An excellent new study finds that climate, habitat loss (in both Mexico and the USA), disease, & insecticides contribute to decline of monarch butterflies. Although one could quibble with the emphasis placed on discussion of various issues, what I appreciate is the quantitative nature and comprehensiveness of the study, and the attempt to include as many factors as possible. Perhaps what is most interesting to me is that it is unclear why some explanatory factors (for example, disease) that have direct impacts on monarchs, are increasing in their severity. Disease incidence appears to have increased 10-fold (from 1% to 10%) over the past 20 years. And it is too bad that we don’t yet have a way to incorporate two factors into such population models: nectar availability during the southern migration and predation during the breeding season, especially by insect predators and parasitoids. We know that nectar is the key for monarchs to fatten up for overwinter survival and that often >90% of monarch caterpillars perish due to insect predators and parasitoids during the breeding season.
My favorite lines from the paper (admittedly taken out of context, but I agree with these statements):
“Ecological processes are often extremely complex and, as a result, it can be difficult to discern putative cause-and-effect relations when there are many more predictor variables than there are samples.“
“Glyphosate application (irrespective of whether measured as cumulative or annual) obviated the need for previous year’s abundance, derogating the role density dependence plays at the scale of our study. Additional data may increase the differentiability of the other collinear variables in the future.”
“Counterintuitively, dicamba and 2,4-D application in the northern regions was positively associated with population size (both herbicides declined in use in the North at the time that monarchs were also in decline); however, in the South, the use of these herbicides was negatively associated with population size (both herbicides increased in use in the South over the period of study).”
“Further declines in population size because of glyphosate application are not expected. Thus, if remaining threats to habitat are mitigated we expect climate-induced stochastic variation of the eastern migratory population of monarch butterfly around a relatively stationary population size.”