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Regardless of the mechanisms, it is certainly true that all levels of biodiversity are greatest in the tropics. Additionally, the rate of endemism is highest, and there are more biodiversity hotspots. However, this richness of diversity also means that knowledge of species is lowest, and there is a high potential for biodiversity loss.

Conservation of biodiversity

In 1988, British environmentalist Norman Myers developed a conservation concept to identify areas rich in species and at significant risk for species loss: biodiversity hotspots. Biodiversity hotspots are geographical areas that contain high numbers of endemic species. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots ( [link] ) containing large numbers of endemic species, which include half of Earth’s endemic plants.

 Biodiversity hotspots are indicated on a world map. Most hotspots occur in coastal regions and on islands.
Conservation International has identified 34 biodiversity hotspots, which cover only 2.3 percent of the Earth’s surface but have endemic to them 42 percent of the terrestrial vertebrate species and 50 percent of the world’s plants.

Biodiversity change through geological time

The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are ongoing: speciation and extinction. Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase; likewise, the number of species will decrease when extinction rates begin to overtake speciation rates. Throughout Earth’s history, these two processes have fluctuated—sometimes leading to dramatic changes in the number of species on Earth as reflected in the fossil record ( [link] ).

The graph plots percent extinction occurrences versus time in millions of years before present time, starting 550 million years ago. Extinction occurrences increase and decrease in a cyclical manner. At the lowest points on the cycle, extinction occurrences were between 2% and 5% percent. Spikes in the number of extinctions occurred at the end of geological periods: end-Ordovician, 450 million years ago; end-Devonian, 374 million years ago; end-Permian, 252 million years ago; end-Triassic, 200 million years ago; and end-Cretaceous, 65 million years ago. During these spikes, extinction occurrences approximately ranged from 22% to 50%.
Percent extinction occurrences as reflected in the fossil record have fluctuated throughout Earth’s history. Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times.

Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million years ago). In most cases, the hypothesized causes are still controversial; however, the most recent event seems clear.

The five mass extinctions

The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the rock strata. [link] provides data on the five mass extinctions.

Questions & Answers

A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and th
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Chromosome is the thread-like structure containing DNA and found in the nucleus of a cell. DNA contains gene which has genetic information.
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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