A few days ago, a viewer watching the planes rehearse for Fleet Week spotted a different kind of aerial phenomenon: Monarch butterflies, fluttering around over Twin Peaks.

“I was driving by Twin Peaks,” they wrote, “and paused to watch the stunt airplanes… and I saw several Monarch butterflies. I had a hard time getting a photo, but here’s a blurry one.” They ended with a plea to photographers armed with something more than an iPhone:  “Someone, please get some better shots?

Will these butterflies overwinter in the eucalyptus forests on Mount Sutro and Mount Davidson? They may very well do so. It’s excellent habitat. We’re reprising below part of an article written after a visit to Natural Bridges State Park, where these insects regularly roost.

This is particularly crucial in view of UCSF’s plans to cut down thousands of trees on Mount Sutro – according to the 2017 Draft Environmental Impact Report, as many as 4,500 trees in the first year alone, and 6,000 in Phase I (the first five years).

Excerpts from

THE PLANTS THAT MONARCH BUTTERFLIES NEED

We’d read that Monarchs might feed from eucalyptus flowers, which provide a winter resource to so much wildlife. That proved to be true. Looking through binoculars, we saw the eucalyptus flowers far overhead were full of butterflies.

We knew, of course, that Monarchs depend on eucalyptus trees along the West Coast. What we also learned was that other so-called “invasive” plants also help Monarch butterflies to survive. The boardwalk trail has interpretive signs with infographics. We found this one, Nourishing Nectar, particularly interesting.

English ivy is one of the key plant species of Sutro Forest – and it’s under attack in both the old and new Plan for Sutro Forest. Since the Monarch butterflies found its nectar a nourishing snack, surely this would also be true of other nectar-feeders. Are UCSF – and the Sutro Stewards who do much of the actual work – aware of the habitat destruction killing this ivy causes?

Cape Ivy provides cover and habitat

We didn’t see English ivy in bloom at Natural Bridges State Park this December. Maybe as a fall-flowering plant, its season was over. But Cape Ivy around the tree trunks had small yellow flowers that offered the Monarch butterflies another snack.

Monarch Butterfly nectaring on Cape Ivy Flower

Cape Ivy is also one of the plants in the understory of Sutro Forest, and it’s being targeted for destruction along with English ivy. Yet its nectar clearly has nutritional value for insects and thus, it has a place in the food chain. And it’s obviously important as habitat and cover for small birds.

THE MONARCH’S WESTERN MIGRATION

We were interested to learn that, unlike the Monarchs east of the Rockies (which migrate from Canada to Mexico and back), the butterflies in the West migrate between the interior and the coast.  The butterflies east of the Rocky Mountains go south to Mexico in winter. The butterflies on the Western side come to the California coast.

The Monarchs in the Western migration depend heavily on non-native, naturalized species of plants. Some 75% of the Monarch roosts are mainly eucalyptus. In fact, eucalyptus, Monterey Pine, and Monterey Cypress (which nativists consider an “non-native” species outside the Monterey Peninsula) account for around 90% of the tree species they use.

But as we saw, they also draw sustenance from Fall and Winter-blooming ‘weeds’ like English ivy and Cape ivy. This can be critical – the Fall generation of these butterflies is the longest-lived, and makes the migration to the Coast and back again that preserves the species. The extra nutrition improves their likelihood of surviving.

Of course, it’s not just naturalized plants. Milkweed is the Monarch’s nursery plant; that’s where it lays its eggs and where its larvae feed. The main threat to the Eastern migration – which has been declining sharply – is that farmers use more pesticides than ever to eradicate weeds, and the butterflies are simply not finding enough of it. There’s a move to grow more milkweed.

Meanwhile, it’s crucial to preserve the Western migration, which seems robust for now. What would happen if the eucalyptus that shelters these butterflies are felled?

[Edited to Add: Apparently it’s NOT robust. See the comments.]

It could happen. We don’t think that anyone would cut down trees that are well-known as Monarch roosts – as in Natural Bridges State Beach. But in some years, the butterflies spill out into other trees, and could potentially establish new overwintering sites. Two years ago, they even came to San Francisco.

NATURALIZED PLANTS – RESILIENT HABITAT

The whole area we visited was a lively habitat of naturalized plants. Eucalyptus. Ivy. Blackberry. Grasses. It was full of wildlife, and not just butterflies. We saw a lot of birds, and heard the rustles and twitters of even more. A Townsend’s warbler made a brief appearance before diving back into cover. And high above, we heard a woodpecker – maybe a Downy – in a eucalyptus tree.

Downy Woodpecker?

Native plant advocates say that native plants provide superior habitat for native species of animal life. There’s no evidence that this is true. Some few species of insects are tied to particular plants, but even then, the rapid reproduction rates of insects suggests that they would evolve to use new plant species once they’re plentiful enough. A great example: How the soapberry bug adapted to use a new food source – in only 100 generations (about 20-50 years).

All the evidence is that naturalized plants provide a resilient and rich habitat for a range of animal species – and with no extra effort from land managers. We only have to avoid destroying them.