Sunday, June 24, 2012
But many changes to water system carry seeds of disaster.
The Aswan Dam on the upper Nile, completed in 1963 and fully effective by the 1970s, was meant to regulate the river’s flow and provide hydro electric power. However, the traditional fertility of the Nile valley was dependent on over 100 million tones of silt deposited annually as the river flooded. The silt is now silting up the artificial Lake Nasser, forcing farmers downstream to rely on fertilizers, and robbing local brick makers of raw materials; around 35 percent of Egypt’s cultivated land is suffering from salination. Deprived of nutrients, the fish stocks of the eastern Mediterranean are declining while the Nile delta is being eroded steadily. Schistosomiasis is a disease common in newly irrigated areas, has spread explosively, causing debilitation and death; in 1990 between five and six million people were affected.
Surviving wetlands are often badly damaged: by engineering schemes; by eutrophication, when nutrients flow off the land causing huge growths of weeds that over whelm other plant life; by pollution from sewage or toxic industrial waste; or by thermal pollution from water used for cooling.
Tuesday, June 19, 2012
Sometimes wetland areas are seen as places where disease carrying insects breed, so they are drained without thought of their importance to the environment as a whole. Forests are felled because of the value of the wood they contain, regardless of the fact that they act as a water holding system, without which rivers may run faster in rainy seasons, causing soil erosion in their upper reaches and problems of silting as the river approaches the sea.
In some areas, even nonessential human demand is regarded as paramount –for example, the breeding beaches of marine turtles in many parts of the world have been sacrificed to holiday homes and hotels, used by a comparatively small number of wealthy visitors. Although experience should show that environmental change often has far reaching effects not foreseen in the early stages of change, mankind seems to be at many instances incapable of learning.
Sunday, June 17, 2012
By 1990, the numbers had risen to over five billion people will be looking for shelter and food somewhere on the earth. One species cannot increase explosively without affecting all the others. Since about 1700, at least 300 species of vertebrates have become extinct. Now a days we can observe in detail most creatures that are being lost, and the estimate is between 50 and 100 species a day. If the present trend continues at least a quarter of all living things will have disappeared before the human population reaches its levels out, probably in the middle of 21st century.
Monday, June 11, 2012
Plant extinctions have followed the same pattern as animal extinctions. Destruction of environment, particularly on isolated islands, has led to the most dramatic losses. On Hawaii, for example, about 300 plant species have become extinct within the last 300 years. On St. Helena, only 20 of over 100 species of native plants survive and of these 15 are in danger of extinction. It is said that of Barbados all native trees were felled within 40 years of the island’s discovery by European.
Just as an animal species may depend on one plant for survival, the reverses may also be true. For instant, the dodo tree, once abundant in Mauritius, has been reduced to 13 very aged trees since the extinction of the dodo, which ate and dispersed its seeds.
Commercially valuable plants have often been exterminated by over exploitation. Mauritius ebony and Sanfernandez sandalwood are both extinct, examples of felling without provision for regrowth. The wine palm of Dominica, tapped to make spirit in a way that killed it, became extinct in the 1920s.
Habitat destruction is the greatest threat to plant and animal species worldwide. In some cases, such as felling of tropical forests or draining of wetlands, damage is frequently done under the impression that the environment is being improved for human food production or habitation. In others, lack of understanding of the importance of particular environment in the life cycles of certain species has led to their downfall. Drainage of mangrove swamps, nurseries for many kinds of fish.
Tuesday, June 5, 2012
When an animal species is domesticated, it is changed in many ways from its wild ancestors. it may become bigger, stronger and more fecund than the ancestral form. Remaining wild animals are regarded as weeds to be intractable and may have characteristics that have been bred out over generations.
Camel: Date of first domestication uncertain, perhaps by 4000BC. Dromedary, one humped camel unknown as a wild animal. Bactrian camels endangered.
Cat: First domesticated before 1500 BC. Now greatly reduced in the wild.
Cow: Firstly domesticated about 400BC. The wild ancestor, the aurochs, became extinct before 1630.
Donkey: First domesticated about 3500BC. Now extinct over almost all of its former range. Remaining wild asses endangered.
Goat: First domesticated about 7000BC. Now found only in the most remote mountainous areas.
Pig: First domesticated about 7000BC. Almost, the only domestic animal that still survives in significant number in the wild.
Sheep: First domesticated about 8000BC. In Europe reduced to wild populations in Corsica and Sardinia. Elsewhere found in remote mountain areas.
Wolf: The first animal to be domesticated, before 10,000 BC. Once the most widespread mammal now reduced to strongholds in remote places. No longer found in most of Europe; numbered in tens in those western European countries where it survives.
Domesticated animals would seem to be in the least danger of extinction. However, in a world where efficiency of production rules supreme, many domestic breeds, slower in growth or less productive in the amounts of meat or milk they provide, have disappeared in recent years. Attempts are now being made to rescue the genetic heritage they represent, for they may be of use in the development of future breeds.