The decline and recovery in western monarch numbers from 2020-2022

In a previous post to this Blog (The Western monarch puzzle: the decline and increase in monarch numbers), I presented a long explanation for why the number of monarchs counted at the overwintering sites in California numbered a mere 1849 in 2000 yet gave rise to an overwintering population of 247K in 2021. I offered two related explanations for this outcome. First, that the extremely hot conditions in September – November 2000 along the California coast prevented monarchs from reaching the typical overwintering locations or staying non-reproductive if they did so. Rather, I suggested that many of the monarchs in that fall migration overwintered in small, scattered clusters at elevations of 1500-3000’ in the low ranging hills inland from the coast. Second, that a sufficient number of the overwintering monarchs, and perhaps some breeding butterflies from the coast, produced a large first generation in the spring of 2021 that dispersed to colonize the inner-mountain west to the east of California and to a lesser extent the states to the Northwest. Thereafter, the 2-3 generations that followed culminated in a fall migratory population of 300k (or more) that resulted in a Thanksgiving count of over 247K monarchs.

My calculations suggested that such an increase would only be possible if the breeding population that gave rise to the first generation consisted of at least 10,000 females. While this interpretation seemed reasonable, it lacked support until now. Recently, when going through iNaturalist records to answer a particular question, I decided to see what those records said about what happened during the last few breeding seasons in California – BINGO! The iNaturalist records are consistent with the expectation that the population in the fall of 2020 was much larger than the 1849 tallied during the Thanksgiving counts that year. Further, they are consistent with the hypothesis that there was a robust breeding population in the spring of 2021. In fact, the numbers in 2021 (Tables 1,2) are similar to those of 2022 that resulted in an even larger migratory and overwintering population. These records also suggest that the number of monarchs overwintering along the coast in the fall of 2023 are likely to be lower (Table 2).

In the records cited in Table 1, the March-15June records represent the beginning of the spring breeding season which effectively stops with the end of directional flight (migration) by first generation monarchs in mid-June. The total for all records ends in mid-September. This interval covers all breeding and the first 3 weeks of the migration but stops before migratory monarchs usually reach the coastal areas.

The number of iNaturalist records for the Central Valley in 2022 contrasts strongly with other years as can be seen in the maps below. The early movement into the Central Valley in that year was evidently due to high temperatures in March. The deviations from the long-term means in March were as follows: 2020 -1.2F; 2021 -1.6F; 2022 +2.3F; 2023 -4.5F. These early movements into the Central Valley and the foothills to the east appear to account, in part, for the large Thanksgiving count in 2022 – the largest count since 2000.

Table 1. iNaturalist records for the spring and summer breeding seasons for monarchs in California (2017-2023). The low percentage for March-June for 2019 may have been due to the low temperatures in February. The mean temperatures that month along the coast (45-46F) were the coldest since 1966. Higher percentages, as in 2018 and 2020, may have been due to a combination of more favorable temperatures. The records for early season and full season intervals from 2020-2022 are quite similar given that community science records can be influenced by weather and other factors. The counts for 2020 indicate that the migratory population that year was quite substantial.

Table 2. iNaturalist records for March-June 2020-2022. These records suggest that the populations developed in a similar manner from month to month in 2020-2022. The population build up started more slowly in 2023 and the lower number of records for 2023 thus far indicate that overwintering numbers are likely to be lower this coming winter season.

Reference
Taylor, Chip, 2023. The Western monarch puzzle: the decline and increase in monarch numbers. Monarch Watch Blog, monarchwatch.org/blog, May 2023.

iNaturalist maps for monarch records for the years and intervals indicated.