Live for ever?

A landowner, whom I know, tried to post the following comment on last Thursday's blog but failed.  Because he raises some interesting points I am posting his comments here and commenting on them.

PM wrote: Mark -  It sounds as if Hope Farm is going really well.  Many congratulations. However you say that “winter counts aren’t the most important counts – the real measure of success is in breeding numbers” .  Do you really mean this ?  Surely the real measure of success is the number of  chicks that are successfully fledged ? 

Interesting that Trimbush comments that his lapwing have returned.  With spring in the air, doubtless they have one thing on their minds.  Your fellow scientists tell us that each pair of lapwing needs to fledge an average of  0.7 chicks per year in order to maintain a biologically stable population. You will know better than me that RSPB monitoring work on 25 breeding lapwing sites a few years ago, revealed that only two of these sites produced more than the 0.7 chicks per adult pair average.  Which means that 23 of the 25 sites were acting as ecological traps (population sinks). 

And you will remember that research work that we commissioned last season on the North Kent Marshes showed that 3 out of the 4 breeding lapwing sites monitored were likewise not producing a viable number of chicks.  Which we concluded resulted from the breeding wader management prescriptions being politically correct but biologically perverse.

Isn’t this lack of chick productivity the real problem for some farmland birds ?  Shouldn’t fledgling numbers be your real measure of success ?

(Believe it or not, this is the first time that I have posted a comment on any blog).

No, I meant that the real measure of success is breeding numbers.  Those breeding numbers derive from an interaction between how long individuals survive and how successful they are at reproducing. 

Imagine a newly created piece of perfect habitat which is colonised by some birds.  The rate at which that population will grow depends on how good the breeding birds are at producing young but also how long the adult birds survive.  Lots of young but low survival can be as bad as low breeding success and living for a long time.  It's a bit like the RSPB membership number depends on how many new members we recruit but also on how good we are at retaining the members we already have.  If you score high on both then your membership (population) is booming, low on both and you are heading for extinction.  But it's difficult to say that productivity is more important than survival and that's one reason we measure breeding numbers - they integrate those two important population parameters.  And it's difficult to say that membership retention is more or less important than recruitment and that's why we tell you our year end membership each year as that figure integrates those two important population parameters.

These things are clear when you compare species - albatrosses live a long time, breed at a late age and don't have many young each year even when they do breed, whereas great tits have a short life on earth and attempt to fill the woods with their young if they can.  We wouldn't say that great tits have it right and albatrosses have got it wrong.

It is the quality of the habitat that ultimately determines the breeding densities of any species - better habitat leads to higher densities being achieved before the pressures of competition for food or nesting sites limit numbers.  That's a great oversimplification but it captures the gist.

If your habitat is 'full' then if you produce lots of young they will have nowhere to go and they will die.  That is essentially the struggle for existence which Darwin noted and which drives evolution by natural selection.  And that over-production of young is what enables game shoots potentially to be sustainable without the population of their prey crashing.

So, I agree with you that productivity is important but it isn't 'the real measure of success' - unless you run a pheasant shoot and then it's pretty important. 

But we are concerned about the productivity of some birds on some of our nature reserves - of course we are.  With 200+ nature reserves and getting on for 200 breeding bird species on them then it would be a bit surprising if we weren't.  And increasing productivity is something we are trying to do on some of our nature reserves.

And so ideally, when looking at any population to see how it's doing we would see how its density compared with what we would expect - is the site a 'good' place or a 'bad' place for that species.  And then ideally we would look at the balance between production of young and adult survival.  We can rarely measure survival and productivity is pretty difficult too!