Adult breeding monarch relative abundance

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 1993-2018 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. Our data also suggest that
increases of summer monarchs in some regions, especially in the west and
far 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.