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From: jday@spam.ua.oz (Jemery Day)
Newsgroups: rec.birds
Subject: Re: Importing parrots for genetic variability
Message-ID: <847@spam.ua.oz>
Date: 17 Jun 91 01:19:58 GMT
References: <1991Jun12.131020.22423@zoo.toronto.edu> <1991Jun15.154957.3243@sci.ccny.cuny.edu>
Organization: Statistics, Pure & Applied Mathematics, University of Adelaide
Lines: 55

In article <1991Jun15.154957.3243@sci.ccny.cuny.edu> christ@sci.ccny.cuny.edu (Chris Thompson) writes:
>       An Australian theoretical geneticist (Warren Ewens) has spent some
>time talking about the problem of genetic variation, and minimum viable
>population sizes.  ("Minimum viable population" is the current hot catch-
>phrase in conservation biology).  Anyway, the problem here is that when you
>want to calculate a minimum viable pop. size, you DON'T use the actual
>population size.  You have to include the effective population size.  I don't
>recall the formula offhand, but it has to do with sex ratios, and the number
>of animals that actually breed, and survival rates.  Now, this can obviously
>all be manipulated with captive populations, but still, the effective
>population size is always much smaller than the actual population size.

  Genetic variation is not the only consideration when trying to find a
minimum viable population. The effects of demographic stochasticity -
i.e. the chance nature of birth and death in a population; environmental
stochasticity - i.e. random fluctuation in weather patterns with
resulting in variation in the availability of food and water; and
catastrophe - i.e. bushfire which can destroy food sources and habitat.
All four factors are important in performing a population viability
analysis on a given population. ("Population Viability Analysis" is also
"hot" in conservation biology).

>       Also, there has always been this mystique associated with the number
>500.  This has been bandied about as a standard minimum viable population
>size---below 500, most species will not maintain enough genetic variation
>to make it for more than about 50 years.  Warren Ewens again (this was the
>subject of a seminar he gave at Columbia University 1.5 years ago)
>thouroughly debunked this notion, showing that the number is almost always
>much greater than 500.  His position was that the number 500 was based on a
>series of "heroic assumptions", which werem't valid.  So I don't think I'd
>spend a lot of time trying to catch wild birds to maintain genetic
>variation in captivity.  I think I'd rather see it remain in wild
>populations.

Population viability analysis is used to try to estimate the probability
that a given population of a particular species will become extinct over
a given time interval. The notion that a population larger than a
particular magic number will be "safe" from extinction is complete
rubbish. You can attempt to associate a probability with a given initial
population, with that probability representing the chance that the
population will become extinct over a given time interval (say 500
years). The point is that these processes are effected by chance events
and as such must give results which can only be interpreted in terms of
probabilities.

In terms of maintaining genetic diversity and decreasing the chances of
species extinctions I agree with Chris, leave the birds in wild
populations!

>Chris Thompson
>Biology Department, CCNY

Jemery Day
Applied Mathematics Department
Adelaide University