Terrestrial ecosystems vary in their resilience, that is, in their ability to withstand abuse and to grow back when the abuse cases.
If protected from unusual erosion and loss of seed sources, most temperate forests can support successional stages and reestablish themselves in a few ‘centuries.
Grasslands seem more fragile, perhaps because it is so difficult to prevent erosion in semiarid areas. Although we have less data concerning the resilience of tropical forests and tundra, they apparently even less stable ecosystems.
In any case, it is sobering to realize how drastically people have altered terrestrial environments. The degree of destruction in most areas seems roughly proportional to the length of time that agriculture has been practiced and to the state of industrial development.
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During the past 10 years, timber shortages in temperate areas have stimulated heavy logging of tropical forests. Also, well-intended but misguided attempts to increase food production have promoted clearing of tropical forest lands. Meanwhile, development of such arctic resources as petroleum is creating traffic that gouges up the tundra. All these assaults on terrestrial ecosystems trace to increasing human populations. More people require more food and more lumber and more raw materials.
In evaluating the impact of growing human numbers, we must include overall effects on the biosphere. Since the continents are a major portion of this great, worldwide ecosystem, reduced resilience in terrestrial ecosystems must contribute to a generally less stable biosphere. This reason alone is sufficient to cause concern. If people are to survive, we must take care of our precious terrestrial ecosystems.