Anthropogenic (human caused)
habitat degradation is the main cause of species extinctions now. The main cause of habitat degradation worldwide is agriculture, with
urban sprawl, logging, mining and some fishing practices close behind. The physical destruction of a habitat, both directly (
deforestation for
land development or
lumber) and indirectly (burning
fossil fuels), is an example of this. Also, increasing toxicity, through media such as
pesticides, can kill off a species very rapidly, by killing all living members through
contamination or
sterilizing them.
Persistent organic pollutants (POPs), for example, can
bioaccumulate to hazardous levels, getting increasingly dangerous further up the
food chain. Disease can also be a factor:
white nose syndrome in
bats, for example, is causing a substantial decline in their populations and may even lead to the extinction of some species.
Overhunting also has an impact. Terrestrial mammals, such as the tiger and deer, are mainly hunted for their pelts and in some cases meat, and marine mammals can be hunted for their oil and leather. Specific targeting of one species can be problematic to the ecosystem because the sudden demise of one species can inadvertently lead to the demise of another (
coextinction) especially if the targeted species is a
keystone species.
Sea otters, for example, were hunted in the
maritime fur trade, and their drop in population led to the rise in
sea urchins—their main food source—which decreased the population of kelp—the sea urchin's and
Steller's sea cow's main food source—leading to the extinction of the Steller's sea cow. The hunting of an already limited species can easily lead to its extinction, as with the
bluebuck whose range was confined to and which was hunted into extinction soon after discovery by European settlers. == Australia ==