Human modifications of environments
While people are affected by their environment, they also have some capacity for modifying it . For example , they can alter the form of a beach by erecting defensive walls and change its quality by fouling it with oil and debris. Again time must enter our analysis, for the impact of human action is often lagged in time. A lagged impact is one that occurs later at the same place or later and at a different place. An example of the first type of lag is where toxic industrial waste is slowly concentrated in the food tissues of marine animals and the birds that feed on them. An example of the second is where protection of a beach on one part of the coast may mean increased wave erosion at another. Likewise, sewage discharged at one point on a river may affect the fish populations downstream.