windfarms: a personal comment

To: birding-aus <>
Subject: windfarms: a personal comment
From: Andrew Taylor <>
Date: Fri, 14 Apr 2006 23:48:40 +1000
On Tue, Apr 11, 2006 at 05:01:57PM +1000, Robert Inglis wrote:
> - how long would it take for a common species to become a rare species
> if 1 or 2 percent of breeding adults (of wild animals) are killed annually
> in addition to those already dying of natural causes?

Wind farms aside this is interesting question.  If you reduce a population
by 1% per year it will be reduced by 3 orders of magnitude (e.g. one
million to one thousand) in 687 years but its not that simple.

Density dependent factors must be important for many bird populations.
Competition for resources such food, nest hollows and territories will
reduce the growth rate of the population as numbers (density) increases.
As a consequence, an increase in mortality might have little impact.

On example is the Silvereyes on Heron Island.  Jiro Kikkawa and his
students collected data on them over decades.  A model fitted to this
data show significant density-dependence.  For the mathematically inclined
the model for annual population change is:

N(t+1) = N(t) * exp(0.97 - 0.0025*N(t) + 0.176*Z) where Z is a unit
normal random variable.

What this means is at low numbers the Silvereyes population can more than
double per year but growth slows as numbers increase with an equilibrium
just over 380.  Installing a wind turbine on Heron Island that killed 2%
of the Silvereyes per year, according to this model, would have little
impact on the population.  Which is not to say the demographics of
other populations aren't very different - and I'd guess there are few
Australian bird populations where this sort of data is available anyway.


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