Wednesday, February 25, 2004

By David Suzuki.

People generally like simple answers. This happened, so this happened. Cause and effect. Simple.

But nature doesn't work that way. Just when we think we've got something figured out, another idea comes along that turns our preconceived notions upside down. In spite of all our scientific advances, we are only just beginning to understand how ecosystems work.

Consider invasive species. Plants and animals evolve to fill particular roles within a given ecosystem. The population of each of these species is usually held in check by other forces, such as climate, predators or food availability. When we take a species out of its native home and transport it elsewhere, it may face new challenges and die off, or it may find itself virtually unencumbered by constraining forces.

When the latter happens, the species can become "invasive." That is, it can flourish, become a pest to human beings and overwhelm native species. In fact, invasive species are believed to be a major cause of the loss of biodiversity worldwide. Biologists have long known that the lack of insect enemies is a key factor in determining if a plant species becomes invasive, but we are finding out that the situation is decidedly more complex. We now know that another, surprising agent may also be involved in the success of an invasive plant - soil organisms.

Most of us tend to think of soil as inert matter, but it is very much alive, replete with microscopic organisms that can either enhance a plant's capacity to grow and flourish, or hinder it. A recent study published in the journal Nature has found that spotted knapweed, which was introduced to North America through imported alfalfa seeds more than a century ago, has likely been able to spread so profusely, not because it lacked insect enemies, but because it lacked microscopic soil enemies. In Europe and Asia, specific soil organisms help keep the plant from becoming an invasive weed, but these microbes aren't found in North American soil.

Just as few people would have thought that soil microbes could have such a profound effect on the success of a plant species, few would likely think that reducing fishing to protect fish stocks would actually harm seabird populations. Yet that is exactly what has happened in Europe's North Sea.

One of the North Sea's top predators, the great skua, has greatly benefited from the leftovers of commercial fishing. Over the years, these birds have learned to scavenge fish guts and undersized fish tossed back by fishing boats. They've eaten well. As a result, the great skua population is now 200 times larger than it was a century ago.

However, fish stocks in the North Sea are in trouble from years of overfishing. New European Union policies have been designed to allow stocks to recover, but reduced fishing also means fewer discards for the skuas. And rather than go hungry, the birds are preying on other seabirds like puffins and kittiwakes.

The impact on other seabird species is not insignificant. A recent study found that a five per cent increase of birds in the skua diet would result in an annual loss of thousands of other seabirds. In some areas, the level of predation by skuas is already unsustainable and the authors conclude that the situation "presents a potentially serious threat to some seabird communities."

We have to remember that nature does not behave in a simple, linear fashion. Our natural systems have, over millions of years, developed complex systems of checks and balances. Humanity is now powerful enough to meddle with those systems and the results are unpredictable. That's something to keep in mind when we consider climate change, genetically modified food and other emerging scientific issues. We have to be cautious. And we can't assume we know how things will turn out, because with nature, the answer is rarely simple.

Source: David Suziki Foundation