Q: "...So if ferrets go feral, what exactly do they eat, and how does that
    differ from what mink eat...and how does it tie to ferrets in Calif?"
 
A: I don't care what mink eat so long as it isn't my ear.
 
In a very simplistic nutshell, no two species can occupy the exact same
niche.  If two try, one will either move, shift the niche composition so the
two niches are not exact (but may still overlap), or go extinct.  The way
this is done is through interspecific competition at both the individual
level; i.e. a mink driving off or killing a polecat, as well as at the
population level; i.e. mink reproducing faster than polecats and slowly
replacing them over time.  In a stable community, competition favors the
established status quo, so the introduction of new elements is difficult, if
not outright impossible.  After all they are already well established, they
have a prey base, and are adapted to the local environment, etc.  For a new
mammal to enter an established community, it has to be *BETTER* that at
least one of the existing members.  In the United States, there are several
mammalian predators that fit that criteria; the coyote, raccoon, opossum,
and mink.  Note three of the four are actually not strict carnivores but
omnivores.  Only the American mink is a strict carnivore, yet it outcompetes
the European polecat and European mink hands down.  All are human tolerant,
oportunistic generalist feeders with responsive reproductive rates.
 
The status quo isn't a problem in communities where the stability has been
disrupted, or in communities with "open" niches. In the former, some sort
of outside force has pushed the community into instability, such as when
Mt. St. Helen exploded, or when a farmer clear cuts a section of land for
plowing or the Fish and Gestapo poisons a body of water to kill off native
"trash" fish so they can restock exotic "sport" fish. In the later
instances, an introduced species has no competition to keep it at bay.
 
It is in these disturbed and/or open areas that introduced species can gain
footholds and invade.  This is not unusual, but is how most species
originally established themselves.  Almost any mammal can invade and
successfully colonize an "open" niche, much like what the ferret (and lots
of other mammals) did in New Zealand.  Lots of food, lots of recruits,
little competition equals a successful invasion.  But it takes a special
mammal to invade and successfully colonize an area with established
predators and built in competition.  One of the best at this job has been
the American mink, who, in less than a century, has colonized most parts of
Europe.  While few really good studies prove that the mink has actually
eliminated European predators from their territories, what evidence that
does exist suggest they have had a major negative impact on *ALL* niche
competitors, including European mink, European polecats, European otters,
and even stoats, martens and small canid species.
 
In Britain, the European polecat was nearly wiped out by human hunting.
Once common throughout the Isles, it is now only found in parts of Wales,
and evidence exists that at least part of the population has interbred with
ferrets.  When the American mink established itself in Britain, concerted
efforts, bounties, and extensive hunting could not make a dent in it's
population, and efforts have been halted because they are futile.  Mink are
awesome predators, and if a population ever became established in New
Zealand....well, it would be an interesting study.
 
Mink and polecats eat about the same stuff, but in different proportions.
For example, both eat voles, but mink eat more voles than polecats, who eat
more anurans than mink.  In areas where they are sympatric, mink force the
polecats to the edge of the niche, and make them shift their eating habits.
At least one study suggests that polecat populations, as a whole, are on a
gradual decline as a direct result of the introduction of mink.  This is
only a recent realization because many early investigations of polecat
ecology are, for the most part, methodologically flawed.  One major problem
in polecat (ferret) diets is they were either done in New Zealand, which is
a poor example for the rest of the world, or in areas of Europe that have
been heavily impacted by humans.  In those areas, polecats eat a lot more
birds that in areas of little impact, where they are generally anuran
specialists (anurans are frogs, toads, etc).
 
The main difficulty I have with the Ca Ca Fish and Gestapo is they simply
have not done their homework.  (I actually feel they are too stupid to do
any homework at all but hate to put down stupid people everywhere by
including the Fish and Gestapo with them).  In Europe, polecats simply
cannot out-compete American mink.  Heck, European mink cannot outcompete
American mink, which is one reason they are considered threatened throughout
Europe.  Ferrets are domesticated from polecats, and are generally
considered less able competitors than their wild kin, so it is unlikely that
they could out-perform mink either.  Both mink and polecats eat about the
same stuff and have niches that have major overlaps.  The only places in
California where polecats could establish themselves already have
established mink and weasel populations.  Now, if the polecat cannot
out-compete the American mink in Europe, how can it be done on the mink's
home ground, with well established populations, environmentally adapted, and
reproductively viable?  And polecats outcompete ferrets.
 
Still not convinced polecats and American mink have similar diets?
 
W. Jedrzejewski etal 1989 "Food niche overlaps in a winter community of
predators in the Bialowieza Primeval Forest, Poland." Acta Theriologica
34(34):487-496.
 
"The polecat's role in the predatory community is somewhat surprising.  The
feeding ecology of this species has primarily been known from studies done
in rural landscapes, where domestic poutry, other birds and rabbits made up
a bulk of this animal's diet (citations).  This led to a misunderstanding
that in its ancestral habitats the polecat was primarily a bird-eater
(citation).  In fact, its place in an undisturbed community is more similar
to that of the mink than of any other mustelid."
 
Bob C and 20 MO Mustelid Monkeys
[Posted in FML issue 2204]