Natural Changes That Can Affect an Ecosystem

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Ecological succession is a natural process that ecosystems undergo as species evolve, outcompete and replace previous ones and reach a stable level within an ecosystem. The Darwinian concept of natural selection usually determines a species' ecological success. For example, if two similar types of bird coexist in an environment and one develops an adaptive trait, like a longer beak for digging insects out of tree bark, this species will find more food. This species will dominate its competitors in an area where insects hide in deep bark crevices, and competitor numbers will dwindle and disappear unless they adapt to a different food source. Often, a change in food supply sparks adaptations in its consumers. If a certain leaf is a beetle's main food source, and it develops a new chemical, beetles that can't digest it will die. Beetles that can digest the chemical will survive and eventually become widespread.

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Ecosystems can only support a certain number of organisms. This maximum is called carrying capacity. Carrying capacity produces short-term changes to organism populations as they fluctuate around the equilibrium of carrying capacity. For example, take a hypothetical forest environment. After an uncharacteristically warm winter, the wolf population is thriving -- there are 50. Since there are so many wolves, they require an unusually high amount of food and reduce the deer population to 20. Now, with only 20 deer left, the burgeoning wolf population cannot continue to survive, unless it turns to other prey, such as cattle. Wolf numbers drop to 10. With only a few wolves roaming, deer can repopulate without much danger. Their population flourishes to around 80. Now remaining wolves have more food available, and their population grows again. This heuristic view of wolf and deer populations is part of a more complex reality. As wolf and deer populations shift, so does the population of the wolf's other prey, the wolf's prey's prey, the plants that nourish the deer, and so on. This dynamic balance of activity composes the ecosystem.

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Shocks are a general name for a variety of dramatic events that severely impact ecosystems. Natural disasters fit this category. A hurricane can devastate a coastal ecosystem by eroding underwater habitats and depositing sand and debris in the ocean. Rarer events, such as volcanic activity, also have massive effects. The colossal eruption of Mount Tambora in Indonesia in 1815 ejected so much ash into the atmosphere that significantly less solar heat entered. Consequently, 1816 has been called the "Year Without a Summer," because of freezing temperatures the year around. Agriculture suffered, and global famine resulted. And perhaps the most infamous population shock is the asteroid that sparked the Cretaceous-Paleogene extinction -- which is believed to have wiped out the dinosaurs and 75 percent of the planet's species.

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Prior to humans exacerbating natural changes in the climate, fluctuations in greenhouse gas levels, the tilt of the Earth, volcanic activity and variations in solar energy that reaches earth all affected global ecosystems over a more gradual time period. For example, eras with relatively low levels of greenhouse gases in the atmosphere have lower average global temperatures and large glacial areas. Small changes in the sun's energy intensity and earth's orbit and tilt affect how much solar heat enters the atmosphere, or where it hits the Earth. As organisms adapt to climatic shifts, or become extinct, whole ecosystems evolve and continue to support life.