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Continental vs. Island extinctions



Conventional wisdom has it that the broader the range or the bigger the
land mass, then the slower is the rate of extinction.  This
makes intuitive sense: niches are broader, more numerous; there
are more places to hide, more habitats to fall back on, etc., etc.  To
this, add the apparent benefit to a species that has bigger populations
and therefore relative immunity to bad luck/stochastic forces--populations
are larger on bigger land masses (generally).

That is why the Blackburn _et al_ paper is so interesting: it finds
that other forces may be more important than land mass.  In fact, they
voice something like surprise when reporting that predator diversity has a
greater correlation with avian extinctions than does land mass.  Michael
Habib argues that while this effect is important on islands it may not
be on continental land masses.  But I'm not sure why.  Is there a good
reason for not extrapolating islands to continents?  After all, continents are
just big islands.  It is true that in our recorded history we haven't seen
anything like the rate of predator-caused extinctions as on oceanic
islands.  But there are reasons other than land mass _per se_
that may account for this observation--or lack of it.

1. Invasions of continents are rare.  However, when a continent has been
invaded, e.g., when NA and SA biotas merged, many extinctions _are_ noted.
The causes of these are not known--but the present paper lends
weight to the notion of predation as a prime cause.  For a second example,
I would think Australia would be relevant, i.e., many species threatened
thanks to cats, etc., etc.

2. Isolation from predatory guilds is not possible (in ecological time,
at least).  Therefore, prey populations do not experience sudden influxes
of new strategies.  Predator/prey arms races are waged, more effective
concealment strategies are evolved, and new diets are exploited without
evidence of elevated extinction rates.  But the species alive today are
different species than those existing in, say, the Miocene.  In that
sense, the older species are extinct.

Why _is_ predator/diversity more important than land mass on islands?

Predators have ways of _finding_ prey.  Prey must live in
certain habitats--and since they are not distributed randomly, predators can
plan/predict their location and eat them.  This is _not_ a stochastic
process.  This locating ability should be in full force on continents as
well as islands.