Allee effect and metapopulations

[Patrick Foley]

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 Chicago
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 of
conservation interest, we would have a much better predictive theory of extinction.
 
    A crude first approach is to assume that there is some Allee threshold
population size NsubA you might say. Below NsubA, the population deterministically
declines. Above NsubA, the population tends to grow upwards towards a carrying
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 Finnish
populations of Melitaea cinxia, the Glanville Fritillary. It is difficult to apply
simple models of metapopulation persistence honestly to real populations. The
biggest reason is that local populations are not often independent in their
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 here
(not me I'm afraid) since their work is unpublished.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 csus.edu
 
 
 
Neil Jones wrote:
 
> In article <000901c07f21$a719ad20$9b0f1218 at gscrk1.sc.home.com>
>            gatrelle at tils-ttr.org "Ron Gatrelle" writes:
>
> > Yes, I know of a couple. The bad apples that spoil the bunch. I am very
> > tempted here to name one name in particular. The person lives below the
> > Mason Dixon line and is fairly well known. I also admit Neil that just one
> > 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 "other
> > 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" as
> 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 everything
> I sent myself filed away.
>
> >
> > Neil, not only do I agree that rare species should not be collected. I am
> > the rare collector who does believe that one collector can eliminate a
> > local population. However, I believe this is only done by the aggressive
> > collection of immatures -- e. g. Megathymus larvae and pupae.
>
> There is no question that increasing predation can impact the population.
> 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 are)
> 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 our
> climate is not always reliable in the flight season.
>
> On the site in question the managment of the habitat was only just starting
> 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 them.
> I did this with a colleague and we found 2 groups of larvae. (I had actually
> 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 survive.
> Well in this case the species is known to exist in metapopulations.
> Groups of populations which periodically become extinct to be recolonised
> from their neighbours.
>
> Many of the colonies of this species are small. In fact the majority of them
> 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 few
> 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 number
> of rather complex equations to demonstrate it. Putting it simply though
> It would be possible by impacting enough populations to seriously threaten
> the metapopulation as a whole. In a grossly oversimplified manner
> is boils down to the essence that it can be demonstrated that *in theory*
> and at our *current level of knowledge* a certain number of colonies need
> 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 chances
> 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 nwjones.demon.co.uk http://www.nwjones.demon.co.uk/
> "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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