timelets

http://science.sciencemag.org/content/360/6395/1294.full

For example, the abundance of monarchs was one of the highest in decades in the Northeast during the 2017 breeding season, but in the subsequent winter, the population was well below historic levels in Mexico.
...
Recent evidence points to a decline beginning >45 years ago in another U.S. population of monarchs, which migrates within California (14). Such long-term and broadscale negative population trends suggest continent-wide changes that transcend single explanations such as herbicide-tolerant crops.

Modeling efforts are helping to elucidate the causes of the shrinking monarch populations (13), but as is the case for most declining species, multiple stressors likely conspire.