Invasive creatures trigger deadly chain reactions
There is an old, time-worn proverb that illustrates how one event can lead to another. It is set in the days when men traveled, and battles were fought, on horseback. You may even remember hearing it.
It begins with the loss of a horseshoe nail: “For want of a nail, the shoe was lost; for want of a shoe, the horse was lost; for want of a horse; the rider was lost; for want of a rider the message was lost; for want of a message, the battle was lost; for want a battle, the kingdom was lost; all for the want of a horseshoe nail.”
Chain reactions happen in nature, too.
An article recently appeared in a Michigan newspaper, describing a series of events that has led to the death of thousands of loons, our state bird, many of which migrate through the Great Lakes.
Last year during the fall migration, thousands washed up dead on Lake Michigan shorelines. The number of banded adult birds returning north from their wintering grounds this past spring was down between 20 and 25 percent, following that 2012 die-off.
There were similar mass die-offs in the Great Lakes in 2006, 2007 and 2010.
The direct cause has been attributed to botulism, a disease that attacks the nervous system of its victims, commonly waterfowl of various species, including loons, ducks and geese. The victims experience gradual paralysis and eventual death.
The toxic agent that kills them actually forms when algae decompose in the lakes’ depths, depleting oxygen levels. This allows the toxin to grow and eventually become part of a lake’s food chain.
Though the decaying algae and the botulism toxin are not new to the Great Lakes, the production of this killer has accelerated. The apparent culprit is an invasive species we have all heard much about, the zebra mussel, introduced in the ballast water of ocean-going ships traveling the Great Lakes in the 1980s.
The zebra mussel has gradually spread to our inland lakes, in most cases by those of us who trailer our fishing and pleasure boats from one watershed to another.
This thumbnail-sized, shelled creature consumes great quantities of tiny plankton, disrupting the food chain that supports the sport fish that anglers seek.
Not only that, but this mass consumption of plankton greatly clarifies the water, allowing sunlight to penetrate deeper, spurring the growth of even more algae, which eventually die off and create an environment that produces even more botulism toxin.
One of the fish that carries this toxin to the loon is another foreign species, also brought here in ships’ ballast water, the “round goby.” This bottom-dwelling fish has become widespread in the Great Lakes, including at the deeper depths where loons often feed. Their ability to spawn multiple times a season, consume the eggs and young of other fish, and compete for spawning habit, have made them a major threat to native fish species, and under certain conditions, a contributor to the threat to our own state bird.
Scientists are hoping they can find a way to break the chain reaction that seems to be endangering the loon, a signature creature of Minnesota’s lake country and our far northern wilderness, but no solution is yet in sight.
Another chain reaction that involves invasive species concerns one of the ducks most popular with Minnesota hunters. This species, the scaup, better known as the bluebill, has in recent decades seen its population fall below what waterfowl managers call the “long-term average.” The reasons are not completely clear.
One thing that is clear, however, is that there have been die-offs of scaup by the thousands — coots, too — on Winnibigoshish, Bowstring and Round lakes in Cass and Itasca counties during fall and spring migrations. The invasive species in play here is a snail just a half-inch long, or less, native to Europe, brought to the Great Lakes in the 19th century.
The faucet snail has gradually spread to other waters, believed to be by attachment to boats, anchors, trailers, aquatic vegetation and perhaps other recreational equipment; made more likely by the snail’s ability to live out of water for days.
It is not the snail that kills the birds, but a parasite it carries. Faucet snails play host to a creature known as a trematode. Scaup or other waterfowl eat infected snails, and the trematode — which could be described as a miniature leech — attaches to their intestinal wall and begins to consume their blood, eventually killing the bird.
These infected inland lakes have a large amount of sandy-bottomed shallows, which may make them especially suitable faucet snail habitat.
Only time will tell how often these episodes will be repeated here, or elsewhere. So far, the scorecard has been pretty one-sided in favor of the unwanted newcomers.
But that doesn’t mean that the best and the brightest scientists won’t keep searching for those elusive “magic bullet” answers.