At 11:36 8/02/2001 +0000, you wrote:
>G'Day Mates
>
>I am a pommy cricket supporter and have been watching
>your debate about canon netting. Now i am not an
>educated man but had to laugh when Dr Woodall wrote
>the message below when discusuusing the mortality
>caused by cannon netting. Now i bet the lot of you
>that over a 100 years the mortality rate for wild
>birds is about the same as humans. I see you have a
>namesake of mine (close) on the list.
>
>G'Day Mates
>
>Shane
>
Dear Shane,
I think you'd best stick to watching cricket.
Let me explain.
If we assume that the population is stable, not increasing nor decreasing,
and there is no net immigration or emigration, then
births = deaths.
For a pair of humans (in the west) over their lifetime (65 years):
births = just over 2
deaths = just over 2 (the parents, and a small proportion of their children)
mortality rate 2/65 = 0.03 per year per pair
[even in the Third World the figures would be something like 10 births, 10
deaths]
.... and the population remains stable.
For a pair of birds, say Pacific Black Ducks with a lifespan of say 2 years
on average,
although some live to 10 years in the wild (from banding studies):
births = average clutch of 9 (range 7-13) (I'm only allowing for one breeding
season but there could be more);
deaths = about 9 (the two adults and 7 of the offspring, leaving two to
maintain the population).
mortality rate = 9/2 = 4.5 per year per pair
Small passerines (song-birds) in the northern hemisphere have similarly
large clutches, short lifespans and high mortality. Small birds in the
southern hemisphere tend to have smaller clutches, but live and breed much
longer,
and have a lower mortality
So, over a lifetime, birds have much higher productivity than humans,
and thus much higher mortality. The annual mortality
the difference is even more striking.
Over the 100 year period you mention,
there would be about 3 deaths among the pair of humans and their offspring;
there would be about 450 deaths among the pair of ducks and their offspring.
I don't see that many dead birds around,
thus I stand by my statement that much mortality is unseen.
Do you get my drift/swing or have you caught a flipper?
Cheers
Pete
Dr Peter Woodall email =
Division of Vet Pathology & Anatomy
School of Veterinary Science. Phone = +61 7 3365 2300
The University of Queensland Fax = +61 7 3365 1355
Brisbane, Qld, Australia 4072 WWW = http://www.uq.edu.au/~anpwooda
"hamba phezulu" (= "go higher" in isiZulu)
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