Laboratory Manual for Introductory Geology 4e

FIGURE 18.14 Two bird species hunted to extinction.
(a) The dodo was last seen on the island of Mauritius in 1681. (b) The last passenger pigeon died in the Cincinnati Zoo in 1914.
to protect the Atlantic cod. It would have been inconceivable to an 18th- or
19th-century fisherman that this important food fish could ever be endangered,
because it was as abundant in the oceans as the passenger pigeon was in the
air. But overfishing has decimated the species, and it is not certain at this time
whether there are enough cod left for conservation measures to be successful.
2. Human activities destroy natural habitats. As the human population increases, we
compete for habitable space with plants and other animals. Many forest-dwelling
species, for example, need continuous tree cover for safety from predators, but
that continuity is destroyed when we harvest large swaths of forest for agriculture
or housing (FIG. 18.15). Continuous habitat is also necessary to sustain populations
of some species, especially those with large ranges that travel great distances
to find mates and food.
3. Modern transportation aids invasive species. Plant and animal species compete for
space and food naturally, even where no people are present, but our actions can inadvertently
tip the scales in favor of newcomers. Oceans have protected islands and
continents from invasive species throughout geologic time, as few terrestrial species
can swim or float across the Atlantic or Pacific. But opportunistic “hitchhikers” now
make those crossings attached to the hulls of ships or in water carried by ships as
ballast (for stability). Seeds and insects now fly across the oceans on planes along
with cargo, reaching their new homes even more quickly. Plants imported for landscaping
sometimes take over, pushing out the native flora. Given a chance, bamboo
and bittersweet run rampant in North America, and kudzu and purple loosestrife
are rapidly becoming serious problems in U.S. fields, as is cinquefoil in U.S. lakes.
492 CHAPTER 18 LOOKING TO THE FUTURE