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Integrating new species into the food web

August 21, 2012

Zebra mussels, open underwater with siphons out. Creative Commons

We have been reading panic-stricken news reports about zebra mussels for over 10 years, but we weren’t paying much attention until a recent news report that they have arrived in California.  We decided it was time to educate ourselves about this “invasive species.”

Zebra mussels and their close relative, the quagga mussel, arrived in the Great Lakes Region of the United States in 1988, probably in the ballast water of big ships.  Although they are native to southern Russia and Ukraine, they are now found throughout Europe and England.

The negative side of the ledger

What the mussels lack in size, they make up for in numbers.  Though they are tiny—about the size of a dime–they are prolific breeders capable of creating big colonies rapidly.  They are a fresh-water mussel which means they exist where there are often water treatment facilities that supply our drinking water.  Their larvae are microscopic so they can enter water treatment facilities through the intake pipes and clog the system. 

They filter huge quantities of water, consuming plankton (microscopic plants and organisms) depriving other animals of nutrition.  This filtering of the water also increases water clarity and light penetration, changing the entire ecosystem in complex and unpredictable ways.

The positive side of the ledger

Where the mussels have gained a foothold, they have quickly entered the food web.  A monitoring program was started soon after mussels were found at Long Point Bay in Lake Erie.  The first sampling done in 1991 found mussels in 27% of the sampling stations, an estimated 1,189 tons of mussels.  By 1992, mussels were found in 80% of the sites, an estimated 4,536 tons of mussels.  (1)

In 1992, the monitoring program also started conducting stomach analysis of ducks killed at Long Point Bay.  Three species of duck (Greater and Lesser Scaup and Bufflehead) were found to be feeding heavily on the mussels.  Between 1993 and 1995 the population of mussels declined significantly from the highpoint of 4,536 tons to only 758 tons in 1995.  The population of the duck predators increased correspondingly during the same period of time. (1)

The authors of this study speculate that the mussels were also depleting their food source at the peak of their population and that they had exhausted available attachment sites, but the scientists believe duck predation was the primary reason for the declining population of mussels.  As always, there are many variables operating simultaneously in the ecosystem, and it isn’t possible to isolate one from the others.  (2)

Ducks aren’t the only predators of the mussels.  Crayfish are apparently capable of consuming large quantities of the mussels.  And some fish eat the mussels.  One study found that yellow perch didn’t eat the mussels in 1994, but a later study in 2004 reported that the perch were eating the mussels.  Plankton waste from the mussels settles on the lake bottom and the bottom feeders benefit from that fall out.

There is a downside to this story, however.  Remember that the mussels filter the water as they eat.  In addition to filtering plankton, they also filter pollutants and contaminants.  Researchers assume that the predators of the mussels are consuming those pollutants which then become a part of the food chain.  The mussel-consuming ducks at Long Point Bay apparently had elevated levels of contaminants in their tissue compared to ducks that consume only aquatic plants. (2)

What should we do?

According to the news story about the mussels in a local paper, the California legislature is considering increasing the registration fee for boats which would raise about $5 to $8 million dollars.  Although the news story isn’t clear about how this money would be used, let’s assume for the sake of argument that it would be used to prevent the spread of these mussels beyond the 25 lakes in California where they are now found.  That would apparently involve more inspection of boats being put into the water where the mussels don’t presently exist.  If that’s the plan, we enthusiastically endorse it.  Prevention is the best medicine, as they say.

But once the mussels have arrived, all scientists agree that eradicating them is not a realistic option.  Therefore, dousing them with chemicals—which is one of the recommended treatments—will undoubtedly do more harm than good. 

New species quickly become a part of the landscape.  Our initial reaction to them tends to be negative because we are suspicious of change.  In fact, there may be benefits that aren’t immediately evident and even if there isn’t an immediate benefit, they are often integrated into the environment over time.  Their populations often stabilize once they have exhausted available resources.  We should be patient because nature is resilient and our time frame is much shorter than nature’s time frame.    

Are we learning this lesson?

Broom, Redwood Park, Oakland, California

The California Invasive Plant Council (Cal-IPC) is dedicated to the eradication of non-native plants.  Scotch broom is one of their favorite targets for eradication.  Little progress has been made in this effort (see “Broom:  ‘I’ll be back’” and “Broom:  ‘I’m ba-ack’”) and recently Cal-IPC acknowledged this in their newsletter.  However, they urged their supporters not to lose heart because they reported that broom is now being browsed by herbivores.  So, what native plant advocates could not accomplish with manual labor and chemical warfare, the animals may accomplish by incorporating broom into their diets.  One hopes the animals aren’t eating broom doused with herbicides.

Cal-IPC also acknowledges in this article that broom does not grow in shade:  “Broom cannot tolerate heavy shade.  It usually established following logging or other activities that remove tree canopy.”  Could it be that they have finally noticed that the result of clear-cutting non-native trees in the East Bay hills is more broom, not more native plants?  We can only hope so.

There are pros and cons to every decision we make.  We don’t always know in advance what they are.  So, it pays to be cautious.  If we are patient, maybe nature will sort it out without our interference.  Particularly when our interference damages nature, we should exercise restraint.  We should give nature more credit for healing itself.  It has a much better track record than we do.

*******************************

(1)    Cox, George W., Alien Species and Evolution, Island Press, 2004

(2)    Petrie, Scott A., Knapton, Richard H., “Rapid Increase and Subsequent Decline of Zebra and Quagga Mussels in Long Point Bay, Lake Erie:  Possible Influence of Waterfowl Predation,” J. Great Lakes Research, 25(4) 772-782

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