02 March, 2012: I’ve often said that ecology is not rocket science…it’s far more complex. Ecology is the scientific study of the relationships of organisms to their physical environment and to one another. Sometimes these relationships are immediately obvious and intuitive, and sometimes they’re a bit more, well…complicated.
Ecology and the case of sea otters
Take the well-documented case of sea otters, sea urchins, and kelp “forests” in the North Pacific Ocean. Beginning in the early 1970s, ecologist James A. Estes, his students, and many subsequent researchers, revealed an intriguing set of relationships that help us understand the broad impact of top predators in an ecosystem. The discoveries arose from the patchy distribution of sea otters in the North Pacific. The fur trade in this region had left only a very small scattering of sea otter populations. Otters had been completely removed from some sites, while colonies in other sites persisted. Some populations were reestablished by translocations. This patchy distribution of otters set up a lucky natural experiment for Estes and his colleagues. They found that kelp forests and many organisms that depended on kelp forests had disappeared from areas without otters, but flourished in areas with otters. What do otters have to do with kelp? With additional observation and dozens of experimental studies, ecologists discovered that otters eat sea urchins, which in turn eat kelp as their primary food source. Where otters are absent, sea urchin populations increase 10 – 100 times! At these population levels, urchins eliminated most or all of the kelp, creating what are called “urchin barrens.” Where otters are present, they keep urchin populations down, and kelp forests flourish, supporting a wide variety of invertebrates, fish, and other organisms missing from urchin barrens.
The story becomes even more complicated when we consider the differences in wave pattern and water chemistry between kelp forests and urchin barrens and what happens when killer whales are present to prey on otters. But the point is that predators like the sea otter can have a profound influence on an ecosystem through several levels. This ecosystem effect from top to bottom of an ecosystem is termed a trophic cascade.
We have our own, world-famous trophic cascade in the Greater Yellowstone region, discovered when the gray wolf was reintroduced to the region after being largely absent for nearly 70 years. In a study published last month in the international scientific journal Biological Conservation, William Ripple and Robert Beschta reported that wolves are profoundly changing Yellowstone from what it has been for the last several decades. Wolves have reduced elk populations and changed their feeding patterns, particularly in Yellowstone’s Northern Range. As a result, Ripple and Beschta say that aspen trees and willow shrubs are flourishing, bringing back beaver populations and improving habitat conditions for fish and some songbirds. For example, the authors found that along streams in Lamar Valley, 100 percent of the tallest aspen sprouts were browsed in 1998, compared with only 20 percent in 2010―less browsing generally means more trees to reach maturity. The estimated 6,000 elk that remain on the Northern Range (down from an early 1990s high of more than 15,000 individuals) behave differently with wolves present. These elk tend to avoid foraging in areas where they are more vulnerable to predation and don’t tend to forage in any one place very long. This has been characterized as the “ecology of fear.” Wolves have had other wide-ranging effects in Yellowstone, directly reducing coyote populations. Coyotes, in turn, impact prey populations and repress red fox populations. What do fewer coyotes and more red foxes mean to small mammal and songbird populations? And how do carcasses provided by wolves affect ravens, eagles, grizzly and black bears, and other scavengers? It’s clear that these scavengers benefit from carcasses, but it remains unclear how that will ultimately affect the size of these populations and their effect on other species. Like I said, ecology is complex.
What lesson should we take away from this complexity? For me it is humility and the recognition that we still have so much to understand about nature and how she works. I have to chuckle a little at the folks who stand up at public meetings and stridently hold court in local bars expressing with great certainty their opinions on wildlife from their many years of observation. I’ve hunted wildlife of all sorts and observed nature for nearly 60 years now, and actively studied it professionally for more than 35; the more I learn, the more I realize I have to learn. The wonderful thing is that exploring nature and discovering her mysteries never ceases to amaze, inspire, and rejuvenate anyone who is really paying attention.
For further reading:
Terborgh, J. and J. A. Estes (eds). 2010. Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature. Island Press, Washington, DC.
Ripple, W. and R. Beschta. 2012. Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. Biological Conservation 145:205 – 213.