A species is said to be extinct when it no longer lives anywhere on the planet. Extinction occurs when the last members of a species die because they cannot acquire the food, water, shelter, and/or space necessary to survive. The decrease in population size that typically precedes extinction can be due to environmental change including increased predation, because individuals of the species are outcompeted by other species, and/or because they succumbed to disease. Extinction can also occur when a large number of individuals from a species are killed by a chance event, such as a major drought or flood.
Species are more likely to go extinct when they have limited geographic ranges because many individuals may simultaneously experience chance events, such as droughts, floods, the ill effects of invasive species, or habitat destruction, that harms the population. Species with small population sizes are also more susceptible to extinction because they are more likely to lack the variation in traits and/or the genetic diversity needed to adapt to changing environmental conditions.
Extinction has been pervasive throughout Earth history – every few million years approximately half the species seen in the fossil record either became extinct, or had evolved enough new traits to be given new species names. On evolutionary timescales biodiversity has been maintained by the evolution of new species that replace the extinct species. Rates of extinction have varied among species and through time, but most extinctions are considered “background extinctions”, occurring at rates that did not disrupt entire ecosystems. The phrase “mass extinction” is used to describe one of five major events in Earth history during which many different kinds of species vanished relatively rapidly, over a few tens to hundreds of thousands of years. Today, human activities are causing extinctions at a rate that rivals past mass extinctions.
Extinctions have happened throughout Earth history for a variety of reasons at various rates, including:
- Changes in environmental conditions, including climate (temperature, precipitation, snow and ice cover), ocean acidity, nutrient levels, and ocean oxygen levels.
- Rare events, such as a meteorite impact or a large volcanic eruption, which directly killed organisms as well as caused cascades of changes in the Earth system that changed climates and environmental conditions, including disrupting photosynthesis and thus productivity, which in turn disrupted the food webs dependent on photosynthesis (species interactions).
Humans have caused extinctions and put species at risk of extinction through a variety of activities, including:
- Deforestation, habitat destruction, agricultural activities, and urbanization, which remove organisms from the environment, and limit resources and space available for species populations.
- Fishing and hunting, which directly decrease species populations.
- The release of pollutants and waste that can harm the health of organisms.
- The introduction of invasive species that compete with native species for food, water, or other resources, reducing native species populations.
- Human freshwater use, which can limit the amount of water available for other organisms in ecosystems.
- Activities that cause global warming, such as the burning of fossil fuels, agricultural activities, and deforestation. Increasing average global land and ocean temperatures have altered temperature and precipitation patterns, as well as the distribution of snow and ice cover, which affects the growth and survival of some species.
- Activities such as the burning of fossil fuels, agricultural activities, and deforestation that release carbon dioxide into the atmosphere, some of which is absorbed by the ocean causing acidification. This decreasing pH of ocean waters (along with ocean warming) causes physiological stress for many species, which can cause decreased growth, decreased reproductive rates, and death, leading to decreased species population sizes to the point of extinction.
Can you think of additional cause and effect relationships between extinction and other parts of the Earth system?
Visit the evolution, species interactions, and species population pages to explore more connections between the biosphere and global changes.
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