16.7–16.11: Human activities can have disruptive environmenal impacts

A dense green jungle surrounds Hong Kong.

When you fly over southern New England today, you look down on an undulating carpet of forest that covers hills and valleys. Roads traverse the forest, and here and there you see towns and cities or cultivated land and pastures, but most of what you see is the tops of trees.

If you could have flown over the same route 200 years ago, things would have looked very different. By the early 1800s, most of the forest in southern New England had been cleared and the land was in cultivation. Homesteads consisting of farmhouses, barns, stables, and other outbuildings were scattered across the countryside. Stone walls extended across hills and valleys, separating cropland from pastures.

Going back further, the same flight 400 years ago would have taken you over mostly forested land. Here and there, Native American villages would have stood among cleared fields, but most of what you saw then would have looked similar to what you see today.

The change from forest 400 years ago to cleared land 200 years ago was a major ecosystem disturbance, but now the area has returned to forest. The species compositions are not identical—today’s forests are not exactly the same as those of 400 years ago. Nonetheless, the stone fences found deep in the New England woods today are the only obvious trace of the agricultural history of the area. The fences still snake their way up and down hills, but now they run through solid forest instead of between fields and pastures.

By the middle 1800s, New England settlers were abandoning farms on marginal land and moving west. And, once abandoned, the fields previously cultivated began a process of ecological succession that returned them to forests (see Section 15-15).

In the first year after a field is abandoned, its bare soil provides a harsh environment for plants. Sunlight blazes down, heating the ground surface and baking moisture from the soil. It takes a tough plant to sprout and grow under those conditions, but there are plants that can do it. We call those pioneering species of plants “weeds,” and we see them growing in disturbed habitats—along roadsides, in vacant lots, and even in cracks in pavement.

As weeds grow and die, year after year, their organic matter enriches the soil and helps it retain moisture. And every year, the plants grow more densely so that they shade the ground surface, which no longer gets quite so hot and dry. Additional species of plants can grow in these milder conditions. These plants are taller, so they provide still more shade, and the ground becomes still cooler and more moist. At this stage, the seeds of bushes and small trees can sprout. As these woody plants grow, they make the ground cool and shady enough to allow the seeds of forest trees (oak, maple, or beech) to sprout. More saplings of forest trees sprout and grow every year, and eventually they are so closely packed that little sunshine penetrates their leafy canopies to reach ground level. Undisturbed, the forest stops changing at this point, because the mature trees have created conditions in which only their own seeds can sprout and grow.

An ecosystem disturbance is reversible when the disturbance alters the biotic (living organisms) and abiotic (decaying organisms, soil, rocks) nature of the habitat but does not result in the complete extinction of any species, making it possible for species to re-establish their populations. The example of the New England forests is based on a disturbance that was caused by humans—clearing land for agriculture in the 1700s—but wind storms, forest fires, floods, and volcanic eruptions have been clearing forests throughout the history of life (FIGURE 16-14).

Figure 16.14: The forest returning. Just two decades after an explosive eruption of Mount St. Helens, in Washington State, shrubs and small trees had returned to the plain at the foot of this active volcano. The recovery continues today.

If an ecosystem disturbance involves the complete loss of a species to extinction, however, the disturbance is irreversible. Each species is the result of a long and uninterrupted evolutionary history, involving an interplay of random and selective forces and producing a unique genome. Once lost, a species can never exist again (FIGURE 16-15). For this reason, ecosystem disturbances that involve the loss of species can have more devastating consequences than those that do not lead to extinctions, no matter how great the changes to the abiotic environment might be.

Figure 16.15: The end of a species.