I have very fond memories of the occasional Monarch I would see growing up (and the lengths I would go to in order to ensure they were not the similarly colored Viceroys). Equally sad is the dramatic decline of songbirds and bird biodiversity in general (though I don't know if it's due to similar destruction of breeding grounds).

Monarchs are threatened during migration. It’s about how many individuals survive the round trip to breed more.

Drought and fire are two natural factors that destroy their migration routes’ food supply. Another is agriculture.

Folks like Monarch Watch and Xerces encourage planting of the few milkwood species (ie weeds) that the butterflies depend on for energy or egg-laying.

Restoring prairie is also effective. But there’s still drought and wildfire.

I disapprove of eminent domain but this is a great steelman case for it.