Page 95 - Environment: The Science Behind the Stories

from open water and depositing nutrients in feces that sink,
they shift the community’s nutrient balance to the bottom and
benefit species that feed there. Once they clear the water,
sunlight penetrates more deeply, spurring the growth of large-
leafed underwater plants and algae. In the long term, however,
eutrophication (pp. 126–127, 430) may ensue, bringing harm
to the system.
In recent years, scientists have documented a surprising
new twist: One invader is being displaced by another. The
quagga mussel (Dreissena bugensis), a close relative of the
zebra mussel from Ukraine, is spreading through the Great
Lakes and beyond. This species, named after an extinct
zebra-like animal, is replacing the zebra mussel in many
(a) Zebra mussels clog water intake pipes of power plants locations.
and industrial facilities. In a second twist, today there are signs that the zebra
mussel invasion may be losing steam. In a number of areas,
zebra mussel populations have begun declining, including
in part of Lake Huron, portions of the Mississippi River, the
St. Croix River in Minnesota, Lake of the Ozarks in Missouri,
and other locations. Some declines are due to displace-
ment by quagga mussels, but for others the reasons are
unknown. In some cases, predation by fish or ducks is
being credited for driving down zebra mussel numbers, as
native predators develop a taste for the invader. A similar
process has already occurred in Europe, where waterfowl
began preying heavily on zebra mussels and contributed to
population crashes.
In areas where zebra mussels are declining, some of
the native fish and invertebrates that suffered from their
arrival are now recovering. Ecologists are monitoring popu-
(b) Zebra mussels foul, starve, and suffocate native clams by lations closely to see how the situation develops. No one
adhering to their shells and sealing them shut.
expects zebra mussels to disappear or be eradicated, but
Figure 4.1 Invasive species can have severe economic and there is now hope that some of their impacts might be
ecological impacts. reversed.

Species Interactions Competition can occur when resources
are limited
Interactions among species are the threads in the fabric of eco-
logical communities. By interacting with many species in a When multiple organisms seek the same limited resource,
variety of ways, zebra mussels have set in motion an array their relationship is said to be one of competition. Compet-
of changes in the communities they have invaded. To under- ing organisms do not usually fight with one another directly
stand why invasive species introduced by people can cause so and physically. Competition is generally more subtle and
much disruption, we must first look at how species naturally indirect, taking place as organisms vie with one another to
interact. Ecologists organize species interactions into several procure resources. Such resources include food, water, space,
fundamental categories (Table 4.1). shelter, mates, sunlight, and more. Competitive interactions
can take place between members of the same species, called
intraspecific competition, or between members of different
Table 4.1 Species interactions: effects on Their species, called interspecific competition.
Participants If individuals of the same species are competing for lim-
Type of interaction Effect on Effect on ited resources, then competition becomes more intense when
Species 1 Species 2 there are more individuals per unit area (denser populations).
This is density dependence (p. 86), and it can limit the growth
Mutualism + +
of a population.
Predation, parasitism, + – Whereas intraspecific competition is a population-level
herbivory
phenomenon, interspecific competition affects communities.
Competition – – Interspecific competition can give rise to different types of
94 “+” denotes a positive effect; “–” denotes a negative effect. outcomes. If one species is a very effective competitor, it may

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