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Re: Continental vs. Island extinctions




On Wed, 13 Oct 2004, MICHAEL HABIB wrote:

> 1. I would argue not so much that species on continents should be less
> prone to extinction as much as those species with large geographic
> ranges should be less prone to extinction (ie. the range the species
> actually inhabits).

But predators target species where they are.  Certainly, traveling away
from predators is an option for some species...and this gives them
immunity to some extent.  Geese flying to the Arctic probably do this in
part to _escape_ predation.

> Large ranges can only occur on large land masses (or the
> reverse if the animal is aquatic), but not all terrestrial ranges are
> large.

But island avian species may well have large ranges, and it's only the
breeding that binds them to the island.  I'm thinking of a sea-going bird
that returns to an oceanic island to breed, perhaps the albatross that
rats eat on the nest.  If a bird has a new predator introduced into its
breeding range--and here I'm thinking about arctic foxes given access to
breeding grounds of snow (?) geese, or more edge being created by farming
of the pothole region allowing greater predator access to prviously immune
breeding range--then the result is the same.  But then remember that these
animals are still in existence _because_ they found these islands of low
predator density in the first place.

> It has been shown pretty soundly that geographic range matters a
> great deal, and it generally proves to be a primary predictor of
> extinction risk in extant species.

A large range might be a _result_ of a successful anti-predator tactic.
It may also be a _tactic_ to avoid excessive predation (e.g., geese
nesting remotely).

> In cases such as the NA/SA interchange, I agree that
> there _is_ strong evidence for predation-mediated extinctions.  But it is
> also true that situations such as the interchange happen very
> infrequently, even on a geoglogical timescale.

And yet the major northern continental predators have invaded the entire
hemisphere--and undoubtedly have exerted a selection pressure and had an
influence on species strategies, life histories, and distribution.

> 4. An individual predator finding a prey item is not stochastic, but
> the probability that a predator's geographic range overlaps with a
> potential prey is somewhat stochastic.

Predator species can only exixt where there are prey.  This couldn't be
random.

> My argument for predation/range continuum is difficult to test, but not
> impossible.  If my argument is correct, then Madagascar and other large
> islands should have extinction dynamics somewhere in between.

But the other variable confound, e.g., predation pressure may be more mild
on Madagascar.  Interesting to note the very _high_ rate of threatened
species--lemurs particularly--due to...?