In the past, most scientific study was oriented toward how the environment, or physical factors, affected the living organisms. Rachel Carson's book Silent Spring , published in 1962, made it all too apparent that living organisms, especially humans, have a significant effect on the environment as well. Scientists study ecology at different levels.
Some ecologists concentrate on how a given species responds to physical factors.
They might attempt to determine specific limits of an organism's tolerance of different environmental factors, or carry out experiments to define the physical conditions that promote optimum growth and reproduction. This area, called physiological ecology , emphasizes the experimental aspects of ecology. Scientists also study ecology at the population level . A population is usually defined as all of the individuals of the same species that live in a given area. For instance, we can speak of the population of humans in New York City, or the population of moose on Isle Royale, Michigan.