Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

Abstract

Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. It is well documented that, among migratory populations, numbers of overwintering monarchs have been falling across several decades, but trends among breeding monarchs are less clear. Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association's annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. Negative impacts of agricultural glyphosate use appeared to be counterbalanced by positive effects of annual temperature, particularly in the US Midwest. Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction. We suggest that density-dependent reproductive compensation when lower numbers arrive each spring is currently able to maintain relatively stable breeding monarch numbers. However, we caution against complacency since accelerating climate change may bring growing threats. In addition, increases of summer monarchs in some regions, especially in California and in the south, may reflect replacement of migratory with resident populations. Nonetheless, it is perhaps reassuring that ubiquitous downward trends in summer monarch abundance are not evident.

Do demographic processes change at extremely low population size in western monarch butterflies?

“…It is not uncommon for positive density dependence in one life cycle stage to be counterbalanced by demographic or behavioral changes at other life cycle stages (Levitan 1991, Shuai et al 2020, Stephens et al 1999). Some past studies of eastern monarch butterflies have suggested an opposite pattern of demographic compensation to our results for western monarchs: faster declines in winter than summer in the east (Inamine et al 2016, Crossley et al 2022), and faster recovery of winter population counts than breeding range size in our data. The difference in findings may arise in part because the summer counts used in Inamine et al (2016) and Crossley et al (2022) represent only a portion of the eastern monarch breeding range, and are likely biased toward higher-quality habitat (Pleasants et al 2017).…”

“…It is not uncommon for positive density dependence in one life cycle stage to be counterbalanced by demographic or behavioral changes at other life cycle stages (Levitan 1991, Shuai et al 2020, Stephens et al 1999. Some past studies of eastern monarch butterflies have suggested an opposite pattern of demographic compensation to our results for western monarchs: faster declines in winter than summer in the east (Inamine et al 2016, Crossley et al 2022, and faster recovery of winter population counts than breeding range size in our data.…”

“…The difference in findings may arise in part because the summer counts used in Inamine et al (2016) and Crossley et al (2022) represent only a portion of the eastern monarch breeding range, and are likely biased toward higher-quality habitat (Pleasants et al 2017). Additionally, Crossley et al (2022) compared trends in winter monarch counts for the eastern population to trends in summer monarch butterflies for all of North America, including western and non-migratory populations, so divergent trends might reflect different dynamics of these groups.…”

Extensive regional variation in the phenology of insects and their response to temperature across North America

“…We still know relatively little about how climate change affects the abundance of insects in different populations across entire continents (but see; Crossley et al, 2021Crossley et al, , 2022Outhwaite et al, 2022;Soroye et al, 2020;Warren et al, 2018). To date, most large-scale insect surveys have been designed primarily to examine diversity rather than biomass or abundance (e.g., Steinke et al, 2017), or have focused on specific taxa, such as bumblebees (Kerr et al, 2015;Soroye et al, 2020;Weaver & Mallinger, 2022) or butterflies (e.g., Crone et al, 2019;Forister et al, 2021).…”

Identifying hypotheses for drivers of decline of the bogong moth (Agrotis infusa)

“…Macgregor et al 2019;Thomas et al 2019). Indeed, a recent large study of trends in breeding North American monarch butterflies found no evidence for a ubiquitous decline (Crossley et al 2022).…”