Extinction and Conservation. Reading-Freeman, Chapter 55. Humans have extensively modified the biosphere The human population passed 6 billion in the year 2000, and is growing at a rate of almost 2% per year.
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In aquatic ecosystems as well, an increasing amount of productivity is harvested by humans. Nearly every major fishery in the Northern Hemisphere has showed strain from overharvesting, and many have collapsed.
Most of the grasslands and forests of the Northern Hemisphere were destroyed by the end of the nineteenth century, the grasslands of the southern hemisphere are now vanishing, and tropical forests are disappearing at a rate of about 2% per year. This type of destruction has become the norm for most biological communities, as the human population expands our economic needs require resources from more and more land. The remaining habitat is often broken into many small fragments, which are separated by large areas of land under cultivation or other human uses, effectively reducing a single "continent" into many "islands".
Edge effects fundamentally alter habitat. For certain species, this can be critical to their ability to survive. For instance, places where human habitation borders nature preserves frequently have weedy plants, fire is controlled, domestic cats and dogs escape and prey on native wildlife, and human noise and activity disturb the behavior of certain animals.
Some large mammals, such as coyotes and raccoons, reach much higher densities in edge habitats because they are able to take advantage of human resources (garbage), and return to the safety of the preserve.
Taking this a step farther,raccoons in the US, and red foxes in England, have even penetrated urban areas to become part of the city, reaching high densities.
Other mammal species cannot tolerate edge environments, wolves and mountain lions do not like humans and cannot live on the edge (in cases where they try, very bad things might happen.
Example: The brown headed cowbird is a native to the United States. It is a brood parasite, evicting the eggs of other species to replace them with its own. Cowbirds prefer edge habitats. Now that forests are fragmented, there are few safe areas from cowbirds, and forest interior species such as bluebirds are suffering a major loss of fitness in some areas.
Ladys' Slipper Orchids. There are probably about 25, 000 species of orchids worldwide, and they are being lost faster than they can be classified.
Orchids are typically tightly coevolved in mutualistic relationships with other species, and the loss of any of these relationships can lead to extinction.
Ladys' slippers are a very diverse group that occupy a wide variety of habitats in the Northern Hemisphere. They are in decline even in protected areas, such as Indiana Dunes.
Human activities have altered their ecosystems. Ladys' slippers need a mutualistic fungus to germinate and grow for the first few years. Airborne nitrogen compounds (mostly from automobiles) effectively "fertilize" vast areas of ground and may put the mutualistic fungi at an ecological disadvantage.
Also, the widespread application of pesticides, the human tendency to groom and "clean up" areas of open sand and fallen wood, and the introduction of the honeybee to North America have caused the Andreneaid bees that would normally pollinate these plants to disappear from many areas.
Human activities are creating the worldwide equivalent of the "Great American Faunal Interchange". This is an uncontrolled experiment in community ecology, with the potential result of a massive loss of gamma diversity worldwide caused by the loss of endemic species.
In 1998, the zebra mussel was discovered in Lake St. Claire near Detroit. It was introduced to the Great Lakes from the Caspian Sea, probably in the ballast water from a cargo ship, sometime around 1985. This mode of dispersal is very common, in 1982 the comb jelly (a ctenophore) was introduced to the Black Sea in a similar manner. Comb jellies increased in number until they amounted to an estimated 90% of animal biomass in the Caspian.
It forms dense clusters of individuals, and can clog the water intakes of electrical power stations, water stations, and other industrial facilities.
Zebra mussels are incredibly effective filter feeders. Zebra mussels actually make the water much clearer, but alter native communities of organisms in the process. In the Hudson River, phytoplankton biomass decreased 85% after zebra mussels invaded, zooplankton decreased 70% as a result.
The zebra mussel is a very effective competitor. Extinction of native bivalves will almost certainly result from this introduction.
You may have noticed that Chicago water tastes weird during the summer, that is because the water is now clean enough to allow the growth of cyanobacteria deep enough in the lake to be pulled into the water intakes. The residue of cyanobacteria toxins has an off taste.
Honey bees are native to Europe and Asia. Apis mellifera, is a European species that is widely cultivated for honey, beeswax, and as a pollinator. European immigrants probably introduced the honeybee to North America in the nineteenth century (Native Americans called it "white man's fly".) It is a very effective competitor, and displaces native bee species.
Boiga irregularis was introduced to Guam in the late 1940s, probably by hitching a ride in the wheel well of a plane. Since that time, it has literally eaten most of the endemic birds of Guam to extinction. Since there are no other native snakes in Guam (other than a blind, burrowing species), the bird fauna there evolved no natural defenses.
In Australia, competition and predation keep it in check, but the simpler ecosystem of Guam has allowed it to increase in numbers to up to 20 individuals per square acre of jungle (among the highest ever recorded for a snake).
It also causes other problems in Guam, including numerous power outages resulting from large numbers of snakes resting on power lines.
These are three more cases of an introduced species being too good at what they do. All three plants were introduced intentionally in the nineteenth century. Each of the three has become so common that it is likely to displace other species. For example, in some East Coast marshes, purple loosestrife amounts to 90% of the vegetation, displacing native sedges and other plants.
Rarity-Rarity is not the same thing as endeminsm, endemics can be very common in the restricted area where they do occur. "Naturally rare" species have low population densities, but may be widely distributed and have respectable population sizes. We do not completely understand the ecological factors that make some species "naturally rare", but when a common species gradually becomes rare, it is often a prelude to extinction. "Naturally rare" species can be a challenge to conservation, because they are difficult to monitor and it is very difficult to ensure that sufficient habitat is set aside for them.
Small Population Size-Small population sizes render a species very vulnerable to extinction, through reduced genetic variation via genetic drift, the potential for inbreeding depression, demographic stochasticity caused by random ecological disasters and, for sexual species, the small chance that every individual in the population might be born the same sex.
Ecological Specialization-Ecological specialists are more prone to extinction because there are only a few ways they can 'fit themselves into" an ecosystem. They must have certain interspecific relationships in order to feed, obtain mates, have places to live, or maintain competitive superiority. The loss of other species in the community, or habitat change due to human activity, can change these factors, and render a formerly successful species vulnerable to extinction.
Useful to Humans or A Competitor of Humans-Humans have a way of killing all the pretty things, harvesting all the useful things, and hunting to extinction everything that could be perceived as a competitor. For instance, fishermen in San Francisco are prone to despising the California Sea Otter, despite its important place in the ecosystem of the California Coast, because of its status as a competitor. They are protected now, however, they were nearly hunted to extinction for their pelts. Species that cross the paths of humans sometimes suffer for it.
Any conservation plan that does not take human economics into account is prone to failure. It is very difficult to set aside a habitat and protect it from all human activity. The closest we have ever come are on military bases and nuclear test sites (the conservation effect was unintentional at first), and some private organizations (Nature Conservancy) buy natural land and simply fence it off. Even these exceptional preserves have neighbors, and are occasionally eyed by developers and government reclassification.
The vast majority of preserves must balance the needs of human ecotourists, indigenous peoples, neighbors, and government budget considerations against conservation goals.
It is one of the strange ironies of our existence that, though the actions of our species modify the biosphere to an extent unprecedented in the history of the earth, as individuals, we do not necessarily feel any collective responsibility for our actions.
The future of our own species will depend, to a very large extent, upon decisions we make as individuals, regarding our priorities. It is quite possible for our species to survive for many thousands of years more, but this is likely only if this generation takes additional steps to ensure that the planet will remain habitable to our own species.