When you're talking about the utility of squirrels, what exactly are you calculating? How much you personally value squirrels? How do you measure that? If it is just a thought experiment ("I would pay $1 per squirrel to prevent their deaths") how do you know that you aren't just lying to yourself & if it really came down to it, you wouldn't pay? Maybe we can only really calculate utility after the fact by looking at what people do rather than what they say.