Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [249]
The term “man-made drought” refers to a form of land degradation secondary to land clearance, sheep overgrazing, and rabbits. When the cover of vegetation is removed by any of these means, land that the vegetation had previously shaded now becomes directly exposed to the sun, thereby making the soil hotter and drier. That is, the secondary effects creating hot and dry soil conditions impede plant growth in much the same way as does a natural drought.
Weeds, discussed in Chapter 1 in connection with Montana, are defined as plants of low value to farmers, either because they are less palatable (or totally unpalatable) to sheep and cattle than preferred pasture plants, or because they compete with useful crops. Some weeds are plant species unintentionally introduced from overseas; about 15% were intentionally but misguidedly introduced for use in agriculture; one-third escaped into the wild from gardens where they had been intentionally introduced as ornamentals; and other weed species are Australian native plants. Because grazing animals prefer to eat certain plants, the action of grazing animals tends to increase the abundance of weeds and to convert pasture cover to plant species that are less utilized or unutilizable (in some cases, poisonous to animals). Weeds vary in the ease with which they can be combatted: some weed species are easy to remove and to replace with palatable species or crops, but other weed species are very expensive or prohibitively difficult to eliminate once they have become established.
About 3,000 plant species are considered weeds in Australia today and cause economic losses of about $2 billion per year. One of the worst is Mimosa, which threatens an especially valuable area, the Kakadu National Park and the World Heritage Area. It is prickly, grows up to 20 feet tall, and produces so many seeds that it can double the area that it covers within a year. Even worse is rubber vine, introduced in the 1870s as an ornamental shrub from Madagascar to make Queensland mining towns prettier. It escaped to become a plant monster of a type depicted in science fiction: besides being poisonous to livestock, smothering other vegetation, and growing into impenetrable thickets, it drops pods that disperse far by floating down rivers, and that eventually pop open to release 300 seeds carried far by the wind. The seeds within one pod suffice to cover two-and-a-half acres with new rubber vines.
To the misguided government policies of land clearance and sheep overstocking previously mentioned may be added the policies of the government’s Wheat Board. It has tended to make rosy predictions of higher world wheat prices, thereby encouraging farmers to incur debt for capital investments in machinery to plant wheat on land marginal for wheat growing. Many farmers then discovered, to their misfortune after investing much money, that the land could support wheat for only a few years, and that wheat prices dropped.
The remaining cause of land degradation in Australia, salinization, is the most complex and requires the most explanation. I mentioned previously that large areas of Australia contain much salt in the soil, as legacies of salty sea breezes, former ocean basins, or dried-out lakes. While a few plants can tolerate salty soils, most plants, including almost all of our crops, cannot. If the salt below the root zone just stayed there, it wouldn’t be a problem. But two processes can bring it up towards the surface and start causing problems: irrigation salinization and dryland salinization.
Irrigation salinization has the potential for arising in dry areas where rainfall is too low or too unreliable for agriculture, and where irrigation is necessary instead, as in parts of southeastern Australia. If a farmer “drip-irrigates,” i.e., installs a small