13.Birds and their role in ecosystems. Causes of decline.

Around the world, ornithologists are alarmed at bird population declines and are concerned about what the mean for the worlds ecosystems and our own future. According to estimates about 12 percent of the worlds 9,800 bird species are threatened with extinction and in the near future more species may become threatened.

Its not just the loss of birds that worry scientists, but what their loss could mean for the ecosystems that depend on the many services birds provide: dispersing seeds, pollinating flowers, controlling insect and rodent populations, and eating dead animals. As a result of the bird declines plant species become extinct.

Many declining bird species serve as indicators of growing environmental problems. Aquatic songbirds called dippers, for example, disappear from stream waters acidified by pine plantations and acid rain.

Habitat loss and degradation is the leading danger. Most threatened species depend on one type of habitat, and many of then are forest-depended. Yet forests are being lost around the world at a rapid rate-especially in tropical regions that are particularly rich in bird diversity. Grasslands, shrub-lands, savannas, and wetlands also provide important bird habitat. These systems are being lost or degraded.

Hunting for food or collection for the pet trade is yet another threat to birds which are an important food source for subsistence communities. Birds are also trapped for the pet trade. The colorful and talkative parrots and cockatoos are particularly threatened by capture for the cage-bird trade.

Pollution threatens birds in the oceans, near industrial sites, and in the countryside. Large and small oil spills kill many birds, including penguins and gannets. In addiction to increased tanker traffic, aging vessels make the business of transporting oil even more hazardous. On land, oil and natural gas exploration, extraction, and pipelines threaten some of the worlds most bird-rich habitats in Peru, Ecuador. Agricultural pesticides kill millions of birds, weaken others, and deplete birds food supplies of insects and wild plants.

15.Renewable and non-renewable natural resources.

Natural resources can be categorized as either renewable or non-renewable. Renewable resources are replaced naturally and can be used over and over again. Non-renewable resources,however, are almost impossible to replace and their supplies lessen with each use.

RENEWABLE RESOURCES. For many years people thought that water was one of the most abundant natural resources. Water is fragile resource. Water re-entering the water cycle often carries traces of fertilizers, pesticides, industrial chemicals, and sewage. These contaminants pollute the water and can destroy its value as a resource.

Forests are renewable r. if people plant new trees to replace those cut down. Fish and wildlife are renewable res. If people leave enough fish and wildlife to reproduce and if they preserve natural habitats.

Soil-renewable res.

NON-RENEWABLE RESOURCES. Such important resources as coal, oil, natural gas, iron ore, copper, bauxite, gold, silver. As people use these resources, they cannot be replaced.

Resources depletion can be slowed through recycling. Recycling is the process by which products that have been used and discarded can be reused. People, who work to protect natural resources and natural environments, support recycling because it slows the use of the earths res.

Paper, which is made from trees, is one of the commonly recycled materials.