On Fri, 03 Mar 2017 19:11:25 +0000, Tim Streater
wrote:

In article , Don Pearce
wrote:

On Fri, 03 Mar 2017 15:05:57 +0000, Tim Streater
wrote:

In article , Don Pearce
wrote:

Yes to the first. For the second, sparrows have their predators like
any other bird and as soon as a decline sets in the balance is upset
to a point where the predators make short work of the remaining birds.

There's no such thing as a "balance" as you put it. Populations of
everything rise and fall for a variety of reasons.

There is a balance- it is dynamic, affected by all sort of factors.
The good old unstable Xi+1 = r * Xi * (1 - Xi) equation describes this
more or less. But when suddenly new generations are not being raised
because a food source has all but vanished, that is severely upset.

You're assuming the food source has vanished. And sparrows here eat a
variety of things, not just insects.

You don't know that. The simple equation you give above describes
simple behaviour. The factors influencing the population of sparrows is
going to be correspondingly more complex, and for all you know that may
make the population values vary wildly and unpredictably.

Of course I'm simplifying. But the fact is that sparrows disappeared
at - as far as I can remember - the same time as the flying insects.
Insects are their main summer food. Now we have two options here.
Either the insects vanished and the sparrows starved, or the sparrows
ate all the insects.

I don't think this is the forum for going into the minutiae of the
food chain of the sparrow. Either we keep it simple or we don't
bother.

d

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