Good points. If I understand you correctly, these sorts of benefits from population size (independent of the amount of resources/habitat area available) would be essentially the converse of density-independent mortality factors. I've tried to use the term "density" quite broadly here so that effects like these could be accounted for in a single density-dependent welfare curve.
Hi Flemming, you're right that high survivorship would generally entail a long life expectancy. Sorry, this summary didn't adequately explain how "RWE" is to be calculated. In the RWE calculation, welfare expectancy is normalized around the average annual welfare across all the ages within an individual's maximum lifespan (i.e. the lifespan they might live if all their needs were met and they died of old age), so the average age-specific welfare == 1. This normalized welfare expectancy is then divided by the life expectancy, which always values every year of life as 1. This controls for differences in life expectancy, so in the pre-print linked above, species with life expectancies as different as 1 year and 40 years come out with RWE values pretty near 1 on either side.
RWE is intended to show whether the ages which most individuals live through are especially good or especially bad ones. For example, as kcudding pointed out in an earlier comment, some herbivorous insects seem like they may have higher welfare as juveniles than as adults. This would lead to RWE > 1. For many species, though, the juvenile period involves very high mortality, so most individuals only survive to experience desperate times. They would probably end up with RWE < 1. RWE always tends towards 1 as life expectancy increases towards the maximum lifespan, including in humans (which I can say with confidence since we have actual data on age-specific psychological wellbeing for humans!), which emphasises that it is about identifying a gap between welfare expectancy and life expectancy, not a welfare metric in itself.
I completely agree that the downstream effects on other species should be accounted for. The true ideal would be to manage a population in such a way that maximizes the total welfare expectancy of all sentient life! I hope we'll eventually have the information necessary to do so. In the near term, I'm advocating for populations that are currently managed in such a way as to maximize their net reproductive rate (i.e. 'fecundity expectancy') to instead be managed around total welfare expectancy.
Given the relevant information, I think the welfare expectancy approach makes sense of the example of an herbivorous insect where the juvenile stage offers the highest welfare, but where locking them into that stage would lead to rapid extinction.
(edited to correct numbers:)
For example, assume that the average survival rate and welfare during the juvenile stage is 0.9 and during the adult stage is 0.2; the juvenile stage lasts ~1.8 months while adults survive a further ~0.7 months; and adults produce an average of 5 offspring per month. Assume we could determine a proportion of the larvae to develop into adults while the rest remain juveniles, and that this wouldn't affect any survival/welfare/fecundity rates. The lifetime welfare expectancy of juvenile-locked individuals would be ~8.08 and of "normal" individuals ~1.56. "Normal" individuals would produce an average of 4.86 offspring over their lifetime. A higher proportion of individuals being limited to the juvenile stage increases average welfare expectancy per individual, but curtails the population size. A lower proportion of individuals being limited to the juvenile stage means average welfare expectancy is lower, but the number of individuals experiencing it is higher.
Plugging in the numbers above, I find the total welfare expectancy is maximized when 38% of individuals are limited to the higher-welfare (but non-reproductive) juvenile stage. Of course, the assumption that such an intervention wouldn't affect age-specific survival/welfare rates seems implausible, and this still isn't accounting for potential effects on other species. However, I hope this illustrates how the usefulness of this approach to wild animal welfare might scale with better ecological understanding!
I agree with your observation about scale. It's interesting to think about where the idea of parents having obligations to their children - or of individuals having a special obligation to their community members/fellow citizens - comes from. I think these might come partially from a notion of neglectedness. My child is not more important, morally, than any other, but I can assume most other children already have parents looking out for them, so my child is counterfactually the most neglected cause (and the most tractable cause among children I could care for).