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Re: Continental vs. Island extinctions
I do think there are very strong arguments as to why extinction dynamics on
continents are expected to differ from those on islands, including the
importance of predation. John Bois makes a good, articulate argument on
several fronts (especially the idea that predators track prey ranges, for which
there is ample evidence), but I do have several responses:
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). Large
ranges can only occur on large land masses (or the reverse if the animal is
aquatic), but not all terrestrial ranges are large. 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. See the work by
Andy Purvis, John Gittleman, Kate E. Jones, Ian Owens, M. Bennett, Georgina
Mace, and Marcel Cardillo for some examples. Others exist as well. However, I
will be the first to note that these studies generally did not include measures
of non-human predation (which is very hard to quantify). So they show more the
importance of geographic range than the unimportance of predation.
2. Part of "importance" with regards to threatening processes is frequency. As
John Bois points out, invasions on continents are rare, and isolation
situations are also uncommon. Therefore, the frequency with which
predation-mediated extinction events occur on continents should be low. 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.
3. The primary reason that islands should not be extrapolated to continents
with regards to extinction processes is simply that on islands, all geographic
ranges are small. On continents, there is a wide variety of range sizes.
Those species with small ranges are going to be effectively island species, and
thus be just as prone to extinction as species on real islands (including
predation effects). However, some species will have huge geographic ranges,
and will be very resistant to any threat process with an area of effect smaller
than their range (including predators with smaller geographic ranges...again
NOT home ranges). On oceanic islands, the threat processes will nearly always
encompass the entire island, and thus the entire range of the species present.
Species normally capable of wide dispersal will still be stuck with small
ranges on islands.
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. On small islands, invasive predators are nearly
guaranteed to overlap with the resident prey species in all areas they inhabit.
The larger the landmass, the less likely this becomes, at least for those
continental prey species with large ranges. There may be an continuous effect.
That is, predation may become more important as an extinction threat the
smaller the landmass becomes, with range becoming less important (land mass is
probably unimportant statistically on islands, by the way, because there is no
variance in the land mass size: everything is small). In the other direction,
the larger the landmass, the more like a continent it becomes, with species
ranges being important and predation being only a rare cause of extinction.
In this sense, continents would be one end of a continuum, and thus would be
"big islands". Treating them exactly the same as true islands is probably not
easily supported, however. 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