Right now, when a species has low resolve, they all behave the same : they leave.
The idea would be to have a different behaviour for each one.
Humans would stay as now and just leave.
Beavers would start eating trees (without producing any wood in the process) quickly opening glades (glades opened that way wouldn't provide any discovery bonus, but would still raise Hostility).
Lizards would burn buildings, prioritizing buildings providing needs to other species.
Harpies would attack other settlers, prioritizing other species. Injured settlers would be incapacitated for X amount of time (could be reduced by building an hospital, with the whole wounded mechanics used by other parts of the game) and their resolve would be lowered (except is they attack Harpies, because they like to fight).
For each species, doing the action, would raise resolve, to avoid falling into a downward spiral.