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Threats to Biodiversity
The core threat to biodiversity and human welfare, is the combination of human population growth and resource exploitation.
The three greatest proximate threats to biodiversity are.
introduction of exotic species. The first two of these are a direct result of human population growth and resource use. The third results from increased mobility and trade.
A fourth major cause of extinction,
anthropogenic climate change, has not yet
had a large impact, but it is predicted to
become significant during this century.
The burning of fossil fuels in recent history
has caused a dramatic increase in the levels
of carbon dioxide in the Earth’s atmosphere,
which have now reached levels never before
seen on Earth. Scientists predict that the addition of this “greenhouse gas” to the atmosphere is resulting in climate change that will significantly impact biodiversity.
Environmental issues, such as toxic pollution, have specific targeted effects on species, but they are not generally seen as threats at the magnitude of the others.
Humans rely on technology to modify their environment and replace certain functions that were once performed by the natural ecosystem.
Other species cannot do this.
Elimination of their ecosystem - whether it is a forest, a desert, a grassland, or a marine environment - will kill the individuals in the species. Remove the entire habitat within the range of a species and, unless they are one of the few species that do well in human-built environments, the species will become extinct.
Human destruction of habitats accelerated
in the of the twentieth century.
Consider the exceptional biodiversity of
Sumatra: it is home to one species of
orangutan, a species of critically endangered
elephant, and the Sumatran tiger, but half of
Sumatra’s forest is now gone.
The neighboring island of Borneo, home to the
other species of orangutan, has lost a similar area of forest. Forest loss continues in protected areas of Borneo.
Habitat destruction can affect ecosystems other than forests.
Rivers and streams are important ecosystems and are frequently modified through land development and from damming or water removal.
Damming of rivers affects the water flow and access to all parts of a river. Differing flow regimes can reduce or eliminate populations that are adapted to these changes in flow patterns.
An estimated 91percent of river lengths in the United States have been developed: they have modifications like dams, to create energy or store water; levees, to prevent flooding; or dredging or rerouting, to create land that is more suitable for human development.
Many fish species in the United States, especially rare species or species with restricted distributions, have seen declines caused by river damming and habitat loss.
Research has confirmed that species of amphibians that must carry out parts of their life cycles in both aquatic and terrestrial habitats have a greater chance of suffering population declines and extinction.
This is because of the increased likelihood that one of their habitats or access between habitats will be lost.
Overharvesting is a serious threat to many species, but particularly to aquatic species.
There are many examples of regulated commercial fisheries monitored by fisheries scientists that have nevertheless collapsed.
The western Atlantic cod fishery is the most spectacular recent collapse. While it was a hugely productive fishery for 400 years, the introduction of modern factory trawlers in the 1980s and the pressure on the fishery led to it becoming unsustainable.
The causes of fishery collapse are both economic and political in nature. Most fisheries are managed as a common (shared) resource even when the fishing territory lies within a country’s territorial waters.
Common resources are subject to an economic pressure known as the tragedy of the commons in which essentially no fisher has a motivation to exercise restraint in harvesting a fishery when it is not owned by that fisher. The natural outcome of harvests of resources held in common, is their overexploitation.
While large fisheries are regulated to attempt to avoid this pressure, it still exists in the background. This overexploitation is exacerbated when access to the fishery is open and unregulated and when technology gives fishers the ability to overfish.
Fishery extinction is not equivalent to biological extinction. At the same time, fishery extinction is still harmful to fish species and their ecosystems.
There are some instances in which true extinction is a possibility. Whales have slow-growing populations and are at risk of complete extinction through hunting. There are some species of sharks with restricted distributions that are at risk of extinction. The groupers are another population of generally slow-growing fishes that, in the Caribbean, includes a number of species that are at risk of extinction from overfishing.
Coral reefs are extremely diverse marine ecosystems that face peril from several processes.
Reefs are home to about 4,000 fish species, despite making up only 1 percent of marine habitat. Most home marine aquaria are stocked with wild-caught organisms, not cultured organisms. Although no species is known to have been driven extinct by the pet trade in marine species, there are studies showing that populations of some species have declined in response to harvesting. There are concerns about the effect of the pet trade on some terrestrial species such as turtles, amphibians, birds, plants, and even the orangutan.
Bush meat is the generic term used for wild animals killed for food. Hunting is practiced throughout the world, but hunting practices, particularly in equatorial Africa and parts of Asia, are believed to threaten several species with extinction.
Traditionally, bush meat in Africa was hunted to feed families directly. However, recent commercialization of the practice now has bush meat available in grocery stores, which has increased harvest rates to the level of unsustainability. Threatened
Additionally, human population growth has
increased the need for protein foods that are
not being met from agriculture.
Species threatened by the bush meat trade
are mostly mammals including many
primates living in the Congo basin.
Exotic species are species that have been intentionally or unintentionally introduced by humans into an ecosystem in which they did not evolve.
Such introductions likely occur frequently as natural phenomena. For example, Kudzu (Pueraria lobata), which is native to Japan, was introduced in the United States in 1876. It was later planted for soil conservation. Problematically, it grows too well in the southeastern United States-up to a foot a day. It is now a pest species and covers over 7 million acres in the southeastern United States.
If an introduced species is able to survive in its
new habitat, that introduction is now reflected
in the observed range of the species.
Human transportation of people and goods,
including the intentional transport of organisms
for trade, has dramatically increased the
introduction of species into new ecosystems.
Sometimes the distances are well beyond the capacity
of the species to ever travel itself and outside the
range of the species’ natural predators.
Most exotic species introductions probably fail because of the low number of individuals introduced or poor adaptation to the ecosystem they enter.
Some species, however, possess pre-adaptations that can make them especially successful in a new ecosystem. These exotic species often undergo dramatic population increases in their new habitat and reset the ecological conditions in the new environment, threatening the species that exist there. For this reason, exotic species are also called invasive species.
Lakes and islands are particularly vulnerable to extinction threats from introduced species.
In Lake Victoria, the introduction of the Nile perch was largely responsible for the extinction of about 200 species of cichlids. The introduction of the brown tree snake from the Solomon Islands to Guam in 1950 has led to the extinction of three species of birds and three to five species of reptiles endemic to the island.
The global decline in amphibian species recognized in the 1990s is, in some part, caused by the fungus Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis.
There is evidence that the fungus is native to Africa and may have been spread throughout the world by transport of a commonly used laboratory and pet species: the African clawed toad (Xenopus laevis).
Early evidence suggests that another fungal pathogen, (Geomyces destructans), introduced from Europe is responsible for white-nose syndrome, which infects cave-hibernating bats in eastern North America and has spread from a point of origin in western New York State.
The disease has decimated bat populations and threatens extinction of species already listed as endangered.
Climate change, and specifically the anthropogenic (caused by humans) warming trend presently underway, is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss.
Scientists disagree about the likely magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species committed to extinction by 2050. Scientists do agree, however, that climate change will alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them.
The warming trend will shift colder climates toward the north and south poles, forcing species to move with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range.
One such unexpected species contact is between polar bears and grizzly bears. Previously, these two species had separate ranges. Now, their ranges are overlapping and there are documented cases of these two species mating and producing viable offspring. Changing climates also throw off species’ delicate timing adaptations to seasonal food resources and breeding times.
Many contemporary mismatches to shifts in resource availability and timing have already been documented.
Range shifts are already being observed: for example, some European bird species ranges have moved 91 km northward.
The same study suggested that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough.
Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, and mammals.
Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations.
The rate of warming appears to be accelerated in the arctic, which is recognized as a serious threat to polar bear populations that require sea ice to hunt seals during the winter months. A trend to decreasing sea ice coverage has occurred since observations began in the mid-twentieth century. The rate of decline observed in recent years is far greater than previously predicted by climate models.
Finally, global warming will raise ocean levels due to melt water from glaciers and the greater volume of warmer water.
Shorelines will be inundated, reducing island size, which will have an effect on some species, and a number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains-a cycle that has provided freshwater to environments for centuries-will also be jeopardized. This could result in an overabundance of salt water and a shortage of fresh water.
adaptive radiation rapid branching through speciation of a phylogenetic tree into many closely related species
biodiversity hotspot concept originated by Norman Myers to describe a geographical region with a large number of endemic species and a large percentage of degraded habitat
biodiversity variety of a biological system, typically conceived as the number of species, but also applying to genes, biochemistry, and ecosystems
bush meat wild-caught animal used as food (typically mammals, birds, and reptiles); usually referring to hunting in the tropics of sub-Saharan Africa, Asia, and the Americas
chemical diversity variety of metabolic compounds in an ecosystem
chytridiomycosis disease of amphibians caused by the fungus
Batrachochytrium dendrobatidis; thought to be a major cause of the global amphibian decline
DNA barcoding molecular genetic method for identifying a unique genetic sequence to associate with a species
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