rain forest tree in Borneo to collect canopy insects. When the collection had been sorted by taxonomists at the Natural History Museum in London, there were more than 1000 species, and yet the area of collecting sheets on the ground was only 20 m2. In total, 4000 to 5000 species of insects were collected and sorted in a similar way from just 10 Bornean trees. For one group, the Chalcidoidea wasps, 1455 individuals were collected, but after sorting it was found that this represents 739 species. Because fewer than 100 chalcid species had been recorded before from Borneo, this indicates how little is known about the diversity of insects in some ecosystems.

Elsewhere, 43 species of ants were collected by canopy fogging from a single tropical tree in Peru, a number approximately equal to the ant fauna of the British isles. Tropical forests may cover only a small percentage of Earth's surface, but they are vital for the global cycling of energy, water, and nutrients. Most terrestrial life is found in temperate and tropical forests and grasslands. Some other vegetation types, such as the fynbos of South Africa, are also extremely species rich. This system supports more plant species per square meter than any other place on Earth, with more than 8500 species in total, 68% of which are endemic.

Perhaps less attention has been paid to the diversity of life in soils and associated leaf litter and dead wood. It is probable that there are at least as many species of insects specific to the soil as to the canopy. The diversity of soil organism assemblages and their importance in ecosystem functioning is just beginning to be understood. Relatively obscure groups such as fungi, springtails (Collembola), mites, and nematodes are all rich in species in the soil and are extremely important in ensuring that organic material is broken down and the resulting nutrients made available for the growth of plants. Earthworms in temperate regions and termites in tropical regions are critical for the production, turnover, and enrichment of the soil. They also help to aerate the soil and increase the through flow of water, hence reducing water runoff and soil erosion.

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