The development and spread of agriculture is a much more serious source of habitat destruction. Sometimes, entire natural ecosystems are converted into fields which raise only one or a few plants. The diversity of populations is thus lost.
When forests in New Zealand were cleared for agriculture, a shrub named the Adams' Mistletoe disappeared. In Swaziland in South Africa, one of the last populations of Kniphofia umbrina, a beautiful lily species, was replaced by a field of corn. In 1978 only a few thousand individuals of the entire species remained. In Ecuador, the Caoba tree has been reduced to about a dozen individuals. Its habitat has been wiped out and replaced by banana and oil palm plantations. The Vuleito Palm of Fiji has also lost land for the raising of bananas.
Billions of plant populations have been plowed under or eaten by domesticated herbivores. The seriousness of this occurrence is greatly multiplied because of the fundamental position of plants in food chains. In addition, because of the specialized feeding habits of most organisms that attack plants, every plant species that goes extinct takes an average of ten to thirty species of other organisms with it. The diversity of plants is the underlying factor that controls the diversity of other organisms and thus the stability of the world ecosystem.
In the Cape Province of South Africa, native plants are hard-pressed by agriculture and urban expansion. The area has over 6,000 kinds of plants, including over 600 species of heathers, weird giant proteas, bulbs, and orchids. The area also holds the world record for endangered plants. Over 1,200 species of the plants that live there are threatened, several hundred are nearly extinct, and 36 recently disappeared entirely.
The grazing of domestic animals is another aspect of agriculture that threatens plants populations everywhere, and the herbivores that depend on them as well. A Russian orchid is threatened because cattle eat it and compact the soil so it cannot grow. Cattle are also wiping out a beautiful Swiss alpine relative of the daisy. In the Balearic Islands, a peony of great horticultural value, whose roots may hold a cure for epilepsy, is being eaten by goats. Cameron's Euphorbia, a valuable succulent food for livestock in arid lands, is virtually extinct around the Horn of Africa because of overgrazing. In New Zealand, Godley's Buttercup is being destroyed by introduced grazers, especially the Chamois. Most of the Mediterranean region had already been stripped of its native vegetation by deforestation and overgrazing centuries ago. The flora of California was so completely changed by cattle grazing and cmpetition from Mediterranean species introduced by the Spanish that the botanists of today are not sure what the original species was really like. Much of Australia's native flora has long since disappeared down the throats of sheep.
The spread of agriculture over the world has also affected the distribution and abundance of animal species over vast territories. The majority of animals that once lived in California's Central Valley have disappeared since the area was converted into one of the richest, most pesticide-soaked agricultural regions in the nation. Overgrazing and conversion of land to truck and cotton farming in Arizona are pushing the Gila Monster toward the brink. And on a small Japanese island near Okinawa, the Iriomote Cat is being extinguished by farmers. This creature may set a modern record for mammals - less than forty years from description to extinction.
In areas such as Kenya lies one of the greatest problems. Its population level is exploding at a rate of over 4% per year. This means that there is more and more pressure to bring marginal and submarginal lands under cultivation and grazing. Kenya's game parks currently occupy those very areas, and are already severely threatened. Demands have been made that sectors of Tsavo National Park be turned over to landless peasants.
Overgrazing caused by agricultural expansion has also caused desertification. About 6% of earth's ice-free land surface is already desert, and another 28% is under risk of being converted to desert. The process is occurring at a frightening pace. Supposedly, a worldwide area equivalent to two Belgiums is being converted to desert annually. In addition, dominance of undesirable plant species, erosion, and declining rainfalls and groundwater supplies are common where overgrazing has occurred. When water is diverted to supply agriculture and people in drier areas, it leads to the drying up of springs and rivers, and the damming, rechaneling, and general development of most others. This diversion of water affects terrestrial and aquatic systems. For instance, an island, the largest known breeding ground of the California Gull, has become a peninsula. The new land bridge lets coyotes have access to the island, threatening the livelihood of these birds. They have consequently been forced out and the gulls have not found a safe substitute breeding area.
Another agricultural activity that has enormous impact is the use of insecticides and herbicides. Two of its many problems is that it leaves the areas where it is applied and threatens populations and species that are not its intended victims. In other words, they are both mobile and nonselective.
In the past, toxic chemicals-especially chlorinated hydrocarbons, such as dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs)-have become concentrated in food webs, the interconnected food chains that circulate energy through an ecosystem. These toxic chemicals affect the crucial species near the top of the food chain. Both DDT and PCBs interfere with the calcium metabolism of birds, causing eggs to have soft shells and producing malformed young. PCBs will also impair reproduction in some carnivorous animals.
Between 1960 and 1963, about 10 to 15 million fishes, including catfish, Menhaden, mullet, sea trout, drumfish, shad, and buffalo fish, were killed in the lower Mississippi. Water birds were also affected. What had happened was a chemical relative of DDT, endrin, had entered the water in runoff from sprayed and dusted agricultural fields and in waste water from a company called Velsicol Chemical Corporation, which manufactured the compounds. Harmful effects from pesticides have also been found in phytoplankton and predatory birds. Fortunately, restrictions on the use of DDT and other hydrocarbons in the US have led to recovery for the Brown Pelican, Peregrine Falcon, and Osprey.
The thing about pesticides is that they are synthetic products of human culture. The natural decomposers that usually break down organic chemicals and recycle their constituent parts have not "learned" how to break them down rapidly, and thus the toxins remain in the environment for a long time, threatening organisms the entire time.
One tragic incident involving pesticides involved an outbreak of the Levant Vole in 1975-76 in the northern Huleh Valley of Israel, causing great damage to alfalfa fields. Farmers used a chemical called Azodrin to kill the animals, not realizing it threatened wildlife because precautions on the original label had not been translated into Hebrew. The result was the deaths of around 400 eagles, hawks, owls, and other predatory birds that had feasted on the abundant voles. They also fed on the smaller birds - pipits, wagtails, larks, thrushes, and buntings - that had been killed by the original spray. The birds also fed on voles and small birds that had eaten contaminated food. The Azodrin also killed Jungle Cats and wild pigs.
Indirect Causes - Habitat Loss
