Bay Checkerspot Butterfly
Edgewood County Park and Natural Preserve is renowned for its spring wildflower displays and biodiversity, but recently the buzz has been about what’s fluttering above the flowers, and munching below. Edgewood’s famous butterfly is coming home.
The Good Ol’ Days
For years and years, Bay checkerspot butterflies enjoyed a comfortable, if short, life throughout thousands of acres in the Bay area. Although fairly picky about their environment, these butterflies thrived in serpentine grasslands. Serpentine, our State rock, breaks down into soil poor in the nutrients that all plants need. But native plants have adapted to these austere conditions, and, in turn, the checkerspot has evolved to depend on a handful of these native plants.
In March and April, female butterflies lay clusters of eggs on California plantain.
Tiny black caterpillars hatch ten days later. The caterpillars feast on the plantain and grow quickly. Some caterpillars move to other food plants, including purple owl’s clover. As the plants die off in the summer heat, the caterpillars stop eating and enter diapause, a resting state.
When plantain germinates after fall rains, the caterpillars resume eating. Once they are large enough, the caterpillars form pupae in early spring. A few weeks later, adult butterflies emerge. In the mere ten days they live as adults, the checkerspots fly about the serpentine grasslands, sipping nectar from native wildflowers. They mate, lay eggs, and die, completing their year-long life cycle.
An Unexpected Turn
During the late 1990s and early 2000s, the checkerspot’s population at Edgewood began to drop precipitously. Conservation biologist Stuart Weiss had been studying the insect for years, and in 2002, Dr. Weiss counted the last caterpillar at Edgewood. Although large numbers of the butterfly could still be found farther south in Santa Clara County, the butterfly had gone locally extinct. What could have caused this population crash?
By 2003, the Bay checkerspot butterfly could no longer be found at Edgewood Natural Preserve. Elsewhere on the peninsula, local populations had gone extinct as well. Scientists discovered a common thread: urban smog and automobile emissions.
This air pollution is a slow-release nitrogen fertilizer: it enriches nitrogen-poor serpentine soils, allowing European grasses to crowd out the native plants. The prime suspect at Edgewood: Highway 280.
Each day, more than 100,000 vehicles speed past, spewing nitrogen oxides (NOx). Catalytic converters introduced in the 1990s inadvertently introduced a more potent fertilizer, ammonia (NH3) — worsening the problem.
The butterfly’s favorite habitat — serpentine grasslands — had been converted to fields of non-native annual grasses, thanks to the enrichment of the soil. These grasses were crowding out the native plants that the butterflies and caterpillars required. Italian ryegrass seemed to be one of the worst offenders.
Scientists confirmed that the closer you got to the freeway, where Edgewood’s nitrogen pollution was measured at the highest levels, the thicker the ryegrass. The farther from the freeway, the more Bay checkerspot host and nectar plants.
Preparing for Restoration
Dr. Weiss realized that there was a good chance that the Bay checkerspot could be restored, if its habitat could be returned to something close to its original state. Thanks to generous grants from PG&E, Weiss began mowing plots of affected grasslands in 2005 where he had counted large butterfly populations in years past. He found that the native plants — tidy tips, goldfields, California plantain, and others — staged comebacks when these competitive invaders were removed.
In February and March, 2007, Dr. Weiss and his assistants collected hundreds of caterpillars from Coyote Ridge in Santa Clara County, where they still can be found in large numbers. This was done with special permission from the U.S. Fish and Wildlife Service, which protects this federally threatened animal.
|Collecting caterpillars from Coyote Ridge, February 2007
Then, with great care, the scientists transported the caterpillars to Edgewood and placed them in the mowed areas, where the California plantain had already begun to emerge.
|Placing caterpillars in new home at Edgewood, February 2007
After pupating, the caterpillars began emerging as adult butterflies.
2008 brought disappointing news. Only one caterpillar was counted. In 2009 there were none.
What could have accounted for this? 2007 was the fourth driest spring since 1895! In all probability the host plants dried up before the new caterpillars were large enough to enter diapause, the dormant stage in which they spend the hot, dry summer.
The biologists re-thought their approach, and came up with an improved plan.
After the disappointing results from the 2007 re-introduction, the biologists knew they had to find a way to mitigate nature’s unpredictable weather. Plan B consisted of acquiring permission from the US Fish and Wildlife Service for a 5-year permit to transfer about 4,000 caterpillars from Coyote Ridge, where the population is booming.
This phased introduction will further increase chances of hitting a good weather year that is conducive to population establishment. Mowing and dethatching continue to take place rotationally throughout the butterfly habitat, creating dense fields of dwarf plantain, gold fields, tidy tips, and other nectar sources.
So in February 2011, about 4,000 butterflies were collected and moved to their new home at Edgewood.
The larger number of caterpillars will increase the critical density of adult butterflies. Butterflies judge habitat quality by the presence of others; they need a critical mass to settle down and develop a sedentary tendency. Introducing too few butterflies, as may have been the case in 2007, may increase the likelihood that they will fly away looking for other individuals.
Unfortunately, the checkerspot butterfly is just one of many victims of nitrogen pollution. Nitrogen is saturating ecosystems worldwide at an ever-increasing rate, largely due to use of fossil fuels. Effects include grasses invading California deserts and coastal scrublands, dwindling floral diversity in European grasslands, and ocean dead zones. Scientists say the threat to biological diversity is comparable to that from global climate change.
Restoration work at Edgewood does more than recreate a home for the threatened butterfly. Brighter spring wildflower displays offer evidence that thriving native grasslands also encourage other native species. This work also demonstrates the feasibility of preserving habitats by dynamically correcting for environmental change.
How Can you Help?
Scientists are using existing technologies to help decrease our collective nitrogen emissions, but we each have a role to play:
- Reduce your emissions: Drive less, and slow down. Ammonia and NOx emissions increase exponentially with speed–so driving the speed limit makes environmental sense.
- Support local organizations working to protect these natural resources, including Friends of Edgewood and the San Mateo County Parks and Recreation Foundation (supportparks.org)
- Stand strong for environmental protections, including the Endangered Species Act.
- Support efforts to regulate nitrogen pollution.