[Pollinator] Early monitoring in California reveals a low overwintering population

[Isis Howard]

The annual Western Monarch Count <https://westernmonarchcount.org/> (WMC),
led by the Xerces Society and count co-founder Mia Monroe, is underway with
the mid-season count beginning tomorrow, Saturday, November 16. The
inaugural early-season count at western monarch overwintering sites
<https://westernmonarchcount.org/map-of-overwintering-sites/> concluded
earlier this month (Oct. 19 - Nov. 3), and suggests that this year’s
overwintering western monarch population is very low.

While these are still initial tallies, approximately 7,000 monarchs have
been observed across the entire range. Numbers should continue to rise as
the season progresses, but early monitoring foretells a story similar to
2018 and 2019 when less than 30,000 monarchs were recorded in the West. For
comparison, over 230,000 monarchs were observed during last year’s
mid-season count. This year, we expect a significantly reduced mid-season
count that will likely fall somewhere between the 2018/19 tallies and the
record low of less than 2,000 monarchs in 2020.

Overwintering site results

Santa Cruz County is home to two significant monarch overwintering sites
this year: Natural Bridges State Park had the largest individual
early-season count with 2,152 monarchs reported on October 29; Lighthouse
Field State Park had the second largest count with 1,078 monarchs reported
on November 1. So far, these are the only sites that we know of hosting
more than 1,000 monarchs this season.

Pismo State Beach monarch butterfly grove reported 354 monarchs on October
30, rising to 406 monarchs as of November 12. Pacific Grove’s monarch
sanctuary had 18 monarchs reported on November 1, declining to 9 monarchs
as of November 7. A sensitive site owned by The Nature Conservancy, is at
19 monarchs – a stark difference from the past several years when it hosted
tens of thousands of monarchs during the overwintering season.

Data for past seasons are available online at westernmonarchcount.org/data,
and it will be updated with 2024-25 data later this winter once data review
processes are complete.

Possible causes of this year’s decline

Why the sudden decrease from last year? Spring and early summer monarch
numbers looked relatively normal across the western breeding range.
However, that was followed by record-breaking heat and drought across the
West from mid-summer through early fall. The number of “where are the
monarchs?” inquiries from the public and fellow researchers came pouring
in. July was California’s hottest month ever
<https://www.latimes.com/california/story/2024-08-08/record-heat-in-july>
and the majority of the western breeding range remains in some degree of
drought <https://droughtmonitor.unl.edu/>, conditions attributed in part to
climate change. An October article
<https://insideclimatenews.org/news/09102024/western-monarchs-record-breaking-heat-waves/>
about monarchs and heat from Inside Climate News cited a 2021 study
<https://www.science.org/doi/10.1126/science.abe5585>published in the
journal Science which identified climate change, particularly warmer falls
over several decades, as a major factor leading to declines of 250
butterfly species across the West.

We know that monarch butterfly populations, like other insects, are
naturally "bouncy" <https://www.xerces.org/blog/bounciness-of-butterflies>
in that they fluctuate from year to year in response to the temperature,
rainfall, the availability of food, and other factors like predation and
disease. However, the migratory western monarch population has undergone a
sustained and significant decline – estimated at over 95% since the 1980s –
far beyond any year-to-year fluctuations. While there are still yearly ups
and downs, western monarch populations are now bouncing around in uncharted
territory.

We witnessed migratory monarchs rebound from severely low overwintering
numbers before, and a similar uptick in future years may be possible but is
not guaranteed. Once again, we call upon all butterfly lovers,
conservationists, land stewards, policy makers and educators to help us
spread the word of western monarch declines and encourage immediate action
to aid this beloved butterfly and its habitat
<https://xerces.org/western-monarch-call-to-action>.

-- 

Isis Howard (she/her)
Conservation Biologist
The Xerces Society for Invertebrate Conservation
503.212.0546 | isis.howard at xerces.org

xerces.org <https://www.xerces.org/>
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