Page 106 - Environment: The Science Behind the Stories

Shifted Shifted
downstream upstream
American shad Figure 2 different fish respond in dif-
Open-water species Gizzard shad the arrival of zebra mussels, the young
Alewife
ferent ways to zebra mussels. Following
Blueback
of open-water fish tended to shift down-
Striped bass
mussels. Young of littoral fish tended to
Shiner White perch stream toward areas with fewer zebra
Pumpkinseed shift upstream toward areas with more
Darter Largemouth bass zebra mussels. Source: Strayer, D., et al.,
Littoral species Redbreast 2004. Effects of an invasive bivalve (Dreissena
Bluegill
Killifish polymorpha) on fish in the Hudson River estu-
ary. Canadian Journal of Fisheries and Aquatic
Smallmouth bass
Stickleback Sciences 61: 924–941. © 2004. Reprinted by
permission of NRC Research Press.
0.1 0.3 1.0 3.0 10.0
Change in number (upstream/downstream)

toward areas with fewer zebra mussels, the Hudson that had crashed after the and ran experiments placing cages (to
whereas littoral fish shifted upstream zebra mussel invaded (likely because of keep out predators) around some areas
toward areas with more zebra mussels competition for food), began to stabilize of mussels. They found that mussels
(Figure 2). Overall, the data supported the and persist at about 4–22% of their within the cages grew larger than mus-
hypothesis that the fish community would pre-invasion population sizes. Popula- sels outside, indicating that predators
respond to changes in food resources tions of crustaceans, flatworms, and were feeding on mussels and prevent-
caused by zebra mussels. other invertebrates also rebounded. ing most of them from growing to their
But then, surprisingly, as Strayer Several types of zooplankton began to full size. Indeed, Strayer’s teams deter-
and his colleagues continued their increase (Figure 3). mined that the zebra mussel’s survival
research, some of the zebra mussel’s To determine why these changes rate had fallen to less than 1% of what
impacts began to reverse. Popula- were occurring, Strayer’s research it was during the early years of the inva-
tions of native mussels and clams in teams stepped up monitoring efforts sion. Predators such as the blue crab
were eating more and more of the mus-
sels, and large mussels were becoming
rare. As the average size of the mussels
Zebra mussels decreased, their filtering capacity fell by
800 800 more than 80%, and more zooplankton
Nauplii began to survive.
Rotifers
Zebra mussels per square meter 600 600 Nauplii per 10 L Rotifers per liter and zebra mussel might just fade away and CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
Do these trends suggest that the
400
400
prove harmless in the long run? Strayer
feels it is too early to answer that ques-
200
tion; much remains unknown, and he
200
is continuing his research. He cautions
0 0 that zebra mussels remain abundant,
1992 1996 2000 2004 2008 that phytoplankton levels have not yet
Year bounced back, and that there is no
guarantee that zebra mussel impacts
Figure 3 Zooplankton can bounce back once zebra mussels decline. Large, mature zebra
mussels have decreased in density since their initial sudden increase upon colonizing will continue to diminish. Nonetheless,
the Hudson River (red line). Two types of zooplankton (blue and green lines) declined the apparent turnaround in the Hudson
following the zebra mussel’s introduction, but they recovered after 2005, once large River is intriguing for ecologists and
mussels disappeared. Source: Pace, M.L, et al., 2010. Recovery of native zooplankton associated with provides hope that the Hudson’s native
increased mortality of an invasive mussel. Ecosphere 1(1): Article 3. systems may recover.

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