By now you have probably heard that the forest area occupied by monarch in Mexico was down from last year, to 2.83 hectares. The area occupied by monarch butterflies represented a 53% decrease from last year.

To put this in historical perspective, we’ve created a graph that provides decade means (with only 7 years included in the first “decade”, due to a lack of earlier data). There are big year to year fluctuations that probably result from year to year weather fluctuations during the breeding, migratory, and wintering phases of the annual cycle. Thus, simply saying that the population decreased 53% from last year is not informative. The long-term average since the colonies were first measured is 5.62 hectares. The mean for the past decade is 2.82 ha, almost exactly what was measured this year. Thus, 2019 was an average year for monarchs over the past decade. But this decade represents a big drop from previous decades—with means of 5.84 ha from 2000-2009, and 9.31 ha for the first 7 years that the colonies were measured.

Figure legend: 1994-2003 data collected by personnel of the Monarch Butterfly Biosphere Reserve (MBBR) of the National Commission of Protected Natural Areas (CONANP). 2004-2020 data collected by the WWF-Telcel Alliance, in coordination with the Directorate of the MBBR. 2000-2001 population numbers as reported by Garcia-Serrano et. al (in Oberhauser and Solensky. 2004. The Monarch Butterfly: Biology and Conservation).

Data from this year are a stark reminder that a single high year does not mean that the population has recovered, and that monarch numbers reflect habitat availability and weather conditions throughout the annual cycle. While monarch numbers were high in much of the breeding range last summer, monarchs produced in the summer need to migrate successfully and survive the winter as well. Data from this year, combined with those from past years, may help us understand population drivers better.