On Tue, 20 Jul 1999, Judy Caughley wrote:
> When we see concentrations of birds or other animals (e.g. kangaroos, mice
> etc), we tend to jump to the conclusion they have migrated from somewhere.
> But one day I sat down and thought if all the animals within a radius of 5
> km moved to the central 2 km what would the effect on density be. The
> answer is quite amazing and suggests that local movements can really explain
> many of the variations in density we see. I'm not denying migration in birds
> like yellowfaced honeyeaters etc - the winter exodus from Canberra to the
> coast is spectacular - but for species that are mobile, concentration on
> food sources is not unlikely.
Well the maths is simple so I'll throw it in. If animals distributed
within a circle of radius of 5km of a point move to within a circle of
radius 1km, density increases by (5/1)^2. In otehr words, density goes
up by a factor of 25. Its a good point.
Andrew taylor
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