Sunday, March 30, 2008

Trophic Cascades

I have written here on a few occasions my fascination with trophic cascades. "Trophic cascades occur when predators in a food chain suppress the abundance of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is an herbivore)" (http://en.wikipedia.org/wiki/Trophic_cascade).

My favorite trophic cascade, which I have described before in this blog, is the role of the Wolf as the Keystone Predator.  In Yellowstone National Park, the return of the Gray Wolf into the ecosystem has caused the Elk to spend less time on the river banks.  This has encouraged more Willow growth, which has in turn encouraged Beaver to return.  In 1995 the Northern range of Yellowstone had only 1 beaver colony, now it has 9.   This in turn should provide more trout habitat, hopefully benefiting some of the endangered species in Yellowstone. Wolves also decreased the coyote population, benefiting coyote prey species such as pronghorn and ground squirrels (see my earlier story on Wolves, Pronghorn, and Coyotes). Its amazing that one species can have such a dramatic effect in the eco-system. This is why the wolf is a Keystone Species.

In my preparation to return to school in the fall, my reading has brought me across some other fascinating trophic cascades.  The first came from Cornell Lab of Ornithology's Handbook of Bird Biology (chapter 9, page 128).  The study presents the relationship of Sapsuckers, swallows, willows, and aspen.  In this environment, the sapsucker, a type of woodpecker, is a keystone species. The conditions must be right to accommodate sapsuckers, but when they are present, the species diversity increases.  The study first demonstrated that the sapsucker preferred to nest in aspen trees that are situated near a certain type of willow, which provides food for the Sapsucker. Additionally, the sapsucker could only bore its nest cavity into the Aspen when a certain type of fungus was present. Thus an Aspen grove with the heartwood fungus, near a willow grove, provided the perfect habitat for the sapsucker. A couple of behaviors of the sapsucker make it a keystone species. First, it drills holes in the willows to get at the sap. These sap wells were visited by numerous other birds, rodents, and bugs that all benefited from the sapsuckers work. The study did not prove that any of these species would be absent without the sapsucker, so this was non-conclusive to grant keystone status to the sapsucker. But another observation was. The sapsuckers bore into the aspen to create their own nest cavity. They seldom use a nest cavity more than once. These nest cavities are used by other cavity nesting birds in subsequent years. The study went on to verify that the cavity nesting birds in the ecosystem were specifically dependent upon the sapsucker for their presence.  That made the sapsucker a keystone species.  Its fascinating that a single bird species, in fact just a few birds, can have so much influence.

The next story comes from a review of some recent research which was posted at the Not Exactly Rocket Science blog call Loss of big mammals breaks alliance between ants and trees. Here the researchers are studying the removal of what appears to be a keystone species. The detailed summary is fascinating, but the high level is that when large mammals eat Acacia trees, the tree appears to recruit a certain species of ant to attack the mammals as they are eating.  In return the tree provides the ants some nectar. It is an example of a mutual symbiotic relationship.  When the researcher fenced off some trees to prevent large mammals from eating the tree an interesting thing occurred.  The tree apparently decreased the nectar it produced for the ants. The specific ant population then decreased. This allowed for two other parasitic ant species to invade and attack the tree. Apparently the original ant species kept these two at bay. When the tree stopped feeding it, it stopped fighting off the other parasitic species.  The result is that neither the tree nor the original ant species were better off. I suspect that if allowed to progress long enough the tree and the ant species would create a new balance.  To me this speaks more to the result of the change (removing large mammals) than the new steady state. We must remember however that most species that go extinct do so as a result of not being able to adapt fast enough to a change in the ecosystem. Thus, it is possible that the Acacia tree must have both large mammals eating it and the presence of a specific ant species to survive. The significance of this story is that the large mammals are in decline in Africa as they are around the world. Primarily as the result of human activity.   The effect is that many more species, such as the acacia tree, are at risk as a result.

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