The estimates of the monarch butterfly overwintering population were announced Thursday February 9th by WWF Mexico. The butterflies are so dense at their dozen or so mountain-top clustering sites that overwintering butterflies cannot be counted. Instead, the area of forest that is densely coated with butterflies (at about 5,000 butterflies per square meter looking up into the canopy) is estimated as a measure of monarch abundance. Butterflies arrive to Mexico around the day of dead in November and stay until March each year.
This winter season (2016-2017), there were approximately 2.9 hectares of forest occupied with dense monarchs (somewhere in the neighborhood of 145,000 million overwintering butterflies). This estimate is down some 27% compared to last year. Nonetheless, the previous two years were a 600% increase over the all-time low populations (2012-2014).
Where does this leave us? The good news is that this year’s population was higher than predicted by many. The season started with a late spring storm that killed an estimated 5-10% of monarchs in March 2016, and many reported low numbers of adults last summer. Nonetheless, the lower numbers this season compared to last are within the range of year-to-year variation, and overall, the population seems to be relatively stable over the past decade. With these 24 years of data, there are various ways to plot and assess the data. Below I have plotted the four year averages for six periods beginning in 1992. Any way you slice it, the trend has been negative, and the population is not nearly what it was. Nonetheless, the downward trend seems to have lessened this last period. Is this the new norm? How dangerously low are these numbers? And what can we do to reverse the trend?