Allee effect and metapopulations

Jean-Michel MAES jmmaes at
Mon Jan 22 06:52:15 EST 2001

That's also very important when a forest is cutted in small pieces, then you
have no more a large habitat but a big quantity of small habitat and each
small habitat can suffer by Allee effect. Very important for protected area
Jean-Michel MAES
AP 527
tel 505-3116586
jmmaes at
----- Original Message -----
From: Patrick Foley <patfoley at>
Cc: <leps-l at>
Sent: Sunday, January 21, 2001 6:46 PM
Subject: Allee effect and metapopulations
> Neil and others,
>     Neil gave one example of an Allee effect. In general, by 'Allee
effect' we now
> mean any tendency for small populations to deterministically decline
further when a
> larger such population would increase. This is a form of density
dependency, but
> opposite from the effect of competition. It was proposed by the famous U
> ecologist (he influenced, oddly enough U Chicago student Ed Ricketts,
author of
> Between Pacific Tides, good friend of John Steinbeck). Allee saw such
effects as
> the breakdown of mutualistic relationships within a population. For
example, in a
> small population it may be hard to find appropriate mates, someone to
watch your
> back etc. If we had a better understanding of Allee effects in populations
> conservation interest, we would have a much better predictive theory of
>     A crude first approach is to assume that there is some Allee threshold
> population size NsubA you might say. Below NsubA, the population
> declines. Above NsubA, the population tends to grow upwards towards a
> capacity. If a population drops below NsubA, it is doomed to extinction
(unless it
> gets lucky).
>     Attempts have been made to predict the extinction of metapopulations
based on
> local population dynamics and migration. Some discussion of this can be
found in
> Ilkka Hanski's excellent book Metapopulation Ecology. Ilkka works with
> populations of Melitaea cinxia, the Glanville Fritillary. It is difficult
to apply
> simple models of metapopulation persistence honestly to real populations.
> biggest reason is that local populations are not often independent in
> population fluctuations due to shared weather etc. I take this to be one
of the
> major problems in the general theory of metapopulation persistence. Some
> approximate success has been achieved by workers who shall remain nameless
> (not me I'm afraid) since their work is unpublished.
>     The basic ideas are clear. Local persistence furthers metapopulation
> persistence. Uncorrelated local extinctions are better than correlated for
> metapopulation persistence. Higher migration rates are good so long as
they don't
> depopulate local populations.
> Patrick Foley
> patfoley at
> Neil Jones wrote:
> > In article <000901c07f21$a719ad20$9b0f1218 at>
> >            gatrelle at "Ron Gatrelle" writes:
> >
> > > Yes, I know of a couple. The bad apples that spoil the bunch. I am
> > > tempted here to name one name in particular. The person lives below
> > > Mason Dixon line and is fairly well known. I also admit Neil that just
> > > of this type is all us pro-collecting types need to go superlative and
> > > build an even bigger straw man. Just as one real poacher is all the
> > > side" needs to impugn all collectors.
> >
> > Yes. I think it is a straw man. What does concern me is that when ever
there is
> > a case of a poacher being prosecuted he gets a lot of sympathy from
> > certain people here. The most obvious case was 6 or 7 years ago when one
of the
> > poachers was on Leps-l posting psychotic drivel (or "historic postings"
> > he put it) about how he had been the victim of a conspiracy.
> > Despite his showing signs of  mental instability (Delusions of grandeur,
> >  paranoia etc. ) he had people supporting his position.
> > I will have to dig out what I have and put it on-line. My archive for
> > this period is probably partly incomplete although I should have
> > I sent myself filed away.
> >
> > >
> > > Neil, not only do I agree that rare species should not be collected. I
> > > the rare collector who does believe that one collector can eliminate a
> > > local population. However, I believe this is only done by the
> > > collection of immatures -- e. g. Megathymus larvae and pupae.
> >
> > There is no question that increasing predation can impact the
> > It is an absolutely standard part of population dynamics theory.
> > Predator not equals Homo sapiens is not written into the equations.
> >
> > It isn't necessarily just collections of immatures. Let me give an
> > example. I knew a population of a checkerspot that was small (they often
> > on a protected wildlife site just a few miles from me.
> > Like all the species in this group it is a batch egg layer. Each female
can lay
> > a theoretical maximum of 288 eggs in one batch on the day she emerges.
> > She may lay more eggs later if she can survive and feed.
> >  It is therefore possible to easily identify the larval web masses.
> > Indeed this has become the standard survey method for the species since
> > climate is not always reliable in the flight season.
> >
> > On the site in question the managment of the habitat was only just
> > and the foodplant was decining. It was actually possible to examine the
> > vast majority if not all of the plants to see if they had larvae on
> > I did this with a colleague and we found 2 groups of larvae. (I had
> > seen one of the egg batches being laid and marked the spot.)
> > *Just two* This colony was in this season reduced to a maximum of two
> > females. Had somebody removed those *two* butterflies the colony
> > would have been wiped out.
> >
> > Now you may argue that if that colony was that small it would not
> > Well in this case the species is known to exist in metapopulations.
> > Groups of populations which periodically become extinct to be
> > from their neighbours.
> >
> > Many of the colonies of this species are small. In fact the majority of
> > are. Finding less than 10 larval webs is very common and the species
> > fluctuates enormously. One site what had an enormous number (279) webs a
> > years ago was down to almost single figures recently.
> > That high number was so exceptional that the BBC came down to do a radio
> > program on it.
> >
> > The next piece of my argument is difficult to make. It would take a
> > of rather complex equations to demonstrate it. Putting it simply though
> > It would be possible by impacting enough populations to seriously
> > the metapopulation as a whole. In a grossly oversimplified manner
> > is boils down to the essence that it can be demonstrated that *in
> > and at our *current level of knowledge* a certain number of colonies
> > to exist over time in a certain area for it to be stable.
> >
> > If you want to be certain of conserving  something you don't take
> > by increasing predation. The ecology of this *single species* is very
> > complicated and difficult to understand but the effect of predation
> > is possible to model. Ecology is complicated, which is why some of
> > us get frustrated when we hear people being convinced by simplistic
> > anecdotal and unproven "creative science".
> >
> > There is also some evidence that smaller populations are more likely
> > to generate colonisers due to the "Allee Effect". It has nothing to
> > do with a French very but is someone's name and it is an effect where
> > small populations tend to loose individuals easily because they sort of
> > wander off.
> >
> > --
> > Neil Jones- Neil at
> > "At some point I had to stand up and be counted. Who speaks for the
> > butterflies?" Andrew Lees - The quotation on his memorial at Crymlyn Bog
> > National Nature Reserve
> >
> >
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