Page 108 - Environment: The Science Behind the Stories

Humans Phytoplankton Figure 4.15 The zebra mussel and the
(economic loss) Zooplankton quagga mussel are modifying ecological
communities. By filtering phytoplankton
and small zooplankton from open water,
Sunlight they exert impacts (a) on other species, both
penetrating negative (red downward arrows) and positive
more deeply (green upward arrows). The map (b) shows
into water
known occurrences of zebra and quagga
mussels as of December 2012. In just two
Clogged intake decades they spread across North America,
pipe of power
plant Plankton- assisted by accidental transportation on the
eating fish Aquatic plants hulls of boats. Source (b): U.S. Geological Survey.
Bottom- Bacteria Native
feeding and benthic mussels
fish invertebrates and clams

Zebra and
quagga mussels

(a) Impacts of zebra and quagga mussels on a Great Lakes nearshore community

Zebra mussel occurrences
Quagga mussel occurrences
Both species occurrences
Zebra mussels eradicated

(b) Occurrence of zebra and quagga mussels in North America, as of December 2012

aquatic plants that become ecosystem engineers; crayfish island-dwelling animals and plants worldwide have been CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
that disrupt the benthic portions of food webs; and disease driven extinct by goats, pigs, and rats introduced by human
pathogens. colonists (Chapter 3).
Examples abound of invasive species that have had Ecologists generally view the impacts of invasive
major ecological impacts (pp. 304–307). The chestnut species—and introduced species in general—as over-
blight, an Asian fungus, killed nearly every mature Ameri- whelmingly negative. Yet many people enjoy the beauty of
can chestnut, the dominant tree species of many eastern introduced ornamental plants in their gardens. Some organ-
North America forests, between 1900 and 1930. Asian isms are introduced intentionally to control pest popula-
trees had evolved defenses against the fungus over mil- tions through biocontrol (p. 274). And some introduced
lennia of coevolution, but the American chestnut had not. species that have turned invasive provide benefits to our
A different fungus caused Dutch elm disease, destroying economy, such as the European honeybee, which pollinates
most of the American elms that once gracefully lined the many of our crops (p. 272). Whatever view one takes, the
streets of many U.S. cities. Fish introduced into streams impacts of invasive species on native species and ecologi-
for sport compete with and exclude native fish. Grasses cal communities are significant, and they grow year by year
introduced in the American West for ranching have overrun with the increasing mobility of people and the globalization
entire regions, pushing out native vegetation. Hundreds of of our society. 107

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