We must make a clear distinction between reversible and irreversible damage to ecosystems. If the damage is within the resilience capabilities of the ecosystem, it is reversible, and the ecosystem will recover.
Occasionally, however, an ecosystem can suffer damage beyond the limits of its resilience, and beyond its capacity for recovery. Some ecosystems are fragile, and they can easily be triggered into collapse. During the Holocene pluvial period (12,000-9,000 years ago), Lake Chad was the size of the Caspian Sea, and the Sahara was lush grassland. But over-grazing by pre-agricultural herders, at a time of declining rainfall, led to soil separation. Once the soil broke down, and separated into dust storms, sand dunes, gravel, stones, and bare rock, the damage was irreversible, at least in historical time.
Similarly, early agriculture has been responsible for much soil erosion. This is seen, for example, in Greece and much of the Middle East. The cedars of Lebanon have long since disappeared, because of both deforestation and soil erosion. Modern agriculture is not entirely blameless in this regard.
The concept of a 'trigger' mechanism is useful here. A relatively minor geological change can produce major consequences. Volcanic activity in Central America produced new land that linked North and South America. It is thought that the change in ocean currents, once the Atlantic and Pacific Oceans were physically separated, may have triggered the last Ice Age, which intensified because of positive feedback.
The extinction of species is also irreversible. The new land bridge linking North and South America certainly led to the extinction of most of the marsupials in South America, as exotic carnivores moved in from North America. The great extinctions that have occurred in various geological epochs are another obvious example, the extinction of the dinosaurs being the most famous.
Our hunter-gatherer ancestors were responsible for the extinction of many species of prey animals because of over-hunting. Such extinctions can have major ecological reverberations. For example, the Clovis Point people in North America hunted many large herbivores to extinction. This led to the extinction of some of the carnivores, such as the sabre-toothed tiger, because of a lack of prey. In its turn, the lack of carnivores led to a gradually increasing population of plains bison. It is thought that these bison destroyed forests by grazing tree seedlings. The forests could not regenerate and, as the existing trees died, the forests became grasslands. It is entirely possible that much of the Prairies of North America are not natural grasslands at all, and that they are the end-result of extinctions caused by excessive human hunting, some ten thousand years ago.
More recently, with the development of trans-oceanic travel, the movement of species from one continent to another by people has caused considerable ecological chaos. Rabbits and cactus in Australia, potato blight in Ireland, Colorado beetle in Europe, and killer bees in South America are obvious examples. Some of the world's worst crop parasite problems are the result of new-encounter parasites (see 3.8). However, although many of these changes are irreversible, some of them are modifiable. For example, the problems of rabbits and cactus in Australia were ameliorated by biological control (see 6.7).
Ecosystem stability is a dynamic stability, maintained by homeostasis, and negative feedback. Stability in a wild ecosystem is clearly essential if that ecosystem is to survive. An ecosystem can normally survive any of the extremes of environmental variation, such as temperature or water availability. Wild ecosystems are vulnerable only to very rare cataclysmic events, such as volcanic eruptions, earthquake-induced tsunamis, or an asteroid collision with Earth.
Modern agro-ecosystems, however, are rather unstable. Much of this instability comes from the failure of a resistance to one or another parasite, and the consequent loss of otherwise excellent cultivars. Another contribution to instability is the loss of crop protection chemicals that have been matched by new strains of crop parasites. And the so-called 'chemical' agriculture of the past half century has led to dangerous losses in soil microbiological activity.
Excellent examples of dynamic stability in plant pathosystems are seen with the n/2 model of the vertical subsystem (see 4.15), and comprehensive horizontal resistance (see 7.2.13).
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