Natural Disruptions to Ecosystems
You have learned that an ecosystem is a community of organisms that interact with each other and their environment. Ecosystems change over time. Sudden disruptions such as volcanoes, floods, or fires can affect which species will thrive in an environment. Other disruptions are caused by human activities. Some disruptions can be devastating for an individual species and may even cause an entire species to permanently disappear in a process called extinction. As species become extinct, the variety of species in the biosphere decreases, which decreases biodiversity, or the variety of life. Every organism plays an important role in the ecosystem to which it belongs. Whenever one species is removed, other species in the food chain are affected. These changes influence a community’s biodiversity and can disrupt an entire ecosystem.
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Fire is a common disruption to ecosystems that can be caused by nature or by human behavior. Natural wildfires can both help and harm an ecosystem. For example, wildfires kill many small animals and displace others that flee to safety. Animals looking to return after the fire will find their homes and much of their food supply destroyed. Also, bare soil that remains after a wildfire is particularly susceptible to soil erosion because the soil is no longer held in place by roots.
Wildfires can help an ecosystem by clearing out much of the dead and dying vegetation allowing surviving plants to benefit from increased light. Ash and charcoal left from burnt vegetation can help add nutrients to depleted soils. These nutrients provide a rich environment for surviving vegetation and sprouting seeds, allowing them to flourish. Eventually ecosystems rebound from fire disruption and are renewed with nutrient-rich soil and flourishing vegetation.
Flooding can occur after a storm and may be disruptive to an ecosystem, depending on the extent of the flooding and how long the water stays. Flooding can result in saturated soils, or soils that are filled with water. Plant roots require oxygen, so saturated soils can kill plants by drowning the plant roots. Flooding may also cause water and nutrients to run off across land surfaces. Rushing water can cause soil to wash away, particularly bare soil. Burrows, dens, and nests can be destroyed, forcing surviving animals to relocate.
Some ecosystems, called flood plains, are made up of species that have adapted to occasional flooding. Flood plains are flat areas along rivers that flood when the river rises above its banks. The flooding deposits nutrient rich sediment along stream banks. Adaptations of flood-plain species allow them to thrive in their ecosystem.
On May 18, 1980, an earthquake under Mount St. Helens contributed to a tremendous volcanic eruption. The north face of the mountain slid away in a huge avalanche, releasing a blast of superheated, rock-filled gas that ripped up the trees in its path. Subsequently, slower flows of gas and rock destroyed trees and living organisms in the soil. Mature forests were turned into ash covered wasteland. Since then, hardy plants have reappeared in the ash field. The plants attract herbivores that drop seeds from other plants in their dung. About three decades after the eruption, the forest began to regrow. The steady progression of species change and replacement in an ecosystem over time, as occurred after the eruption of Mount St. Helens, is called ecological succession.
Human Effects on Ecosystems
Humans impact the natural world more than any other species. Without limits on human activity, humans can damage the environment in many ways. As the human population grows, it requires more resources to keep its members healthy and comfortable. Each new family needs a place to live, food, water, clothing, medicine, and tools.
Habitat destruction occurs when a habitat is removed and replaced with some other type of habitat. No matter how careful the plan, land used to grow crops or build houses is no longer suitable for some of the organisms that once lived there. As a result, the organisms living at the site must move or be destroyed. Habitat destruction is the most important reason species are threatened with extinction today. Habitat destruction can have harmful effects on humans, too. Scientists believe that some of the destruction caused by Hurricane Katrina in 2005 resulted from the removal of wetlands that would normally take up much of the regional floodwaters. Natural coastal ecosystems, including thick vegetation, may be able to buffer the coastline from hurricanes and other storms.
Wherever humans go, they take animals, plants, and microorganisms with them. Tulips-as well as most crop plants and many other highly-valued garden plants-are not native to the United States but rather are introduced species. Introduced species are an important part of the economy and of society. Some introduced species, however, can be disruptive to an ecosystem. An introduced species that has negative effects in its new ecosystem is called an invasive species.
Invasive species can have serious effects on an ecosystem and on the human population. For example, a vine called kudzu was introduced to the southern United States as a way to control soil erosion. However, the kudzu spread rapidly, choking the roots of some plants and blocking light for other plants, effectively killing them and changing the ecosystem.
Throughout history, humans have hunted or killed animals for several reasons. They obtained food and necessary materials from animals and eliminated competition for crops and prey. They protected livestock by trapping or hunting animals such as wolves. Many species were driven to extinction by hunting thousands of years ago. As the human population grew, more and more species of animals were threatened with extinction from overhunting. Laws were passed to limit hunting and to ban the killing of endangered species. Nevertheless, poaching, or illegal hunting, continues to threaten many populations.
Pollution and Environmental Change
As you can see in the graph on the next page, the human population has grown exponentially over the last few centuries. This population growth was a result of increasing resources and new technology to feed and provide for the growing population. Human activities cause pollution-the release of harmful substances into the environment-including air pollution, water pollution, and the production of hazardous waste. Air pollution in the form of carbon dioxide has contributed to global warming.
One effect of today’s heavy use of fossil fuels-including coal, oil, and gas-is air pollution. Air pollution includes noxious compounds and particulate matter. Air pollution can result in respiratory trouble and other human sickness.
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Sources of water pollution include the runoff of fertilizer from lawns and agricultural fields, as well as runoff from oil and other urban pollutants. Water pollution is also caused by oil spills, in addition to smoke from power plants. Fertilizers introduce excess nutrients and result in major changes in water ecosystems, including changing the pH of the water, causing algae blooms, and reducing the oxygen in the water. Rainwater can wash all pollutants on Earth’s surface into rivers, lakes, and the ocean. Oil spills kill animals and can affect several different food webs. Air pollution can lead to water pollution by causing acid rain that can damage-and even kill-living things. Water pollution can also occur when people do not properly dispose of toxic household or industrial materials.
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Climate change can involve global warming, a slow rise in Earth’s average temperature. While global temperatures have changed over time, most scientists agree that the release of carbon dioxide into the air from the burning of fossil fuels contributes to this rise in temperature. An increasing rise in temperatures could affect ecosystems around the globe. For example, rising temperatures can speed up the melting of glaciers and ice caps disrupting or destroying habitats along shorelines as waters rise. Climate change can affect precipitation patterns resulting in droughts or floods. Climate change can also alter ecosystems as populations unable to survive warmer temperatures migrate to cooler regions.