Why didn't the animals die?

I was hoping someone could clear a question I've had in my mind up for me. When Ben released the gas, why didn't the animals on the island die too? Birds can migrate, but what about the boar? I don't think anyone brought more boar on the island after Ben released the gas. The polar bears shouldn't be living either. They should have died along with the gas too. I'm probably missing something, so please explain to me what did or could have happened.

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