Dale Goble asked,

> 1. in a mature, undisturbed ecosystem, are all "niches" filled so
> that the introduction of a new species is necessarily at the expense of
> an established species?
>
> 2. in a disturbed ecosystem -- such as seattle -- is this the case?

Ecology isn't my specialty, but I know some ecologists maintain that it
doesn't make sense to talk about an unoccupied niche--that a niche can
only be defined by seeing how a real species actually lives. It's an
epistemological question: how do you know something is there? Only by
applying an appropriate test to the system. We could probably hypothesize
all kinds of niches, but we only know if any of them are realistic by
seeing whether some species comes to occupy one of them. So if parakeets
move into Seattle, or any other ecosystem, we just have to see how they
fare. If they don't seem to be affecting any other birds adversely, that
would show that, by definition, they have their own niche. If they
compete badly with some native species--for food, say--we'd say they
haven't found a niche of their own. My impression is that one problem
with Monk Parakeets is that they don't just compete with other species
for food but are very aggressive in displacing them (but I don't remember
how--I'm just passing on what I've heard).

There's a classic argument, from Hairston, Smith, and Slobodkin, that the
populations of herbivores aren't limited "from below," by the
availability of food, but rather "from above" by predation. The argument
is that the world is very green, that there's a lot of plant food around,
and it isn't all being eaten, so plants could support larger
populations. I wonder if the argument can be extended to new species
moving into an ecosystem; if there's enough food to support a larger
population of an existing species, does that mean there's enough to
support a second species using the same resources? (Of course, a species
being very aggressive in displacing a competitor is a different kind of
factor.)

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 360-866-6000, x. 6755
Olympia, WA 98505 FAX: 360-866-6794