5.5 Biogeography  (Page 2/6)

Ecologists who study biogeography are especially interested in patterns of species distribution. No species exists everywhere; most are found in relatively small areas of the world. For example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size. Other species are generalists: species which live in a wide variety of geographic areas; the raccoon, for example, is native to most of North and Central America. Some birds (e.g. Osprey, Pandion halietus) are found in appropriate habitats on several continents.

Species distribution patterns are based on biotic and abiotic factors, and are also influenced by events occurring during the very long periods of time required for species evolution. Early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century, and, in fact, observations of these patterns helped Wallace and Darwin formulate the theory of evolution. Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and animals; 92% of the plant species, and 83% of the mammal species in Australia are endemic (found on no other continent). See the Figure below for a couple of examples. This is a consequence of the fact that Australia and Asia have been geographically separated for at least 50 million years.

Sometimes ecologists discover unique patterns of species distribution by determining where species are not found. Hawaii, for example, has no native land species of reptiles or amphibians, and has only one native terrestrial mammal, the hoary bat. Most of New Guinea lacks placental mammals, and prior to human settlement, there were no land mammals in New Zealand except for three species of bats.

Plants can be endemic or generalists: endemic plants are found only on specific regions of the Earth, while generalists are found on many regions. Isolated land masses—such as Australia, Hawaii, and Madagascar—often have large numbers of endemic plant species. Some of these plants are endangered due to human activity. The forest gardenia ( Gardenia brighamii ), for instance, is endemic to Hawaii; only an estimated 15–20 trees are thought to exist ( [link] ).