Versions of this article have appeared in:
The Montreal Gazette
Image courtesy of Dept of Crop & Soil Sciences, Washington State University
Watching worms wends their way
by Tyler Smith
Biologist Joann Whalen studies the seasonal migrations of animals. If
she studied elephants her life would be a lot simpler. There are certain
advantages to working with species you can spot from a helicopter or track
from a land rover.
Despite this, earthworms are totally ignored when it comes time to calculate fertilizer inputs for a crop field. This is because it's nearly impossible to quantify the benefits of earthworms in an open field. "Most of what we know about worms comes from laboratory and greenhouse experiments, and it is very difficult to take those results and apply them to the real world" explains Whalen. "We know that earthworms alter soil nutrient levels; we know that plant growth depends on soil nutrient levels; but we haven't been able to show a direct link between earthworms and plant growth in the field."
The challenge is the scale of the problem. It's easy to measure the impact
of a dozen worms living in a pot on a laboratory bench. It's nearly impossible
to measure the impact of a half-million worms burrowing through a hectare
of soil. A common approach is to carefully study a very small sample of
soil, and then to extrapolate the results upwards to estimate the overall
impact on a field. Charles Darwin was one of the first scientists to use
this technique. By his calculations, 500,000 worms, each consuming 50
grams of organic matter, will eat their way through 25,000 kilograms of
soil in a field the size of a soccer pitch each year.
Using these arguments, Whalen convinced the Natural Sciences and Engineering Research Council to support her study. Now she has the funding, all she has to do is find the worms. "Radio tracking equipment is just too big" she notes, so she'll have to resort to a low-tech solution.
Armed with a shovel and a bucket Whalen and her graduate students will have to dig up the worms by hand. Each of her six quarter-hectare plots will be randomly sampled throughout the growing season of four years. While they collect the earthworms, they'll also be taking soil samples. The resulting data should shed some light on just what it is the worms are looking for as they wander through the earth.
She'll be able to map the distribution of earthworm populations, but not to follow the travels of individual worms, which would require her to be able to recognize each one. But sadly, even for a worm specialist like Whalen, they all look pretty much the same. She is exploring a novel approach however: "I've been looking into tattooing them."
Besides the applications of this research for farmers, it could also be valuable to conservation biologists. As common as they are, earthworms aren't Canadian. They are invasive species, like the notorious zebra mussels that have taken over large areas of the Great Lakes. All the Canadian worms were killed off by the glaciers that covered the country until the end of the last ice age, 10,000 years ago (the few remaining native North American worms haven't yet migrated back into the north). So Canadian forests have developed worm free. Scientists are only now beginning to realize that the worms that crossed the Atlantic with the settlers may be having a significant impact on Canadian ecosystems.
Plants that evolved in worm-free forests had to adapt to poor soils and
the thick layer of dead leaves that worms are so good at breaking down.
A recent study from northern Minnesota has shown that one such plant species,
the goblin fern, may no longer be able to live in forests that have been
invaded by worms.