Conditions for Overcollecting

[Neil Jones]

In article <3.0.5.32.20000103091412.007e1a60 at mail.utexas.edu>
           drdn at mail.utexas.edu "Chris J. Durden" writes:

>   Regarding this population of *Euphydryas (Occidryas)* whether it is
> endemic subspecies or species, its range elswhere in these mountains should
> be assayed before restrictions on sampling are imposed.

What do you mean by "sampling" ? Perhaps you could clarify this?
In biological terms I believe it is equivalent to predation.

> A large portion of
> the mountains are included in the Mescalero Apache Reservation, beyond the
> jurisdiction of the USFW, if I understand the situation correctly. Not too
> long ago a large fire swept through coniferous forest on the reservation,
> destroying cover and opening up the area to sheetflood erosion and burial
> of the rare gulch wetlands under sediment washed off slopes. This I see as
> a far greater hazard to the habitat of this population of *Euphydryas* than
> any of the development at Cloudcroft.

I don't think this is valid. I don't know the area so I can only comment
on what the known population dynamic models tell me should be done.

 I find this difficult to answer here particularly
in text form. The population dynamic systems of checkerspot butterflies
are phenomenally and fascinatingly complex. Niklas Walhberg is an
undoubted expert in this area so no doubt he will correct any inaccuracies
in my less expert analysis.

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. Now there are two important things that this 
says. 

1 EMPTY habitat is a fundamentally inherant component
of the system. So suitable but unoccupied habitat in the vicinity of
occupied habitat must be conserved.

2. If you simplify the equation you find that the equlibrium value of P is
given by 1-(e/c). In other words if you increase the extinction rate the
number of occupied patches will fall. (This is fairly obvious if you think 
about it.)

What we do know from the work that has been done in Finland on Melitea
cinxia (Known in Britain as the Glanville Fritillary but in reality
a checkerspot.) is that one of the major causes of population extinctions
is predation. Specifically in this case it is attacks by the parasitoid wasp
Cotesia melitearum, but it really does not matter what the predation is caused
by. "Predator not equals Homo sapiens" is not written into the mathematics.
Predation leads to an increased extinction rate. If you want to increase the
stability of the metapopulation you avoid anything that increases predation
because it decreases the stability of the system.

Now coming back to why the proposed development at Cloudcroft is harmful.

The equation I showed  can be developed further in order to calculate
the minimum size of a network of habitat patches that will work, and
how many need to be occupied. What the mathematics boils down to
is that 3 times the Square Root of the total number of  habitat
patches must be occupied. If you do the sums the absolute minimum viable
system is 10 patches, all occupied. This is an absolute minimum in perfect
theoretical conditions. Now nature will vary from the perfect mathematical
model because it makes assumptions that all habitat patches are similar
but what it does indicate is that a considerably larger amount of habitat
is required in a given area than might at first seem obvious.
>From this you can also see these occupied patches that are threatened by
development at Cloudcroft are particularly valuable to the integrity of the
system.
The last thing that should be done if this creature is to be conserved
is to allow the destruction of key important parts of its population
before their value is even known. The exact parameters which are required
to know this are difficult to obtain. Do we know, for example, which if
any specialist parasitoids attack this animal? ( The indications are 
that there should be only one, probably a Cotesia species.)
 What are the population dynamics of its host plant? The only safe solution
is to be cautious.

If we get this wrong the result is irreversable. This butterfly will
join the Great Auk, the Xerces Blue and Dodo in oblivion. 

-- 
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