For butterflies, the food source is nectar from flowers. I don't know that flower populations are in decline.

The milk weed dependency is because the butterflies will only lay their eggs on milk weed plants. The caterpillars will only eat milk weed. They tolerate the toxicity, which in turn makes them unpalatable to birds that would otherwise eat them.

callmeed's post gives excellent source material [https://news.ycombinator.com/item?id=7866154]