I have a background in ecology and am interested in animal welfare, including wild animal welfare. I'm also interested in the question of which animals are sentient.
Suffering-focused on pragmatic grounds.
2. I'm not sure I understand your last point. Even if a small change pushes the barely tolerable to intolerable you still have the opposite effect on the other side of the curve, where it provides relief. I am assuming here that in most polygenic traits the curve is not so narrow that there aren't dysfunctional individuals being produced on both extremes.
3. I concede your point that if there is less than perfect competition, this effect doesn't completely negate any effect on average welfare. It would still make such an effect smaller and less relevant as compared to other considerations, like species composition.
I regret not having numbered my points above :P
A change in either direction will put the average away from what the individuals are best adapted to on average and under a symmetric assumption (like approximate normality), I'd expect it to hurt more individuals than it helps.
Suppose a human in a t-shirt and jeans, going for a walk. Preferred temperature: 24 °C. The weather turns and the temperature drops to 14 °C. Is this person half as physiologically stressed and half as miserable as they would have been if the temperature had dropped to 4 °C? I don't think so. The response is not linear. In my original example, most individuals would not end up too far from their preferred temperature. Wouldn't welfare gains and losses be mostly driven by the extremes?
they're just part of the environment and conditions under consideration
They could be, depending on the species, moral patients.
[edited to add numbering]