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 totally get where you are going with this.… but do want to offer the following observation regarding the context dependence of life-stage associated welfare. Many of these species have tight seasonal links for development that in turn are related to factors like plant phenology. So.… just to make it more specific, for the things like juvenile leaf miners (ample food, fairly but not completed protected from predators and parasitoids,) you might “lock” individuals into a declines in both food quality and quantity, and increases in predation and parasitism by preventing them from emerging from the leaf as the the season advances. Thus decreasing the welfare of those “locked” individuals.