food chains, carry disease, prey on native species directly, and as we have already seen,
out-compete natives for limited resources. All of these effects can lead to extinctions of the
native species. In addition, some introduced species hybridize with native species, resulting
ingenetic pollution,which weakens natural adaptations.
Another major cause of extinction is global climate change. As we have already seen earlier
in this chapter, our increasing reliance on fossil fuels in altering the earth’s atmosphere, and
as a result, climate. This has many effects, some of which we have already discussed, but on
a species level, these other effects, including changing air and water temperatures, rainfall
patterns, and salinity threaten species adapted to pre-warming conditions and thus result in
a decline of biodiversity globally.
Overpopulation (already mentioned previously), along with developments in technology,
have added tremendous pressure to resource and land use and add to all of the previously
mentioned threats to biodiversity. The highest rates of population growth are often in third
world tropical countries where biodiversity is also highest. Therefore pressures from local
populations as well as increased pressure from incoming tourists in some areas can produce
enormous consequences for the local plant and animal ecosystems.
A final major cause of extinction is pollution, and mentioned earlier in this lesson. Pollution
adds chemicals, noise, heat, or even light beyond the capacity of the environment to absorb
them without major harmful effects on all kinds of organisms.
One good example of a toxic chemical affecting a species was the use of the pesticide, DDT.
Use of this pesticide in the eastern United States resulted in the effect ofbiological magni-
fication(where many synthetic chemicals concentrate as they move through the food chain,
so that toxic effects are multiplied), with the result of the disappearance of the peregrine
falcon from this area. As a result, DDT was banned in the U.S.
Pollution continues to contribute to habitat destruction and decreasing biodiversity world-
wide, especially in developing countries. Air pollution knows no boundaries and as we have
already seen, its effects on acid rain, ozone depletion, and global warming all affect biodi-
versity.
Water pollution especially threatens vital freshwater and marine resources throughout the
world. Specifically, industrial and agricultural chemicals, waste, acid rain, and global warm-
ing threaten waters, essential for all ecosystems. Finally, soil contamination, mostly from
toxic industrial and municipal wastes (Figure25.32), salts from irrigation, and pesticides
from agriculture all degrade soils, the foundation of terrestrial ecosystems and their biodi-
versity.
Outside the developed world, pollution controls often lag far behind those of the U.S. and
Europe, and some developing nations, like China, are rapidly increasing their levels of pollu-
tion. Many pollution problems are also present in industrialized nations as well; industry and
technology add nuclear wastes, oil spills (Figure25.33), thermal pollution from wastewater,
acid rain, and more to the challenges facing the earth’s biodiversity (Figure25.34).