Increased
Vulnerability to Extinction:
Because of some aspect of their natural history, some species have a higher risk of
extinction than others. Species that fit into the following categories are under
greater extinction pressures than other species. In addition, species that that fall
into multiple categories have an even greater risk.
narrow
geographical ranges--if the entire range is affected by human activity (not
unlikely with smaller, more narrow ranges), then the species no longer has a supportive
habitat and may go extinct
only one or a few
populations--any one population may go extinct from human or natural causes
(earthquakes, fires, disease, loss of habitat); the fewer populations comprising a
species, the greater the risk of a population extinction leading to the species going
extinct
population size
is small--greater vulnerability to demographic and environmental changes and to
loss of genetic variability; fewer individuals to provide genetic resistance to disease or
environmental changes
low population
density--these species will have fewer individuals per unit area and thus tend to
have only small populations remaining if habitat is fragmented;
require a large
home range--these species are more prone to extinction when part of their range is
degraded or fragmented and range is thus reduced; the species will not have the required
habitat size necessary for survival and could be subjected to increased extinction
pressures. Example: the brown bear of North America or the North American mountain
lion both require large ranges for roaming and hunting. When these ranges are
reduced, the species is often forced to continue hunting and roaming in human-occupied
areas. This often results in injury or death of the animals.
low rates of
population increase--these species tend to have delayed reproduction to an advanced
age and tend to produce fewer, larger young; they are often unable to rebuild populations
to a viable size fast enough after a sudden decrease, possibly due to disease or
hunting/predation pressures; Example: elephants produce their first offspring
nearly 10 to 15 years into life and may have only 10 to 12 young in a lifetime. If
the young of an elephant population are killed, then the population has only the older,
less reproductively active individuals to increase the herd size. Once these older
individuals are dead, there may be too few elephants left for the population to remain
viable.
migratory
behaviors--migratory species depend on two or more distinct habitats; if any one of
these habitats is destroyed, reproduction may be seriously impaired because food resources
or reproductive habitats have been destroyed. Example: the sea turtle
migrates back to land for breeding purposes. Many of the beaches used for breeding
have been developed by humans, and the turtles can no longer lay their eggs there or the
young that hatch are in jeopardy because of the human contact.
little genetic
variability--reduced genetic variability can result in reduced adaptability to
changes in the environment; it also reduces resistance to diseases and/or
predators. Example: the cheetah of Africa exhibits exceptionally low levels of
genetic diversity (levels that are expected only after 10-20 generations of brother-sister
matings only). Consequently, the cheetah suffers from a serious lack of disease
resistance, reduced litter sizes, and weak or infertile offspring.
specialized niche
requirements--these species often require one or two particular resources,
generally a habitat type or food item; if these resources are altered or destroyed, the
environment may no longer be suitable for the species survival. Example: the
giant panda of Asia subsists solely on a diet of bamboo. If the bamboo forests are
cleared for farming or livestock, the panda will go extinct.
hunted or
harvested by people--the selfish, utility-maximizing behavior of humans often leads
to overhunting and overharvesting because of the "more is better" and
"now" mentality. Example: the buffalo of the American West were
hunted to near extinction because of the economic value of their hides and tongues.
Habitat
Destruction, Degradation, and Fragmentation:
Habitat is necessary for any species to survive. It provides the necessary elements
of life--shelter, food, etc. Without a healthy habitat, the species will not have
those necessary elements.
A study by Elizabeth Losos found that
"natural resource extraction activities are wholly or partly responsible for
endangering 62 to 68 percent of all species formally listed as at risk of extinction"
in this country. These extraction activities are subsidized at an estimated $1
billion annually. Taxpayers also fund the species protection activities, costing
many millions more. This perverse policy amounts to a "double burden" on
society.
Exotic
Species Introduction:
All too often exotic species (species that are not native to the area) are introduced into
habitats in which the native species cannot compete. The exotic species monopolizes
the resources for survival and the native species, without access to required resources,
cannot reproduce.
The School of Public and Environmental Affairs
Copyright 1999 Indiana University Bloomington
Comments: kenricha@indiana.edu
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