Phenology and Abundance of Migratory and Breeding Populations of Monarch Butterflies in the Pacific Northwest 2012–2024

TL;DR

Monarch butterfly populations in the Pacific Northwest show cyclical changes linked to overwintering numbers in California, with community scientists providing valuable real-time data for conservation.

Contribution

Demonstrates the utility of community science data in tracking monarch population dynamics and detecting sudden declines in real time.

Findings

Monarch populations in the PNW fluctuate on a 2 to 4-year cycle linked to overwintering numbers in California.

A late-summer population collapse in 2024 was detected through community scientist sightings, possibly due to a July heatwave.

Community science platforms and personal observations provide comparable and valuable data for monitoring monarchs.

Abstract

Reduced numbers of overwintering monarch butterflies in California indicate a decline in the species in western North America, but little attention has been given to the status of warm-season populations in the west. Using sighting data contributed largely by community scientists, we evaluated the phenology and abundance of monarchs during spring, summer, and fall in the Pacific Northwest (PNW) over 13 years. Periods of higher and lower abundance corresponded with overwintering numbers, confirming population linkages between the PNW and California. Sighting data showed a late-summer collapse of monarch populations in 2024, highlighting the value of real-time online reporting by community scientists. Effective strategies for the conservation of monarchs in western North America will benefit from high-value, current information on population dynamics, which we show can be provided by community science data.

The decline of the monarch butterfly population in western North America has been documented by 28 years of community scientist counts from overwintering sites in California. However, no long-term studies on the status and dynamics of monarch populations during spring to fall are available. Using multiple datasets from online recording sites, social media, and personal observations, our study over 13 years shows increases and declines in warm-season populations in the Pacific Northwest (PNW) on a 2 to 4-year cycle that corresponds with fluctuations in overwintering populations. Overall, there was little evidence of a systemic decline in the abundance of monarchs in the PNW. Monarchs were reported in all PNW states and territories in all years, with the earliest annual first appearance in Oregon and the latest in British Columbia and a maximum seasonal range of 200 km north of the US/Canada border. Relatively equal numbers of unique sightings originated from each of three community scientist online recording platforms, with a larger number of unique sightings from personal observations/communications and multiple social media sites. The value of community scientist sightings for the rapid detection of substantial changes in populations was highlighted in August 2024 when weekly sighting data showed a substantial decline in PNW populations that may have been caused by poor survival of immature stages during a July heatwave. The conservation of monarchs in western North America will benefit from contemporary and high-value data on phenology and population dynamics, which we show can be provided by community science observations.

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