Natural habitat seeing a return around Sign Hill in South San Francisco

The effort was meant to improve conditions specifically for Mission blue and callippe silverspot butterflies, both endangered species whose habitat only spans from the Marin Headlands to the Peninsula. Since their decline, they only live in about 10 known sites within that area, including Sign Hill and San Bruno Mountain, said Parks Manager Joshua Richardson.  

But an unexpected fire in 2020 caused both challenges as well as unanticipated opportunities. While the conservation efforts required a shift in focus from strictly butterfly tracking to tree removal as a result of fire damage, it also opened up the possibility of restoring the area to what it used to be before tens of thousands of trees were planted on Sign Hill over the course of three decades starting in the 1960s.

Though the effort was well-intentioned, the city’s Natural Resources Specialist Candace LaCroix said the tree-planting trend that took hold of many environmentally-focused movements during that time produced unintended consequences for grasslands such as Sign Hill, whose wildlife can be negatively impacted from heavy tree canopies.

“The 2020 fire that happened in October allowed us to step beyond just restoring what habitat exists and start to reclaim habitat that used to exist. The idea is that clearing these trees that burned is now opening up large swaths of grassland,” she said. “The effect of [the trees] was they were taking away habitats for Mission blue butterflies and making the park more high-risk for dangerous wildfires.”

The unnatural abundance of trees meant that many of them, especially eucalyptus, used up large amounts of water, taking away resources from other now-rare plant species, such as coast irises, which is part of the city’s flag and is the reason longtime residents said the hill looked like it was painted purple during the summertime. 

“The eucalyptus and black acacia trees have actually excluded the coast iris completely from the areas where they’re growing and actually have even reached beyond that by affecting the water table across the entire hill,” LaCroix said. She added there have also been more amphibian sightings, such as tree frogs, spotted in the area since the water table is higher, largely a result of increased tree removal.    

Creating a hospitable environment for rare plant species also goes hand in hand with protecting  the endangered butterflies. The insects are noted for having a canary-in-the-coal-mine effect when it comes to anticipating the impacts of climate change, and on Sign Hill, the Mission blues and callippe silverspots are no exception. 

“Butterflies are a good species to look at as far as the effect that climate change is going to have on biodiversity, since they are a particularly sensitive group of species,” LaCroix said. “Climate change can cause plants to bloom at different times, so if a butterfly is emerging at a different time than the flower it needs, they can fall out of sync, which can be detrimental, or worse, catastrophic for the species. So it’s really their reliance on the whole plant that adds to their sensitivity.”

Since restoration efforts began, LaCroix estimates hundreds of eucalyptus trees have been removed, and the first recorded callippe silverspot sighting in about 40 years occurred in 2021, with two recorded the following year. LaCroix leads volunteer outings open to the public three to four times a month not only for extra sets of helping hands, but as a way to expose both children and adults to the area’s natural ecosystems and what it means to care for them. 

And to Richardson, the effort of maintaining such an important part of South City’s character is not just a symbolic gesture but a practical one too. 

“My big goal with the Sign Hill Restoration is obviously the habitat restoration but really engaging the community — especially the youth but adults as well — to really learn about Sign Hill because it’s South San Francisco’s only true open space,” he said. “A very small percentage of people know you can actually go on Sign Hill, and an even smaller percentage have even been to Sign Hill. It’s kind of this little gem of an open space that is highly underutilized.”