Farther from the toxic source, trees may be able to survive. Changes in vegetation with distance from a toxic source define the ecological gradient. Ecological gradients may be found around smelters and other industrial plants that discharge pollutants into the atmosphere from smokestacks. For example, ecological gradient patterns can be observed in the area around the smelters of Sudbury, Ontario, discussed earlier in this chapter. Near the smelters, an area that was once forest was a patchwork of bare rock and soil occupied by small plants. The ability to resist or withstand stress from exposure to a pollutant or harmful condition is referred to as tolerance.
Tolerance can develop for some pollutants in some populations, but not for all pollutants in all populations. Tolerance may result from behavioral, physiological, or genetic adaptation. Behavioral tolerance results from changes in behavior. For example, mice learn to avoid traps. Physiological tolerance results when the body of an individual adjusts to tolerate a higher level of pollutant. For example, in studies at the University of California Environmental Stress Laboratory, students were exposed to ozone (O3), an air pollutant often present in large cities.
The students at first experienced symptoms that included irritation of eyes and throat and shortness of breath. However, after a few days, their bodies adapted to the ozone, and they reported that they believed they were no longer breathing ozone-contaminated air, even though the concentration of O3 stayed the same. This phenomenon explains why some people who regularly breathe polluted air say they do not notice the pollution.