Position and Evidence

A position (or main idea) reflects the main point an author wishes to make about a subject. A stated main idea, or topic sentence can appear in the beginning of a piece of writing as part of the introduction. The author’s discussion presents evidence to explain or support that idea. Evidence can appear as main supporting ideas, which could be reasons or proof, or as minor supporting details, which may be examples.

The following section from “Explore Natural Sounds” highlights the position, the main supporting ideas, and the minor supporting ideas to help you understand how these work together to present the author’s argument.

“Effects on Wildlife” is part of an article called “Explore Natural Sounds” posted by the National Park Service, a part of the U.S. Department of the Interior. The highlighted sections show the position, main supporting ideas, and minor supporting ideas. Roll over each highlighted sentence for an explanation. After you have identified the position, major supporting details, and minor supporting details, evaluate the ways in which supporting details define and prove the position.

Effects on Wildlife

Functioning ecosystems depend on natural acoustical environments. Many animals, insects, and birds decipher sounds to find desirable habitat and mates, avoid predators and protect young, establish territories, and to meet other survival needs.

Scientific studies have shown that wildlife can be adversely affected by sounds and sound characteristics that intrude on their habitats. Although the severity of the impacts varies depending on the species being studied and other conditions, research has found that wildlife can suffer physiological and behavioral changes from intrusive sounds and other human disturbances. Some sound characteristics have been associated with suppression of the immune system and increased levels of stress hormones in animals. Studies have shown that songbirds that live in places with increasing sound levels have to sing louder than birds in quieter environments, and not all species have the ability to adapt in this way. Birds forced to sing at a higher volume have to expend increased levels of precious energy to attract a mate or warn of predators. Bighorn sheep are less efficient at foraging for food when they are exposed to aircraft, and mountain goats often flee from the sound of helicopters and airplanes. Still other research has demonstrated that intrusive sound properties can adversely affect reproductive success in caribou and communication in whales. When these effects are combined with other stressors faced by wildlife such as winter weather, disease, insect harassment, and food shortages, sound impacts can have important implications for the health and vitality of wildlife populations.

As you can see, “Effects on Wildlife” carefully supports its position with both main supporting ideas and minor supporting ones. The minor supporting details reflect a wide range of different kinds of wildlife in different environments, and the author is careful to show that research supports each minor detail. The closing sentence ties together the position statement and the evidence to reach a conclusion.


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