Question

Search for Extinction, and scroll for statistics. Click on “Known Causes of Animal Extinction since 1600.” You will find four general categories of causes. Thinking about history from 1600 through today, how do you think humans have changed their impact on species in each of these categories? Has the impact increased or decreased over this time period? Give specific examples of changes in this timeframe to support your answers.

Answer 1

There are five major causes of extinction: habitat loss, an introduced species, pollution, population growth, and overconsumption. Through the activity, students will create a list of reasons why animals can become extinct.

Humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. Explosive, unsustainable human population growth is an essential cause of the extinction crisis. As regards the future, consideration of the main mechanisms of human-caused extinctions (overhunting, effects of introduced species, habitat destruction, and secondary ripple effects) indicates that the rate of extinction is accelerating. The basic reason is that there are now more humans than ever before, armed with more potent destructive technology, and encroaching on the world's most species-rich habitats: the continental tropical rainforests.
Most threatened or endangered species face multiple threats. Here is a list
that deals with some aspect of human caused or accelerated extinction
V. Threats to Diversity and Losses of Species1.) Habitat Destruction
2.) Habitat Fragmentation
3.) Habitat Degredation
4.) Global Climate Change
5.) Overexploitation of Species for Human Use
6.) Invasion of Exotics
7.) Increased Spread of Disease
Habitat Destruction, Fragmentation, and Degradation
Examples include deforestation, desertification, road building, pollution,
erosion
All of which are most prominent where human populations are high and
most of the original habitat has been destroyed. More than 50% of primary
wildlife habitat has been lost in many Old World countries to agriculture,
logging, and development. Over 50% of the earth’s species are found in
tropical rain forests even though they only cover 7% of the total land
surface.
Fragments (board) are isolated land patches where more edge has been
created. Large chunks of land are reduced and divided into multiple
fragments. Such increased edge leads to fluctuations in light, temperature,
humidity, and wind. Fragmented landscapes can also limit dispersal and
colonization in both animals and plants (salamander example) especially
plants that depend on animals to disperse their seeds. Eventually it leads to
division of existing populations into smaller restricted subpopulations that
are vulnerable to inbreeding depression and genetic drift.
Global Climate Change
The Greenhouse Effect has lead to increased global warming which in turn
has multiple consequences. The most important in terms of species diversity
is that it is happening so fast that most species do not have enough time to
evolve with or adapt to the changes. Changes that will effect many species
include rising sea levels, earlier arrival of the spring season, shifts in species
ranges, increases in drought, fires, and heat waves.
Overexploitation
One example would be the harvesting of animals from their natural
environments. Removal can occur for many reasons including hunting,
subsistence, commercial use, poaching, and for trade. It usually becomes a
problem when a commercial market develops for a previously unexploited
species. Such commercial markets bring money and resources to more
underdeveloped countries. The resources are then often extracted so
thoroughly that it becomes rare, forcing the market to turn toward another
species. These markets may include legal or illegal trade of fur, organs,
food, and/or animals as pets.
One example here in the US would be the extinction of the passenger pigeon
(overhead). It was once thought to be the most abundant bird on earth but it
was so thoroughly hunted with very few controls that the entire population
of these birds was wiped out.
(read passage)
Exotic/Invasive Species (already discussed)
Exotic = species occurring outside of its natural range due to human actions,
but they do not become established because the new environment is not
suitable to their needs.
Invasive = species occurring outside of its natural range due to human
actions that do establish themselves. They tend to increase their abundance
at the expense of native species usually through the competition for a limited
resource.
Invasive species are a threat to 49% of all endangered species in the US
Human activity that has lead to the introduction of invasive species includes:
-European colonization (release of 100’s of bird and mammal species to
remind them of home, good example is the starling)
-Horticulture and Agriculture (ornamentals and crops that escaped
cultivation)
-Accidental transports (weed seeds, rats, and diseases all in ship ballast {soil
and water})
These new species do so well with the absence of their natural predators,
pests, and parasites. They also tend to do well in altered or disturbed sites
by better adapting than the native species.