Robert Mesibov mesibov at SOUTHCOM.COM.AU
Thu Sep 5 10:35:16 CDT 2002

Doug Yanega wrote:

"Clearly, every organism has habitat preferences, but these
preferences can be overridden in many cases. It seems to me that any
model based solely on preferences is going to miss many marginal
and/or suboptimal areas where a species could (or does) survive, and
for endangered species, especially, that's a VERY risky approach to
What does one do then?"

To be fair to the modelers, the GARP approach does not, in fact, define
preferences and ignore suboptimal areas. What you get from the model is
something like a probability map of species occurrence. CORTEX, for
example, puts colours or grayscale tones in 1 km squares to show several
classes of land: very like where the species is found, like where it's
found, a bit like where it's found, etc.

The idea of "like" here is complex. You might be picturing a graph with the
y axis showing rainfall and the x axis showing soil pH, and an irregular
blob on the graph defining the preferences range for growth of a particular
plant. The GARP output is not simply a multidimensional version of this
environmental enveloping, and it's not very easy, in fact, to get from GARP
just what it is that the plant finds homey.

In any case, how you prioritise field work or other action after seeing the
likelihood map is up to you, and there are two other useful tools you have
available for prioritisation. The first is contiguity. If species A is
found at place X, it's more likely to be found at Y, close to X, than at Z,
which is far from X. You can buffer the known occurrences of A on the map
with a surrounding belt, say 10 km wide, and have a look there. This is the
approach used in Tasmania for rare invertebrates:

Mesibov, R., Bonham, K.J., Doran, N., Meggs, J., Munks, S., Otley, H. &
Richards, K. 2002. Single-species sampling in Tasmania: an inefficient
approach to invertebrate conservation? Invertebrate Systematics 16(4): 655-663.

The second tool is (dare I say it?) biogeographical analysis. If you know,
for example, that Texas is divided up by its snails and other
slow-dispersing invertebrates into 5 biogeographical regions, and if you've
just recognised a new snail in one of those regions, then a rational basis
for prioritising field work is to look elsewhere in that one region.

Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 03 64371195

More information about the Taxacom mailing list