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Exotic zebra mussels ruin coevolution

Whether you believe it's proof of God's guiding hand or the result of random combinations of chemicals, coevolution is an amazing phenomenon.

One of my favorite examples is the lancewood tree in New Zealand, which, until it reaches about 10 feet, seems custom-designed as a spear shaft. Its straight, smooth-barked trunk is about two inches in diameter a foot above the ground and tapers to an inch at the top. It also bears tough, sharp, unappetizing leaves.

But when it reaches 10 feet, the lancewood's trunk swells and develops textured bark, limbs push out, and new soft greenery covers the top. Juvenile and mature lancewoods are so different that they were listed as separate species for many years.

After they figured out that it was the same tree, botanists began wondering why the lancewood grew that way. It dawned on them that New Zealand once was home to several species of moa, the world's biggest birds, which grew to be more than 10 feet tall.

Looking like an African ostrich on steroids, moas browsed on small trees and bushes, and by the lancewood making itself unappetizing until it was taller than a moa could stretch, it increased its chance of reaching adulthood. The last moa was killed and eaten by the Polynesian Maoris between 200 and 500 years ago. But the lancewoods don't know that, so they continue to live their two-part lives.

Not all examples of coevolution are adversarial, however. Some are cooperative, like the long, curved bills developed by tropical hummingbirds that pollinate and feed on the nectar of long, curved flowers.

But now we can also see horrible examples of what happens when man unbalances the scales by introducing creatures to environments where they didn't evolve. One of the best examples is what has happened to Lake Huron.

The food chain in that lake once saw small creatures like diporeia, a tiny freshwater shrimp, move up and down the water column and provide food for small fish, which in turn fed bigger fish such as lake trout, pike and walleyes. When the prey fish and bigger fish eventually died, their bodies were converted into energy that fed plant life, diporeia and countless other small critters, keeping the system balanced.

Then about 20 years ago, a ship from a lake 4,000 miles away arrived with some zebra mussels in its ballast water, and those mussels got into the lake.

Exotic species usually have very little chance of surviving in a new environment. But the few that do often overwhelm native species that never developed ways to cope with them.

That's what has happened in Lake Huron, which apparently is the Great Lake most vulnerable to zebra mussels. The mussels were able to overrun diporeia and other small creatures, and diporeia, which we now know were a base link in the food chain, effectively have disappeared.

As a result, huge amounts of energy that once moved up and down the food chain and supported an enormous web of life are now locked up on the bottom of the lake in the form of zebra mussel shells, resulting in a collapse in baitfish numbers, followed by a collapse in salmon numbers, which has seriously affected the sportfishing and tourism industries.

We should understand by now that we can't fix the problems of the lakes if we continue to leave them vulnerable to exotics. We need think of the biological structure of the Great Lakes not as a tree of life but as a multi-legged tower, and realize that we have chopped off one leg and the tower is leaning perilously.

The next oceangoing ship to enter the lakes could bring a creature that takes out another leg. How many can be removed before an edifice that was millennia in the making collapses completely?