4.Causes of Biodiversity Reduction

Several times during the Earth's history there have been periods of mass extinctions , when many species became extinct in a relatively short time period (a few million years is a relatively short time when compared to the age of the Earth). Scientists are unsure of the causes of both background extinction and mass extinction. Possible explanations for mass extinctions include climate changes or catastrophes such as the Earth being hit by a meteor. Since the beginning of time, five or six mass extinctions periods have occurred that eliminated between 35 % and 96 % of all species on Earth (see Table 2.1 below). Further, it is believed that of all species that ever inhabited the Earth over 99 % of them are now extinct.

Table 2.1: Major extinction events during the Phanerozoic.

Despite the vital importance of biodiversity , scientists believe that human activity is responsible for the Earth entering another period of mass extinction, the greatest in all history. Currently, biologists estimate that at least one species becomes extinct each day. Unlike the mass extinction periods of the past, this one is very different. Firstly it is due almost entirely to human activity. Second, species extinction is taking place at an unprecedented time scale, a few decades to centuries rather than millions of years as in past mass extinction periods. Also, unlike the past when mainly vertebrate species, such as the dinosaurs were wiped out, many plant species are also becoming extinct. It is believed that as many as 25 % of the plant families may be extinct by the end of the 21st century. Scientists estimate that if destructive human activities continue then at least 500,000 species will become extinct between 1975 and the year 2000.

It is usually extremely difficult to pinpoint the factor(s) responsible for today's decline of a species numbers. In most cases, species extinction occurs from more than one factor. The following list describes some of the key reasons for the current decline.

4.1 Habitat Loss

The greatest threat to most wild species is destruction, fragmentation, and degradation of their habitats. In the United States, tall-grass prairies have been reduced by 98 %, virgin forests by 95 %, wetlands by 50 %. About 50 % of tropical rainforests have been deforested by 1995. Habitat loss can be the result of two general processes:

Direct Human Impacts on Habitat. Habitat destruction is mainly caused by population growth which pushes people into new areas and requires their destruction for housing, food and industry. Forests are cut down for lumber and paper products. Tailing ponds from mining can degrade wildlife habitat. Hydro-electric development floods out large areas of land, wiping out habitat. In order to feed a growing population, wetlands are drained or forests are burned in order to grow crops. Roads are built to connect one area to another which fragments the habitat. The breaking up of natural landscapes into smaller pieces does not allow species to travel, especially the larger species. Because these habitat fragments are surrounded by dissimilar habitat they are often referred to as habitat islands. With restrictions for traveling, populations become isolated, reducing genetic diversity so those species become vulnerable to changes.

Indirect Human Impacts on Habitat. Human activities that result in pollution can also result in habitat loss even without coming into contact with an area. For example industries and activities that result in the production of acid rain or photochemical smog can have a dramatic impact on both plants and animals. Industrial wastes, mine acids and excessive heat from thermal electric plants have wiped out some species of fish. The threat of the enhanced greenhouse effect may cause rapid changes in climate. Changes in climate that are so rapid that species will not have enough time to adapt, resulting in extinctions.

4.2 Species Introductions

As long as people have had the ability to move about the planet they have been introducing new species, either intentionally or accidentally, into new geographical areas. The introduction of an alien species can cause the extinction, near extinction or displacement of native species. Although some introduced species may provide food, game, beauty or help to control pests, many can cause serious reductions in biodiversity. Foreign species compete with native species for food or habitat or they may even prey on them. Many times there are no natural predators for these species so their populations are not controlled and their numbers can take over enough of the available habitat to limit native populations. Further, they can bring diseases and parasites that are unknown to native species which may be vulnerable to them as they lack natural immunities. Domestic livestock has dramatically changed the biota of many areas as it can overgraze native ecosystems providing ideal conditions for invasion of other species.

4.3 Pollution and Climatic Change

Toxic chemicals degrade wildlife habitats, including wildlife refuges, and kill some plants and animals. Acid rain has caused a 40 % decline in fish species in Canadian lakes by the early 1990s. Soil erosion and siltation is believed to be responsible for the decline of numerous species of aquatic plants and fish in many of the world's rivers and streams.

Oregon State University researchers have recently discovered in laboratory experiments that low levels of nitrates can cause physical abnormalities and death in tadpoles and young frogs. Large quantities of nitrates are added to agricultural cropland through the process of fertilization. Only a portion of this fertilizer is actually taken up by the growing vegetation. The remainder leaves agricultural soils through leaching , throughflow and groundwater flow . Some of this nitrogen enters wetland habitats where frogs and toads are abundant. Over the last few decades, scientists have documented significant declines in the number of frogs and toads present in these habitats. It is quite possible that the population declines in these organisms is related to the presence of nitrates.

In the tropical oceans of the world, more that 25 % of coral reef ecosystems have been destroyed by sediment pollution and climate change. Scientists also believe that these factors could kill most of the remaining coral reefs by the year 2020. Coral reefs are very productive marine ecosystems and have high levels of species diversity that are only second to the tropical rainforests. Some of the highest levels of ecosystem damage are occurring in the Indian Ocean. Coral reefs found around the Maldives and Seychelles islands have had up to 90 percent mortality over the past two years. Some of the coral reefs in decline have been alive for more than 2 million years. These coral deaths are believed to be primarily due to warmer ocean temperatures driven by climatic change. When sea temperatures become two hot, the coral polyps expel their microscopic algae symbiotic partner. Without the algae, the coral polyps die because these plants provide the coral with nutrients.

Wildlife in even the best-protected and best-managed wildlife reserves throughout the world may be depleted in a few decades because of climatic change caused by projected global warming . Global warming will cause the various natural habitats of the world to shift geographically. If the species found in these habitats have limitations in terms of mobility they may become extinct as their surrounding environment changes.

4.4 Hunting and Poaching

Legal and illegal commercial hunting has led to the extinction or near extinction of many animal species, such as the American bison and the passenger pigeon. These activities continue today, especially in less developed countries where species protection programs do not exist or are poorly funded. In India, Bengal tigers face extinction despite multi-governmental backed protection, since a coat made from their fur sells for up to 100,000 dollars in the Far East. The profit made from selling just one of these animals far out ways the potential consequences of being caught poaching.

There are several different reason why humans hunt animals, including subsistence, sport and commerce. Subsistence hunters kill animals for food. This type of hunting caused the extinction of animals in the past but is not a very big concern today as most people no longer rely on hunting for their food supply. Sport hunting is when hunters kill animals for recreation. Like subsistence hunting, this caused extinction of animals in the past. Today it is of little concern as countries control sport hunting with licenses and regulations. Commercial hunting is when hunters kill for profit. This type of hunting is what causes extinction of animals today. Tigers, cheetahs, snow leopards, all of which are valued for their furs, are some of the animals that are endangered because of commercial hunting. Although these species and many others are now protected from commercial hunters, some still hunt them illegally as they can be sold on the black market for a high price. Commercial fishing is also a very big business that threatens the diversity of fish stocks as over-fishing is quite common. Canada is a prime example of this destruction. The fishing industry off the Grand Banks of Newfoundland was shut down to allow cod stocks to recover from over-fishing.

4.5 Pet and Exotic Plant Trade

International trade in plants and wildlife is a 5 billion dollar a year industry but it is threatening species across the globe. Each year millions of birds, fish, amphibians and reptiles are captured for sale in other countries. These species end up in zoos, aquariums and pet stores. Many of these species die in transport and others are killed or abandoned when owners realize they do not make very good pets. Plants are also at risk, particularly cacti and orchids that are very valuable on the black market.

4.6 Scientific Research

Most animals that are used for scientific research are not endangered. These animals, however, are threatened when habitat loss is also taken into consideration. The chimpanzee and orangutan are two such examples that are threatened for these reasons. Up to six chimps die when captured or during transport for every one that ends up in a laboratory.

4.7 Predator and Pest Control

Extinction or near extinction can also occur when people attempt to exterminate pest and predator species that compete with humans. In Europe, humans exterminated thousands of wolves as they were seen as pest to farmers. The use of pesticides to control insect pests, especially DDT and dieldrin, have caused populations of some bird and amphibian species to decline in North America.

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