NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/environm/14738

Internet Message Format | 1993-01-22 | 6.2 KB

From: [e-mail] (Alan McGowen)
Date: Thu, 21 Jan 1993 22:39:42 GMT
Subject: Re: Temperate zone habitat loss
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Island biogeography is a theory which attempts to explain observed species
area relationships in terms of processes of immigration, extinction and
saturation. This theory is applicable in mainlands since at some level of
resolution habitat islands are ubiquitous -- they are not synonomous with
oceanic islands. The fact that the theory can be applied does not mean that
a single species-area curve holds everywhere *that* the theory can be applied.
It means that the same underlying biogeographic processes are at work producing
the various curves.

As Andrew Taylor pointed out, my remark that diversity and endemism tend
to increase towards the tropics is just a biogeographic fact, having nothing
to do with the applicability of a particular species-area relation.

Like all models in science, models of MVP (minimum viable population) involve
omission of more details than they include, and are not overthrown simply
by pointing out that something has been omitted. Instead, what this means is
that a multiplicity of models, abstracting different aspects of reality, have
to be considered to understand any natural phenomenon. MVP is no exception.
It results from genetic factors, breeding biology, and environmental fluctation
acting together. The MVP when all these factors are taken together -- the
one which should be the basis for a conservation policy for that population
-- is increasingly thought to be set mainly by environmental fluctuation
rather than by genetics. However the MVP for the *specific* kind of genetic
information I mentioned (neutral variation), considered in isolation from
other factors, can be determined by the criterion that Ne >> (1/2u), where
u is the mutation rate of the variation and Ne is effective population size.

By "MVP" in this case I mean the minimum size for persistance, not against
population extinction, but just of population polymorphism. The result is
robust and can be derived in numerous ways, both by starting with diffusion
models of genetic drift and by simpler means (one of which I gave in ECO
CENTRAL 24, I think it was). Derivations can be found in any standard text
on population genetics. The result is also experimentally very well confirmed
by electrophoretic measurements of single-locus polymorphism in Drosophila
and other species, demonstrating that what was left out of consideration when
the model was formed nevertheless left a phenomenon of importance behind.
[This is how real science is done -- not by aprioristic rejection of ideas
for failure to achieve instant universality, but by creative construction
of ideas that can survive observation and experiment in some insight-producing
area of application. Universality is *extremely* elusive in biology. But
universality is not at all the same thing as *understanding*.]

A fairly large amount of neutral polymorphism can probably be lost in any
population without too much cause for alarm if the overall metapopulation
structure permits eventual recovery. Genetic drift simultaneously decreases
*within*-population diversity while increasing *between*-population diversity
-- a major source of geographical variation. Intermittant gene flow between
populations can reestablish higher levels of population polymorphism.

However, there is cause for worry when the entire *metapopulation* is too
small for mutation and gene flow to balance the effects of drift. Most
metapopulation processes appear to reduce effective population size of the
entire species, so that the actual *total* numbers of the species must
be greater than the Ne >> (1/2u) to preserve polymophism *over all*, even
when intermittant gene flow is included. Again, for neutral variation we
have an Ne in the range 10^5 - 10^7, and of course many species are well
below this in actual numbers as a result of human range restrictions.
This cannot improve their future prospects on average, though it is
impossible today to predict a mean time to extinction based on loss of
polymorphism alone. It also very probably reduces intrinsic speciation
potential.

More dangerously, *many* species are now below levels required for persistance
for geologically modest amounts of time (e.g. 1000 years) with high confidence
(e.g. 95%) in the face of normal environmental variation. This is a nongenetic
result.

When a population size is small (Ne < 500, or for most species, population
size < a few thousand) genetic factors can be of enormous importance.
In that case, loss of immediate factors of fitness (a phenomenon
called "inbreeding depression") is possible, and loss of *selectable*
(not neutral) variation can cause great damage to maintenance of adaptation.

For more info, see Michael Soule's (ed) _Conservation Biology: the
Science of Scarcity and Diversity_ and _Viable Populations for Conservation_
for more about MVP.

Note that none of the above points about MVP takes into account the ripple
effect of extinctions in an ecosystems, resulting from connections of
organisms via food webs, as competitors, and as mutualists. This is another
idealized omission of the MVP models, which tacitly assume that extinction
probabilities can be calculated independently for populations.  Work in
blending this population-oriented approach with community ecology and food
webs is an active area today. See Pimm's the _Balance of Nature_ for efforts
in this direction.

Major efforts to obtain direct measurements of how extinction rates vary
with fragment size are now underway [e.g. the Minimum Critical Size of
Ecosystems project.] The results to date are consistent with the broad
predictions of island biogeography theory and with the idea that a roughly
inverse exponential decay to the new equilibrium is common when area is
reduced.

------------
Alan McGowen

Incidently, for those that care, the above is intended as an example of
a "constructive post".


"Models are tools for thinkers, not crutches for the thoughtless."
-- Michael Soule