"When we try to pick out anything by itself, we find it hitched to everything else in the Universe."
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| Wolf Reintroduction, January 1995. |
Grey wolves were extirpated from Yellowstone by the mid-1920s. During the winters of 1995-96 wolves were re-introduced to the park after a 70-year absence.
This ecological tinkering has allowed scientists to study a textbook example of a trophic cascade.
Trophic cascades occur when a predator reduces the population of its prey. This population reduction cascades to release the next trophic level from predation. Grey wolves are apex predators. Grey wolves have gradually surpressed the population of elk in the park since their reintroduction.
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| Eleven-member wolf pack in winter, Yellowstone National Park, 2001 |
Elk are ruminant mammals. They consume grasses, plants, leaves, and bark.
The suppression of the elk population has enabled plant species like aspen, cottonwood, and willow to flourish. Scientists have observed the top-down impact of the grey wolf since its reintroduction in Yellowstone 15 years ago. The abundance and diversity of plant species have increased.
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| Source: Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction |
Aldo Leopold first described the concepts of trophic cascades after observing the overgrazing of mountain slopes following the extermination of wolves.
“We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes – something known only to her and to the mountain. I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.”Bottom-up cascades also occur. Changes in the abundance and diversity of plants will directly impact higher level predation and population.
― Aldo Leopold
“Only the mountain has lived long enough to listen objectively to the howl of the wolf.”Wolves howling at the moon is a compelling myth. A howling wolf is a plaintive reminder of our existential connection to all living things. Seeing constellations in the night sky while wolves howl harkens us to consider our place in the cosmos. We are bound to a thin band of life that covers the Earth. Everything is hitched.
― Aldo Leopold
REFERENCES
- Bottom-up trophic cascades and material transfer in terrestrial food webs, Hideki Kagata and Takayuki Ohgushi, Ecological Research, January 2006, Volume 21, Issue 1: 26-34.
- My First Summer in the Sierra, John Muir, June 1911.
- Top-down and bottom-up diversity cascades in detrital vs. living food webs, Lee A. Dyer and Deborah Letourneau, Ecology Letters, (2003) 6: 60–68.
- Trophic cascade, Wikipedia.
- Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction, William J. Ripple and Robert L. Beschta, Biological Conservation, 3 November 2011.
- Wolf Reintroduction, Wikipedia.
