Most are introduced accidentally when they hitch

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Most are introduced accidentally when they hitch rides on boats, trains, planes, trucks, and cars.The pet and garden industries bring new species to our homes and gardens,from which they may escape or be released. Most do not become established, but some do. Some are useful, welcome additions. Honeybees are not native to North America, nor are most of our ornamental plants, nor are ring-necked pheasants. Many other nonna- tive species, once established, especially away from our homes and cities, become pests. In the absence of factors that normally control their populations, they thrive. Sometimes their numbers increase exponentially. Sometimes they outcompete and replace native species. Sometimes they overgrow whole communities. Worldwide concern with alien species is growing.A major challenge in the 21st century will be to develop techniques to stop their spread and control their populations. The Aliens Have Come What are some of the alien species in your region? How serious a problem are they? What is being done to control them? Often, our concern is not to reduce population numbers, but to increase them. Man- agement of game species is an important and growing branch of applied ecology. So is the management of endangered species. In both cases, overall management goals cen- ter on the maintenance of viable populations, often in human-dominated regions. Throughout much of the 20th century, management focused on single species or even populations. Game laws focused on harvest limits.The Endangered Species Act focused on populations that were fast approaching extinction. Often, focusing on single species or populations did not have the desired effects. In the 1960s, for example, it was realized that the bald eagle, symbol of the United States, was becoming endangered in much of North America.Laws were passed forbidding the killing of any eagle for any reason.Still, bald eagle numbers dwindled. New laws were passed restricting the use of DDT in the United States. This pesticide had been found to accumulate in the tissues of predatory birds, including bald eagles, and to adversely affect their ability to produce viable eggs. Still, eagle numbers failed to recover. Eagles needed more than protection:They needed habitat. New laws and regulations preserved nesting sites, restricted human activities around those sites during breeding seasons, and established areas in which eagles could roost, rest, and hunt. Only then, when not only individuals were protected, but so was their habitat, did eagle numbers begin to recover. Management emphasis has shifted from focusing on isolated species and popula- tions to the preservation of habitat. Our attention, too, must shift from populations to larger entities of which they are a part. In the next chapter, we address the ecology of whole systems. R EVIEW Q UESTIONS 1. Briefly, what is ecology? How is the term “ecology” different from “environment”? How is the science of ecology differ- ent from natural history? 2. How are population ecology, systems ecology, and applied ecology similar? How are they different?
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