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1 Tyou have mainly learned about human activities that cause ecosystems to change and some that allow them to resist change. These include overfishing, the introduction of invasive species, and runoff of chemicals, and more responsible interventions, such as sustainable fishery management and aquaculture. However, a single event, whether natural, such as a volcanic eruption or forest fire, or human error, such as a large oil spill, can also bring longlasting changes to ecosystems.

2 Use the Read, Think, and Take Note strategy as you complete the following reading. When you have completed the reading, share your Read, Think, and Take Note responses with your partner. If possible, answer any questions your partner had about the reading. Note any similarities in your responses. Choose one response to share with the class. Share your response to the reading in a class discussion, as directed by your teacher. In today s world there are few, if any, ecosystems left untouched by people in some way. In some cases this has resulted in major changes, and in others there have been very minor changes. Humans have influenced ecosystems for thousands of years, but current technology and large population size have intensified human impact. Yet while scientists previously thought that ecosystems would basically stay the same in the absence of human intervention, decades of research have shown that ecosystems are constantly changing and shifting. Many things cause disturbances in an ecosystem. Ecologists use the term disturbance to refer to an abrupt event in an ecosystem that suddenly and significantly changes the resources available, the number or type of organisms, or the kinds of species present in an ecosystem. Natural disturbances include volcanic eruptions and fires. Two examples of disturbances caused by humans are oil spills and clear- cutting of forests. The response of an ecosystem to a disturbance depends in part on how major the disturbance is and the extent of the damage done.

3 In responding to a disturbance, an ecosystem undergoes what is referred to as ecological succession. Ecological succession is the natural process in which a disturbed area is gradually taken over by a species or groups of species that were not there before. For example, if a volcano erupted and the molten lava smothered a large area of land, eventually that area would undergo succession. In Hawaii and other volcanic locations, barren expanses of hard lava rock have turned into thick forest ecosystems in fewer than 150 years. Primary succession is ecological succession that starts in an area where there are essentially no living organisms. In the lava-covered area, the first life you would see would be small organisms, such as spiders, that can live without soil. Eventually, dust and leaves from surrounding areas would collect in the cracks and crevices of the lava, and some of the more hardy plants, introduced by wind-blown seeds or birds, would begin to grow there. As the decades passed, more of the surface would become covered in plants, as leaves and other debris decayed and formed soil. The level of biodiversity would rise as more soil developed. When an ecosystem undergoes a disturbance and the soil remains, secondary succession develops instead of primary succession. This might occur after a wildfire, or in a now-unused field where generations of ranchers had grazed cattle. Often there are still plants that survived the disturbance. If there are no further disturbances, those areas tend to return to their previous states. In Yellowstone Park after the huge wildfires in 1988, some of the first plants to grow back were grasses, small flowering bushes, and a bush called fireweed (named for its ability to grow well after fires). By 2000, small pine trees were beginning to replace the trees that had burned down, and today there are sections of burn areas in Yellowstone where it is difficult to tell that such a large fire ever occurred.

4 An ecosystem s ability to return to a stable state after a disturbance is a measure of its resilience. If an ecosystem has a high level of resilience, it will be much more likely to recover from a major disturbance. The level of resilience depends on several important factors, one of which is the native biodiversity within and surrounding the disturbed area. Because different species occupy different habitats and perform different roles within an ecosystem, the more species that survive the event the more likely those habitats and roles will be quickly filled again after a major disturbance. Another critical element for ecosystem resilience is the presence of species in nearby areas that might repopulate the disturbed area. For example, if a coral reef has been severely damaged by a large storm, fish and other organisms in nearby areas might help to repopulate the reef. Major disturbances, however, also increase the chances that an area will be occupied by invasive species, because there is so much available habitat. The resistance of an ecosystem is one factor that can help prevent this. The resistance of an ecosystem results from the natural factors within an ecosystem that help it to withstand external pressures and maintain normal functions. For example, if an invasive species of fish were released in a lake where there were native turtles that preferred to prey on those fish, the resistance of that lake to the invasive fish is high. As ecologists learn more about ecosystem disturbance, resilience, and resistance, they have found that while ecosystems are often able to recover from major disturbances, they also have a point after which they cannot. One concern is that evidence suggests that many of the world s fisheries are at or beyond this point. Fisheries biologists and ecologists are working with many of the world s commercial fisheries to try to determine what, if anything, can be done to prevent our ocean ecosystems from being fished past the point of resilience. As you will see in the next activity, there are several possible solutions that may help to repair commercial fisheries around the world.

5 What is the difference between primary and secondary successions? How does the native biodiversity of an ecosystem affect its resilience? How is the resilience of an ecosystem different from its resistance to disturbance? What factors would you investigate to help you decide whether an ecosystem is nearing the point where it is no longer resilient and, therefore, unable to recover from a disturbance? Explain why you chose those factors.