Checkerspot on the news

[John Shuey]

Neil at NWJONES.DEMON.CO.UK wrote:

> Well the developer's argument about the species' absence has little
weight
> when the known mathematical models of the species population dynamics
are
> taken into account.
> It actually matters little whether the butterfly is present in any
period
> rather whether the site is suitable and reasonably close to an
occupied one.
>
> As I have said before checkerspot populations exist in
metapopulations.
> These are linked groups of colonies where individual colonies become
extinct
> to be eventually colonised from their neighbours. The classical model
of
> metapopulation
> dynamics postulates that  the number of occupied patches P at a given
time
> t is given by dP/dt=cP(1-p)-eP where c and e are colonisation and
> extinction rates.
>
> What can be clearly seen from the mathematics is that EMPTY habitat is
a
> fundamentally inherant component of the system. So suitable but
unoccupied
> habitat in the vicinity of occupied habitat must be conserved.
>
> In fact if the mathematics is expanded on and worked up then the most
modern
> experssion of it is the Incidence Function Model developed by
Professor
> Ilkka Hanski at Helsinki. I know at least one of his team is
subscribed
> so if I get it wrong I will be corrected.
> The model is far too complex to state in an email but it can be
simplified
> to an approximation that states that 3 times the Square root of the
habitat
> patches need to be occupied in order for the system to be stable.
> It can be seen from this that there could be an awful lot of empty but

> necessary habitat in the system.
>

Unfortunately, this is a simplification based on traditional
metapopulation models, applied
to an animal that doesn't fit the traditional model.

As I remember, most folks think that Bay Checkerspots are structured as
a Source-Sink
metapopulation as opposed to the traditional (albeit very rare) true
(extinction-recolonization) metapopulation.  The persistence of a
source-sink metapopulation,
for example, depends on the viability of the source population.  The
smaller, occasionally occupied sinks contribute little, if anything, to
the long-term
persistence of the metapopulation (Susan Harrison 1991). Thus, only the
factors influencing the viability of the source population (e.g., patch
area and population
densities) are of management concern.  The Bay Checkerspot is usually
heralded as the stereotypical source-sink metapopulation animal in the
US (and Harrison
originated the concept after working with the species as part of
Ehrlich's
group).

Conversely, in an extinction-recolonization structure, all local
populations are vulnerable
to extinction, and thus, influential in the persistence of the
metapopulation.  In this type of structure, the factors controlling
extinction and
recolonization rates  (e.g., number of occupied and unoccupied patches,
patch
size, patch population densities, arrangement of patches and corridor
availability) (Harrison
1991, Neve et al. 1996) are of management concern. In the US, the
Karner Blue is the poster child for this type of population structure.

The implications are severe from a conservation standpoint.  With a
source-sink model it is
fairly easy to argue that small "sink" populations are not important
for survival.  In this case, the source population (associated with a
landfill and managed
for the butterfly) is supposedly secure.  Because other occupied or
unoccupied habitat patches presumably do not influence population
persistence at the source,
they may be deemed non-vital for the conservation of this animal.

I'm not endorsing this approach (in part because I think that there are
other, genetic
implications to population persistence in a dispersal friendly world),
but
it is an easy one to argue based on the math.

--
John Shuey
Director of Conservation Science
Indiana Office of The Nature Conservancy

phone:  317-923-7547
fax:  317-923-7582
email:  Jshuey at tnc.org