11 June 2010

The Zen of Ecological Niches

From Out of Eden: An Odyssey of Ecological Invasion, by Alan Burdick (Farrar, Straus and Giroux, 2005), pp. 135-136:
For Charles Elton and many of his successors, biological invasions were a way to probe and characterize the way that ecological communities are assembled and held together. The ecosystem was studied as a sort of organic machine—a system—of semi-interchangeable parts, or, to borrow another analogy, a kind of corporate economy maintained by organisms with defined ecological jobs. By studying the arrival of foreign workers and the consequent displacements, the notion went, a scientist could figure out the overall corporate structure: the various job descriptions, the interoffice competition, the company bylaws, the glue of market success and longevity. Critical to this schema is the job itself, the ecological niche—a concept that has receded from meaning over the years with every new attempt to clarify it. Today, one can speak of a habitat niche (the range of habitats in which a species can and does occur) or a functional niche, the "role" or "place" of a species in a community—a notion further divisible into trophic niche (the relationship of the species to its food and enemies) and resource niche (which spans things utilized by the species, like nesting sites). As a conceptual tool, the niche has effectively dropped from the modern ecologist's belt. "Niche," Mark Williamson summarizes in his book Biological Invasions, "is useful in a preliminary, exploratory description, but becomes difficult to pin down in particular situations."

Whatever a niche is exactly, successful invasion, in Elton's schema, allegedly involves occupying an empty one. But, many biologists counter. what does it mean for a niche to lie "empty"? If a niche is an ecological job that takes up food or resources, and such a job is going unfilled in an ecosystem, the community would show side effects; it would soon be overwhelmed by waste or unused food. So for such a job opening to exist yet not harm the community, by definition it must be a job that involves no interaction whatsoever with other community members—like one of those jobs the boss's kid fills on summer vacation, only less productive. But if that is the case. any invader entering this empty non-job would have no impact on the system—which clearly isn't what happens with many invaders. "If you take the view that there are no empty niches," Williamson writes, "the invasion of communities cannot involve occupying empty niches," Some ecologists contend that in saving that an invader occupies a vacant niche, what is meant is that an invader plays a new functional role in the community, not that it doesn't use resources previously used by other species. The brown tree snake fits this definition; in coming to Guam, it declared itself top predator of an ecosystem that for eons had run perfectly well without such a top executive. Under these terms, Williamson writes, successful invasion becomes a matter of always, often, or sometimes entering a niche that can be full, empty, partly full, or partly empty—terminology that begins to suit the term itself. Williamson concludes, "It is to some extent a matter or the meaning you want to put on the word 'empty.'"

At the very least, invasion biology has made it clear that the conflicts and interactions that transpire between species in an ecosystem are too fluid and dynamic to be meaningfully described by a static term like niche. To introduce niche theory is to propose a koan: Does an ecological niche exist before on invader arrives to fill it? Meditatively interesting, perhaps, but useless in forecasting. "We are still unable to recognize a vacant niche except by carrying out the tautological experiment of introducing a species and seeing if it becomes established," one biologist notes. Williamson adds, "The extent to which a niche is vacant is, in practice, a post hoc explanation. Post hoc explanations are neither intellectually satisfying nor much use in prediction."
This book is engagingly written, but contains a lot of little factual errors.

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