PROPIEDADES DE LOS COMPACTOS EN VERDE Densidad en verde y porosidad

Old-growth forestsare found primarily in northern climates, although there are small caches of untouched trees in remote locations like Tasmania.

Deforestation is the large-scale destruction of trees in a land area by disease, cutting, burning, flooding, erosion, pollution, or volcanic activity.

Old-growth forests are those that have never been harvested. They contain a variety of trees that are between 200 and 2000 years old. Forest floor leaf litter and fallen logs provide habitat for a complex mix of interdependent animals, birds, amphibians, insects, bacteria, and fungi that have adapted to each other over geological time. When the forests are cleared, the animals, birds, and insects that live under their protective cover are displaced or destroyed as well. Genetic uniqueness (biodiversity) of these affected species is permanently lost.

Redwood National Park and three California state parks contain some of the world’s tallest trees: old-growth coastal redwoods.Living to be as much as 2,000 years old, they grow to over 91 meters tall. Spruce, hemlock, Douglas fir, and sword ferns create a multilevel tree canopy that towers above the forest floor. The local shrubs include huckleberry, blackberry, salmonberry, and thimbleberry. Park animals include raccoons, skunks, Roosevelt elk, deer, squirrels, minks, weasels, and black bears.

Redwoods live so long because they are particularly resistant to insects and fire. Scientists have discovered that giant redwoods contain high levels of bark tannins that protect them from disease. Additionally, redwoods can grow either from seeds or new sprouts from a fallen tree’s root system, stumps, or burls. The economic clearing of fallen trees therefore limits new growth.

The most important factor in redwood survival is their biodiversity. Forest floor soils play a big role in tree growth. A healthy redwood forest includes a variety of tree species, as well as ferns, mosses, and mushrooms. These are important to soil regeneration. Fog from the nearby Pacific provides cooling and moisture for the trees.

The redwoods, however hardy, have lost a lot of ground. Of the original 1,950,00 acres of redwood forests growing in California, only 86,000 acres remain today. Three percent of these acres are preserved in public lands and 1 percent is privately owned and managed.

It comes as no surprise that when trees are lost through deforestation, bio- diversity drops. When forests or grasslands are cleared and planted as a single cash or food crop, the number of species drops to one, plus a few weeds. But this is only part of the problem. Since forests support animal species with food and shelter, these species are also eliminated. Often, new species replace the originals, but generally the number of species goes down. When plant cover is removed, other area populations (mammals, birds, and insects) are greatly affected.

Old-growth forestsare made up of trees that are often several thousand years old and take at least 100 years to regrow to maturity if cut down.

A species that has survived, while other similar ones have gone extinct, is called a relict species.A relict species, like the European white elm tree in west- ern Siberia, may have had a wider range originally, but is now found only in particular areas. Other relict species, like horseshoe crabs or cockroaches, have survived unchanged since prehistoric times, even as other species become extinct.

HOTSPOTS

In 1988, British ecologist Norman Myers described the biodiversity hotspot

idea. Although the tropical rain forests have the highest extinction rates, they aren’t the only places at risk. Myers wanted to point out the resource problem facing ecologists. Since they were unable to save everything at once, they needed a way to identify areas with endangered species.

Globally, there are hundreds of species facing extinction because of habitat destruction and loss. Myers identified 18 high-priority areas where habitat cover had already been reduced to less than 10% of its original area or would be within 20 to 30 years. These regions make up only 0.5% of the earth’s land surface, but provide habitats for 20% of the world’s plant species facing extinction. Table 2-4 lists the world’s top 10 hotspots as designated by

Conservation International.

Two factors are weighed heavily in identifying a hotspot: (1) high diversity of endemic species; and (2) significant habitat impact and alteration from human activities.

Plant diversity is the biological basis for hotspot designation. In order to qualify as a hotspot, a region must support 1500 endemic plant species, 0.5% of the global total. Existing natural vegetation is used to assess human impact in a region.

Since plants provide food and shelter for other species, they are used in rat- ing an area as a hotspot. Commonly, the diversity of endemic birds, reptiles, and

An ecological region that has lost more than 70% of its original habitat is considered a hotspot.

If a species cannot find new habitat or adapt to changed land use, it often becomes extinct. Its unique genetic information and position in the ecosystem are lost.

animals in hotspot areas is also extremely high. Hotspot animal species are found only within the boundaries of the hotspot, since they are often specifically adapted to endemic plant species as their main food source.

In recent hotspot designations by world conservation agencies, 25 biodi- versity hotspots—containing 44% of all plant species and 35% all terrestrial vertebrate species in only 1.4% of the planet’s land area—were listed. Hot- spots target regions where the extinction threat is the greatest to the greatest number of species. This allows biologists to focus cost-effective efforts on critical species.

Endemic species have been isolated over a long period of geologic time. Islands, surrounded by water, have the most endemic species. In fact, many of the world’s hotspots are islands. Mild environments, like Mediterranean regions, give shelter to the greatest diversity of species. Topographically different, mild environments, like mountain ranges, allow the greatest ecosystem diversity.

Several hotspots are tropical island archipelagos, like the Caribbean and the Philippines, or big islands, like New Caledonia. However, other hotspots are continental islands isolated by surrounding deserts, mountain ranges, and seas.

Endemic Endemic % plants vertebrates Remaining Endemic Endemic per per natural Hotspot plants vertebrates 100 km2 100 km2 vegetation

Madagascar and 9,704 771 16.4 1.3 9.9 Indian Ocean Islands

Philippines 5,832 518 64.7 5.7 3.0 Sundaland 15,000 701 12.0 0.6 7.8 Atlantic Forest 8,000 654 8.7 0.6 7.5 Caribbean 7,000 779 23.5 2.6 11.3 Indo-Burma 7,000 528 7.0 0.5 4.9 Western Ghats and 2,180 355 17.5 2.9 6.8 Sri Lanka

Eastern Arc Mountains 1,500 121 75.0 6.1 6.7 & Coastal Forests

Peninsulas are key regions for hotspots. They are similar to islands and some, like Mesoamerica, Indo-Burma, and the Western Ghats in India, were islands at some time in the past. Other hotspots are landlocked islands isolated between high mountains and the sea. The Andes Mountains, which separate South America from north to south, are an impassible barrier to many species. On the western coast, the lowlands form a thin, isolated ecosystem from the eastern side of the continent.

The Cape Floristic Province in South Africa is isolated by the extreme dryness of the Kalahari, Karoo, and Namib deserts, and large rivers like the Zambezi and the Limpopo.