In a world with millions of life forms, the unbalanced expansion of any one organism can cause trouble. Duckweed provides an example. Duckweed is a small plant that floats on ponds and streams. It has a single oval leaf less than a quarter of an inch in length, and a tiny root (or two) that hangs down into the water. In suitable conditions, each leaf will grow another, which, when mature, separates and drifts off on its own to grow yet another like itself.
As long as the population of these plants is small, a doubling of their numbers is of little consequence to the living community. However, as the area covered in duckweed expands, a crucial change takes place. Let us say, for this illustration, that it takes one day of summer sun for the duckweed population to double. It has been growing all summer and covers just one sixty-fourth of a pond's surface. That's not much, but in less than a week, that small area becomes a thirty-second, a sixteenth, an eighth, a quarter, a half and on the last day the population of duckweed goes from where half of the pond is open, to where the entire surface is covered over. At that point, sunlight and oxygen are cut off from life below the surface. The exponential speed with which this change is taking place helps explain why, all of a sudden, by historical standards, human beings are plagued by environmental problems when, throughout time, the possibility of disrupting planetary life was unimaginable.