Rates of range expansion increase with population growth rate and with movement speed. Monarch butterflies are an interesting system for studying range expansion: populations expand every year from narrow overwintering ranges to fill extensive breeding grounds over multiple generations. Western monarch populations overwinter on the California coast and fill breeding grounds west of the Rocky Mountains. We test whether changes in abundance of western monarchs are associated with changes in the rate of range expansion. Overwintering counts were millions of individuals in the 1900s, 200,000 to 300,000 in the 2000s, then crashed to ~28,000 in the spring of 2018, with very low abundance apparent on the breeding grounds in 2018 and 2019. This change in population size could lead to a reduction in population growth rates (Allee effects), but could also reduce movement rates if dispersal is density dependent. We used more than 100 years of citizen science data to compare summer range expansion of monarchs between time periods, using a novel approach that combines quantile regression and nonlinear models. Comparing historic (20th century) and recent (2000-2017) data could reveal potential factors associated with long-term decline. Comparing recent and post-crash (2018-2019) data could reveal Allee effects or density-dependent dispersal behavior.
Results/Conclusions
Monarch range expansion in the 21st century was similar to those in 20th . Maximum distance from overwintering sites of the 90th quantile observations was ~1200 km, and occurred on XXX July. Range expansion was slower in 2018 and 2019 than either previous period. Maximum distance was ~800 km and occurred on YY July. The day of peak expansion (when Western monarchs are estimated to be furthers from their overwintering sites) has been steadily advancing through time; this could be a consequence of changes in abundance, but could also reflect a response to a warming climate. Our results suggest that expansion rates are one of the key population processes that have changed as this populations declined. The breeding population did not decline dramatically from 2018 to 2019, suggesting that reduced movement is a more likely explanation for changes in range expansion than lower population growth rates. Monarchs may mitigate Allee effects through slower movement in small populations. This project suggests an underappreciated change in how western monarch butterflies use the landscape. It also demonstrates the power of inference from citizen science data, when combined with suitable statistical models.