Assessment of Western Monarch Butterfly Winter Population Completed

The Xerces Society for Invertebrate Conservation, Portland, OR

More monarchs reported in Northern California this year, but monarchs are not yet recovered

PORTLAND, Ore., February 4, 2016 — Results from a survey of monarch butterfly overwintering sites in California show that there are more monarchs overwintering in the state this year than last. Volunteers with the Xerces Society’s Western Monarch Thanksgiving Count visited 187 sites and tallied a total of 271,924 monarchs. Although more monarchs were counted, the average number of monarchs per site is not significantly different than last year’s count, and this year’s population estimate represents a 39% decline from the long term average. The number of monarchs counted this year is but a fraction of the 1.2 million monarchs recorded in the late 90s.

Monarchs from as far away as Idaho and Arizona converge on tree groves along the California coast to spend the winter. Because so many monarchs from the western United States are clustered together on the coast, the Xerces Society’s Western Monarch Thanksgiving Count provides one measurement of the health of western monarch populations.

“This extraordinary volunteer effort has recorded more monarchs in California this year than last year,” said Sarina Jepsen, endangered species program director. “However, monarch butterfly populations are far from recovered. We will still need a focused effort to address the many threats that monarchs face—from pesticide use and habitat loss to climate change and disease.”

There are positive indications from this year’s count results. Fifteen sites that have been continuously monitored had the highest numbers of butterflies in a decade. Several sites that had not seen monarchs for years were occupied, and there were a number of sites, such as the Berkeley Aquatic Park, that hosted overwintering monarchs for the first time. In Marin County in the northern extent of the overwintering range, 11 sites had increased numbers and two new sites each supported more than 8,000 butterflies.

Less positive is that in southern California, the majority of the sites surveyed had fewer monarchs than last year.

The Xerces Society’s Western Monarch Thanksgiving Count began in 1997 and is the longest running effort to monitor overwintering monarchs in California. The count happens during a three-week period centered on Thanksgiving. Biologists, land managers, and citizen scientists visit overwintering sites year after year to monitor the butterflies. This year, more than 100 volunteers participated.

“This count would not be possible without the amazing work from so many volunteers,” said Emma Pelton, count coordinator for the Xerces Society. “We’re so grateful for their effort—in particular from Mia Monroe, who spends countless hours organizing and training others.”

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