Dear colleagues,
I'm trying to quantify influence of bear predation on the Steller's Sea Eagle nestlings. This factor is one of main causes of nesting failure: about 20% of eagle offspring is depredated by brown bears. Other causes together are responsible for about 10% of offspring loss, referred as nestling mortality. Simply speaking, there are three possible nestlings fates: fledged, depredated, died from other causes.
The question is, how to calculate correctly sea eagle productivity loss due to bear predation. I see two possible ways.
1. Ratio of the number of depredated nestlings to the number of all nestlings
Loss1 = Ndepredated / (Nfledged + Ndepredated + Ndied)
However, in this case the loss by predation would correlate to the Ndied: the more nestlings die from other causes, the less will be the loss by predation.
2. Another option is to exclude dead nestlings from the index:
Loss2 = Ndepredated / (Nfledged + Ndepredated)
Now the index does not correlate with Ndied, but it seems a bit complex and counterintuitive. For example, let's suppose that we have 99 nestlings, 33 of which successfully fledged, 33 were depredated and 33 died. The first index Loss1 = 1/3. However, the Loss2 = 1/2, which means that if no predation occurs, 66 nestlings would fledge, and bear predation reduces this number by 50%.
Which of the indices, on your opinion, is more relevant, or maybe it depends?
Article Measuring Health-Related Productivity Loss
Michael S. Romanov
Thank you, Palem Harinath, but this paper didn't help much. It considers qualitative parameters, and, furthermore, another kind of the productivity: the human labour efficiency. I don't see its relation to my problem.
Hello Michael; One factor not mentioned in the question is any measure of K. If there was no bear predation at all would there be more recruitment into the adult population? Hmmm. The question is a tricky one! Cheers, Jim Des Lauriers
James Des Lauriers, thank you for the answer, but I didn't catch what means "K". Is it the carrying capacity of the ecosystem? There is no density-dependent mortality in young eagles, so I'd suggest, the answer is YES, the more eaglets fledge, the more or them would achieve maturity (over six years). However, there is another population regulation mechanism, Moffat's equilibrium (see Hunt, 1998). It operates through productivity (floater to breeder ratio).
Michael; Yes K = carrying capacity. The number of variables is growing rapidly! Your second expression seems to be most useful if bear predation is the principal source of loss after fledging. However I get stuck thinking about what else limits the eagle population.
The floater/breeder ratio expresses something about the number of acceptable nest sites available. If the local population is limited by nest site availability and there are floaters present, does that mean that the pop. is at K? If so, then bear predation does not influence eagle population density. In your last paragraph you imply that the bear losses occur before fledging. Is that correct? Jim Des Lauriers
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