Newsgroups: sci.environment

$Ghpindda:sci.environment / dean@vexcel.com (Dean Alaska) /  8:59 am  Jan 22, 1993 /
Alan McGowen writes:
>>
>>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.
>
>When one considers this in light of projects such as the one to save
>the California condor (population = 7?), it seems that the limited
>resources available may not be going to the most important and
>productive uses.  I once commented on the long term viability of
>a population so low - that its potential for future evolution may be
>lost.  Someone (was it Andrew?) commented that the first limit a
>population reaches (as it decreases) is that which threatens its
>survival in the face of rare natural (or anthropogenic, I guess)
>disasters.  In other words, the population needed for adequate genetic
>diversity was less than that required to survive rare disasters (such
>as a major forest conflagration) that spans much of its range.

A very crude rule of thumb which readers might keep in mind is this:

1) In general, the first "limit" to be hit as a population reduces in size
is that for loss of neutral polymorphism, hit around population sizes
of 500,000 to a few million.

2) The second "limit" to be hit is that for (moderately) rare natural 
catastrophies. 

3) The third limit is that for normal environmental fluctuations such
as severe winters or violent storms.

4) The fourth limit is that for loss of adapation, thought to be
hit at population sizes of around a couple thousand. [*Effective*
population sizes around 500, but the number has many assumptions in
it.]

5) The fifth limit is that for loss of fitness through inbreeding, hit
around a couple hundred.

6) The sixth is extinction through a small fluctuation or bad demographic
luck (e.g. the last female has only male progeny).

There is a spectrum from small, common environmental fluctuations up to 
huge, extremely rare catastrophies. The "minimum population size" to survive
an asteriod impact might well be larger than 1), but for most purposes
we probably can think of 1) as the upper limit for discussions about MVP.

The limits 2) and 3) often range in the tens to hundreds of thousands or more. 
These limits arise in answer to the question "how large must the
population be to have probability p of survival for t years?" E.g. "how large
must the population be to have a 95% chance of persistence for 1000 years,
given historical environmental fluctuation?" , i.e. Ne = MVP(p,t). 
As Dean says, these Ne are generally significantly higher than the genetic 
limits 4) and 5). The genetic limits, by contrast, are not estimated
as functions of t and p, but of the natural mutation rate u of a given kind 
of variation. Simulation models which include both genetics and enviromental 
fluctuation are also being explored. See the Soule refs for more info. Also
see my post on "Mammalian Persistence."

This means that the MVP -- and thus the MAR (minumum area requirement) is 
more often set by these stochaistic considerations than by genetic ones.
In essence then the problem of MVP consists of establishing a best estimate
of the environmental fluctuation spectrum and the resultant population 
-- and metapopulation -- survival probabilities for various choices of
p and t. *Which* choice of p and t should be adopted by society is a 
political question, but it then potentially determines the MVP and MAR 
objectively. However, which choice of p and t we take also affects the
overall extinction rate: if we have a goal of reducing this back to 
geological normality it potentially constrains p and t, and through them
MVP and MAR (and tacitly then also the amount of land available for human
intensive use).

>I think this all emphasizes the need for the the coherent approach
>towards such problems that would be more likely with the type of
>biology institute that Alan has mentioned.  Crisis management of
>biodiversity issues by emotional politics does not seem to match
>the priorities that would emerge from a triage approach.
>(Surprise, surprise)

On the topic of condors and triage, I'll reiterate the "public awareness"
argument. The fact is that for every one of us who would protect the habitat
of an endangered plant or invertebrate there are probably a hundred people
who will get upset about condors or mountain gorillas or shooting wolves. 
That is a political fact which we would be very stupid to overlook. We need 
to turn that sentiment to legislative benefit -- and we *can* -- but we also 
need to keep it alive. The condors do that. Their PR value is enormous.

Now, would all that go by the wayside if we had a National Institutes of
Biological Diversity? Maybe, but I think it would be a very bad idea to let
it. The success of such an institute will depend to an enormous degree on
the public support for it. I would hope that not only would a certain 
amount of highly-visible condor-type efforts go on, but that some nontrivial
part of the budget would be spent on educational outreach of various kinds.

Some of the projects which migh fail a triage test might be staffed by
volunteers, for example. Or conceivably by an expansion of Clinton's
community service idea. 

I'd very much like to see ecosystem restoration projects come to be 
viewed as a community service. A National Institutes of Biological 
Diversity, in addition to its research, information-clearinghouse,
and critical restoration biology work should also help coordinate such 
community service. Let's not just enthusiastically cluster-bomb the place
with random species of trees, but do real, needed, coordinated restoration
work -- and do it as a civic duty, not just as a minor activity buried 
in some bureaucracy.

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

"By the faces we show each other and the world... we force the spring." 
-- Bill Clinton.