Extinction is a natural event and, from a geological perspective, routine. We now know that most species that have ever lived have gone extinct. The average rate over the past 200 million years is 1-2 species per million species present per year. There have also been several episodes of mass extinction, when many taxa representing a wide array of life forms have gone extinct in the same blink of geological time.
In the modern era, due to human actions, species and ecosystems are threatened with destruction to an extent rarely seen in Earth history. Probably only during the handful of mass extinction events have so many species been threatened, in so short a time.
What are these human actions that threaten biodiversity? There are many ways to conceive of these; let's consider two.
1. Increase in population
2. Effect of human actions.
The sinister sextet examines the second type.
Over-hunting has been a significant cause of the extinction of hundreds of species and the endangerment of many more, such as whales and many African large mammals. Most extinctions over the past several hundred years are mainly due to over-harvesting for food, fashion, and profit. Commercial hunting, both legal and illegal (poaching), is the principal threat. The snowy egret, passenger pigeon, and heath hen are US examples. At US $16,000 per pound, and US $40,000 to US $100,000 per horn, it is little wonder that some rhino species are down to only a few thousand individuals, with only a slim hope of survival in the wild. The recent expansion of road networks into previously remote tropical forests enables the bushmeat trade, resulting in what some conservationsist describe as "empty forests" as more and more wild animals are shot for food.
While over-hunting, particularly illegal poaching, remains a serious threat to certain species, for the future, it is globally less important than other factors mentioned next.
2.Habitat loss, degradation, fragmentation
Habitat loss, degradation, and fragmentation are important causes of known extinctions. As deforestation proceeds in tropical forests, this promises to become the main cause of mass extinctions caused by human activity.
All species have specific food and habitat needs. The more specific these needs and localized the habitat, the greater the vulnerability of species to loss of habitat to agricultural land, livestock, roads and cities. In the future, the only species that survive are likely to be those whose habitats are highly protected, or whose habitat corresponds to the degraded state associated with human activity (human commensals).
Habitat damage, especially the conversion of forested land to agriculture (and, often, subsequent abandonment as marginal land), has a long human history. It began in China about 4,000 years ago, was largely completed in Europe by about 400 years ago, and swept across the US over the past 200 years or so. Viewed in this historical context, we are now mopping up the last forests of the Pacific Northwest.
In the New World tropics, lowland, seasonal, deciduous forests began to disappear after 1500 with Spanish and Portuguese colonization of the New World. These were the forested regions most easily converted to agriculture, and with a more welcoming climate. The more forbidding, tropical humid forests came under attack mainly in twentieth century, under the combined influences of population growth, inequitable land and income distribution, and development policies that targeted rain forests as the new frontier to colonize.
Tropical forests are so important because they harbor at least 50 percent, and perhaps more, of the world's biodiversity. Direct observations, reinforced by satellite data, document that these forests are declining. The original extent of tropical rain forests was 15 million square km. Now there remains about 7.5-8 million square km, so half is gone. The current rate of loss is estimated at near 2 percent annually (100,000 square km destroyed, another 100,000 square km degraded). While there is uncertainty regarding the rate of loss, and what it will be in future, the likelihood is that tropical forests will be reduced to 10-25 percent of their original extent by late twenty-first century.
Habitat fragmentation is a further aspect of habitat loss that often goes unrecognized. The forest, meadow, or other habitat that remains generally is in small, isolated bits rather than in large, intact units. Each is a tiny island that can at best maintain a very small population. Environmental fluctuations, disease, and other chance factors make such small isolates highly vulnerable to extinction. Any species that requires a large home range, such as a grizzly bear, will not survive if the area is too small. Finally, we know that small land units are strongly affected by their surroundings, in terms of climate, dispersing species, etc. As a consequence, the ecology of a small isolate may differ from that of a similar ecosystem on a larger scale.
For the future, habitat loss, degradation, and fragmentation combined is the single most important factor in the projected extinction crisis.
3.Invasion of non-native species
Invasion of non-native species is an important and often overlooked cause of extinctions. The African Great LakesVictoria, Malawi and Tanganyikaare famous for their great diversity of endemic species, termed "species flocks," of cichlid fishes. In Lake Victoria, a single, exotic species, the Nile Perch, has become established and may cause the extinction of most of the native species, by simply eating them all. It was a purposeful introduction for subsistence and sports fishing, and a great disaster.
Of all documented extinctions since 1600, introduced species appear to have played a role in at least half. The clue is the disproportionate number of species lost from islands: some 93 percent of 30 documented extinctions of species and sub-species of amphibians and reptiles, 93 percent of 176 species and sub-species of land and freshwater birds, but only 27 percent of 114 species and subspecies of mammals. Why are island species so vulnerable, and why is this evidence of the role of non-indigenous species? Islands are laboratories for evolution.
Domino effects occur when the removal of one species (an extinction event) or the addition of one species (an invasion event) affects the entire biological system. Domino effects are especially likely when two or more species are highly interdependent, or when the affected species is a "keystone" species, meaning that it has strong connections to many other species.
A keystone species is one whose influence on others is disproportionately great. A seminal study of marine invertebrates in the rocky intertidal region of Washington State found that the top predator, a starfish, facilitated the coexistence of many other invertebrates by selectively consuming mussels, which otherwise would crowd out other organisms. Thus a keystone species is one whose presence or absence both directly and indirectly influences other species through food web connectivity. Contrary to what some may think, not all species are "keystones", and it requires careful experimental studies to identify keystone species.
Pollution from chemical contaminants certainly poses a further threat to species and ecosystems. While not commonly a cause of extinction, it likely can be for species whose range is extremely small, and threatened by contamination. Several species of desert pupfish, occurring in small isolated pools in the US Southwest, are examples.
A changing global climate threatens species and ecosystems. The distribution of species (biogeography) is largely determined by climate, as is the distribution of ecosystems and plant vegetation zones (biomes). Climate change may simply shift these distributions but, for a number of reasons, plants and animals may not be able to adjust. The pace of climate change almost certainly will be more rapid than most plants are able to migrate.
The presence of roads, cities, and other barriers associated with human presence may provide no opportunity for distributional shifts. Parks and nature reserves are fixed locations. The climate that characterizes present-day Yellowstone Park will shift several hundred miles northward. The park itself is a fixed location. For these reasons, some species and ecosystems are likely to be eliminated by climate change. Mountaintop species are especially vulnerable. The plants and animals found on high mountains of the American West include many remnants of a Pleistocene fauna that long ago was displaced toward the arctic, or upslope. With further warming, many of these mountaintop species likely will be eliminated.
A changing climate will have many other effects. The southern extent of the Everglades, today the site of the most ambitious and expensive restoration project ever undertaken, may be underwater, along with significant areas of human habitation. Agricultural production likely will show regional variation in gains and losses, depending upon crops and climate. Some coral reefs will expand, and others will contract or die off. Ecological changes due to an altered climate are difficult to forecast, but expected to be serious.
As a consequence of these multiple forces, many scientists fear that by end of next century, perhaps 25 percent of existing species will be lost.