Springjack Pasture's last ability is a mana ability. The whole ability — including the life gain — doesn't use the stack and therefore can't be responded to.
01-08-2008
wotc
You can sacrifice any Goats you control to activate the third ability, not just Goats created by Springjack Pasture.
01-08-2008
wotc
If you sacrifice no Goats, you'll produce no mana and you'll gain no life.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.