Normally I would take stackable as meaning cumulative, but in this case (when compared to Studious) I'm not so sure.
If I have Fluttershy, Beastmaster and Winona, on the Scent at a problem
Fluttershy's only gains +1 power for her caretaker ability. (Total power being +3, Fluttershy, +1, Winona, +1 Caretaker bonus = 4)
If I add Hummingway, Fine Feathered Friend,
Fluttershy's still only gains +1 power for her caretaker ability. No matter how many Critter Friends she's accompanied by her Caretaker ability will only ever give her a +1 bonus. (Total power being +3, Fluttershy, +1, Winona, +1, Hummingway +1, Caretaker bonus = 6)
However, if we add other characters with the caretaker ability
like Mane Cureall, Veteran Vet and Lemon Hearts, Sweetheart, (both of which have Caretaker), each character with Caretaker gains the +1 bonus. (+3, Fluttershy, +1, Winona, +1 Mane Cureall, +1 Lemon Hearts, +3 Caretaker bonuses = 9).
Is or isn't this defined as "stackable"?
Although I have no problem with this (it makes multiple Caretakers powerful and dangerous though!), it - again - poses some questions and concerns for the Studious keyword ability.