“The Big March of Animals” applies its effect at resolution of the card and does not recalculate during the turn. Its effect lasts as long as the affected monsters remain face-up on the field, or until the End Phase.
“The Big March of Animals” does not affect monsters Summoned after it resolves.
“The Big March of Animals” does not count or affect face-down monsters.
This is not an effect that targets. It is applied to all face-up Beast-Type monsters on your side of the field.