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The history of life on Earth is punctuated by a series of mass extinctions, during which the number of species plummeted. Each of these extinctions changed the landscape of global diversity and allowed new types of organisms to thrive in the aftermath. The first mass extinction occurred in the Ordovician period, about 500 million years ago (500 mya). A second extinction occurred in the late Devonian period, about 360 mya. This extinction paved the way for a diversification of land plants, insects, and amphibians in the Carboniferous period. The next mass extinction took place at the end of the Paleozoic era in the Permian period, about 250 mya. After this extinction, marine and reptile diversity increased greatly, eventually leading to the "Age of the Dinosaurs." The dinosaur age crashed to a halt in the late Cretaceous period (70 mya), when the most recent mass extinction eliminated the last dinosaurs and allowed mammals to become a dominant form of life. While these extinctions were caused by a variety of natural phenomenon, human activity can also drive changes in ecosystems that lead to extinction. In fact, we are in a new mass extinction event caused by humans and the way we are altering the environment.
Extinctions may happen quickly in response to ecological stress or they may happen slowly as ecosystems change over time. Either way, they provide newly available niches that can be used by different species and lead to speciation. The diversity and species richness of an area is a product of both extinction and speciation. Scientists have developed the species richness equilibrium model that represents this. The model begins with an uninhabited "island" that can be either a literal island or an area of like habitats completely surrounded by unlike habitats. All species available to colonize the new area are called the "species pool." As more and more new species enter the new area, the species pool becomes smaller and smaller, and the immigration rate (the probability that any given species moving into the area will be a new species) decreases. At the same time, the island becomes more and more crowded and supplies become scarce, causing the extinction rate to increase. The point at which the extinction rate and the immigration rate balance is called the equilibrium point. The model predicts that changes in extinction and immigration rates will tend toward the equilibrium point, which is different for every island, depending on resources and degree of separation from other areas. This is shown graphically in the figure below.
Figure 7.07: Rates for arrival of new species versus extinction of existing species. The intersection of these two lines (S) gives the equilibrium number of species
