You can compare a “butterfly just doing its thing” to a human though you are right in that don’t need anywhere near as much resource even when factoring in size and lifespan. Even still it follows the same idea, take the plants around it for example. You could argue for many insects and animals this is a limited renewable resource (property albeit unclaimed). The butterfly must work to acquire the nectar. If another animal comes and eats those plants the butterfly must now work harder to acquire the same amount of resources.