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Trophic Cascade


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Wednesday 24th February, 2010

A bit of a quick and sciencey FP today, since I’ve been running short on time lately. I’m trying to get myself relocated into a new (and cheaper) habitat here in Portland, and between phone calls, showings, and applications, I’m glad that I don’t have to move very often.

Lars FP about cormorants got me thinking about other ways that human actions inadvertently and often indirectly alter habitat for other creatures.

What a lot of people call “The Butterfly Effect”, where the simple and theoretical act of a butterflies wings flapping might alter air flow in a way that causes a hurricane on the other side of the world, is know to biologists as the “Trophic Cascade”. When one animal is effected, or eliminated, the result cascades through an entire ecosystem, often with negative consequences for something like a trout.

This is an example of how eliminating wolves is bad for trout. What is particularly interesting is that the link is so indirect, that unless you studied this system (or perhaps witnessed the cascade in a real place like Yellowstone), you might never even know that it exists.

The quick and dirty explanation goes something like this in Yellowstone.

A long time ago people thought that wolves were horrible creatures that killed far too many “game animals” and livestock, and therefore competed with humans for food or deprived us of “sport”. Now, as history has shown, annoying humans (especially Euro American settlers) is a very bad idea if you are an animal, plant, river, or indigenous race. The story played out such that most of the wolves in the western US were shot, trapped, poisoned, or otherwise removed from the ecosystem.

With the wolves out of the way, the elk, which had been required to move around all the time in order to stay ahead of the wolves, ended up settling down into the easy life in the river valleys. Big rivers like the Lamar migrated and meandered across the valley floor, taking trees from the outside bends into the river and turning them into habitat. The natural progression is that new trees – cottonwoods, and willows that stabilize banks, provide shade, and add complex habitat when they fall into the river – grow up in the bars on the inside bends of the big rivers. But elk consider cottonwood shoots a delicacy, and with no wolves to keep them on the move, the elk have managed to essentially stop the regrowth (recruitment is the scientific word) of new, young trees. In time, only a few trees remained in the valleys and the trout habitat has suffered. This is hard to believe because the fisheries in Yellowstone are so very impressive still, but imagine what they could be like.

In the late 90s, wolves were reintroduced to Yellowstone, and they are now thriving. The elk are moving again, looking more like Lance Armstrong than the average couch potato. And the cottonwoods, and willows are beginning to return, along with the balance.

Another twist to this story is that the trapping of beavers (they weren’t annoying to settlers, but their pelts made some very fashionable and completely ridiculous looking hats) also plays a role in the Trophic Cascade. Without beavers there were no more beaver ponds. Many alpine meadows dried up, and the riparian plant communities died off. If they tried to come back, the lazy elk ate them all, and with no trees, it would be hard for any beaver that found its way into a river valley to build itself a new dam, house, or find a snack of bark. This too hurt trout populations since the alpine beaver ponds were sources of groundwater recharge and cold flows to the streams throughout the dry summer at lower elevations. Warming water temps opened the door for competition from non-native trout, or simply made some streams uninhabitable. And on and on. We actually talked about this back in 2008 on the FP and the Board.

It turns out that scientist have documented similar links between wolves, elk, and riparian trees like cottonwoods and aspen in Olympic NP in Washington State, and up in Jasper, BC, Canada.

Since I probably oversimplified things in the sake of time (and to reduce the boring science part of this), here are a bunch of urls to websites and papers on this topic to help you fall asleep.,%20Olympic%20trophic%20cascades.pdf Beschta & Ripple, wolves & riparian recovery YNP.pdf Yellowstone aspen with wolves.pdf

Be Well & Fish On,

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