Today, sea otters are the poster children of cuddle appeal, but their endearing behaviors were lost on the fur hunters of the 1800s. Otter fur lined jackets (and the fur trade lined pockets), but soon otters no longer lined the Pacific Coast.
The sea otter, however, is a “keystone species” — its impact on our coastal ecosystems is disproportionately large compared to its natural abundance in the marine community — so its removal had profound effects that we only noticed recently, as the otter staged a dramatic return over the last 70 years.
Otters like shellfish. So much so that where otters live, abalone and sea urchins are rare. Urchins eat kelp,so once otters devour the urchins, kelp grows in abundance. That’s good news from a restoration perspective: the return of the otter has also meant the return of kelp forests, historically “more natural” than rocky urchin barrens. And it makes the seaweed industry happy (kelp products turn up in ice cream and toothpaste, among other things). But plenty of fishermen find the change less palatable: the (now defunct)abalone industry, for example, arose largely because the loss of otters led to a population explosion in the snails. In the sea otter’s absence, we humans partially assumed the role of top predator, a role we’re hesitant to relinquish.
Although the otter drama has died down, other predator comebacks remain controversial. Wolves, which exerted top-down controls on herbivore populations all across the northern portion of our continent, are slowly (under the stewardship of biologists and conservationists) regaining their ecological foothold in places like Yellowstone National Park. Where once an ecosystem was falling apart at the seams (overgrazing by elk decimated forests, leading to a lack of proper dam-building materials and many unhappy beavers; shifting vegetation changed the face of the park and its complement of animal species), wolves have almost magically stabilized it. For biologists observing the system, the reintroduction of wolves must have been like finding the missing key and unlocking a treasure trove.
Of course, most of the nearby ranchers would rather the key had been permanently lost. Their treasures are their herds, and wolves are often blamed for any loss of stock. I, for one, am glad the wolves are back to playing their native role — and hoping they extend their range East, to control a deer population explosion that’s decimating my childhood forests. Hiking past the leafless stalks of what should be the next generation of trees, I’m frequently willing to shoot Bambi myself.
The dramatic and complex effects stemming from the loss of top predators (and the reversals associated with their return) is not unique to these systems. Such “trophic cascades” (“trophic” for food chain, “cascade” for the direct and indirect ripples spreading downward through the system) have been found around the world. As we continue to monitor the accidental experiments created by human impacts, we’ll doubtless find many more examples.
Will this knowledge help us predict the effects of future species losses?
Probably not. Ecology is a complex science, and its overriding conclusion is that, well, “it’s complicated.” In some cases, apex predators and their top-down cascading effects rule the system. In other situations,though, the controls are bottom-up, and the community is limited by nutrients, the rate of plant growth or some other fundamental factor. And, because we’re seeing all these systems as snapshots (often heavily impacted by human activity), it’s hard to guess where the real balance between these two regimes lies.
One thing is clear, though. We are an inextricable part of the system. Of course, we always have been.But before we learned to use oil for cheap energy, before we domesticated crops and settled onto farms, before we organized ourselves into hunter-gatherer clans, our own cascades were much smaller.
Today, though, we are the keystone species. Our top-down effects cascade through the system when we fish out top predators like sharks or when we shoot the one mountain lion found roaming in Redwood City. Our bottom-up effects transform ecosystem processes when we add fertilizers or pollute landscapes.
Our ability to modify the world has evolved faster than the world’s ability to deal with our modifications. Of course, there’s a growing feeling that we should try to limit these modifications — not least because they’re putting our future existence on this planet in serious jeopardy.
Ina few pet systems, especially here in America, we entertain dreams of systems “restored” to the way they “must have looked” before us. But unless our population shrinks dramatically, it will be very hard to avoid pressing all of our accessible land area — and most of our coastal waters — into direct human service.
So perhaps the real moral from the story of the otter, or of the wolf, is to impose our will with impunity, understanding that ecosystem cascades, like true waterfalls, are incredibly powerful, sometimes beautiful and often impossible to control.
Holly welcomes fully-clothed reader comments and suggestions via email at firstname.lastname@example.org. This column was originally published in The Stanford Daily on May 12, 2011.