Monarch Population Status

The last year has been tough on monarchs. Hotter than normal conditions for returning monarchs in the spring of 2009, followed by the fourth coldest summer since 1928 in the breeding areas and less than optimal conditions during the fall migration, resulted in the smallest overwintering population to date, 1.92 hectares (the longtime mean is 7.44 hectares).

This small population was of some concern at the outset and the development of El Niño conditions in the Pacific late last summer was unsettling. Such conditions have been known to be associated with moisture-laden systems that have lead to catastrophic mortality at the overwintering sites. The freezing weather that followed heavy rains in the winters of 2002 and 2004 killed roughly 80% and 70% of the populations in those years. Were something similar to happen this winter, the returning population might be only the equivalent of 0.6 hectares, or less.

We have no prior experience with such a small returning population and it is uncertain whether monarchs in such numbers could recover in one year. Assuming that 80% of the overwintering monarchs died in 2002, and that there was some additional mortality due to normal causes, the returning population that year was probably equivalent to 1.4-1.6 hectares. So, a surviving population of 0.6 or less would certainly be unprecedented, and seemingly vulnerable unfavorable conditions as they move northward into the breeding areas in March and April.

The priorities in the monarch area involve dealing with refugees, the massive cleanup, and the damage to the infrastructure of the region due to recent disastrous storms. News of the monarchs and how well they have survived the storms of January and February are fragmentary and confusing. One of the smallest colonies, Herrada, initially only 0.06 hectares, was hit hard by high winds, rainfall, and freezing conditions. The number of surviving butterflies is said to be quite low but no precise estimate of mortality or survival is available. Similarly, masses of dead butterflies were reported at Cerro Pelon, yet other post-storm reports indicate strong flights.

The colonies at Sierra Chincua and El Rosario are reportedly intact with good flights reported for Chincua. But these reports mean little, since 80% of the butterflies could have been killed while the surviving 20% put on a show good enough to impress the tourists. Detailed assessments are needed of the living and the dead. I’ve been told that such studies are planned and should be completed by the end of the month. Let’s hope that in spite of the winds, hail, rain, and freezes, the number surviving will be enough to repopulate our fields and gardens this summer!

ADDENDUM – 21 February 2010
Unable to wait for an official report on the numbers of monarchs that have survived the onslaughts of the repeated storms at the overwintering sites, I have been soliciting reports from those that lead tours and tourists that have visited the monarch colonies in recent days. The reports are fragmentary but none have been positive and it is keeping me up nights. It seems certain the returning population will be less than a 1-hectare equivalent – perhaps 0.6 hectares or even much less.

The returning numbers make a difference because, even though monarchs have a relatively high reproductive rate, there is a limit in how much recovery we might expect in one year. Let’s look at what happened again after the 80% loss in the winter of 2002. The returning population the following spring was probably close to 1.4 hectares and the following winter the population was 7.64 hectares, a 5.45-fold increase. This is the highest increase seen from one year to the next in the 16 years we have good data on the sizes of the overwintering colonies.

However, even if we suppose that monarchs are capable of a 10-fold increase in one year, once the returning population drops significantly below 0.6 hectares (ha), the chances of a recovery in one year diminish substantially. A returning population of 0.6 ha, with favorable conditions, might get us back to overwintering numbers of 5-6 hectares but a returning population of 0.3 ha would only get back to 2-3 hectares – again with favorable conditions. In both cases, these numbers would be below the long-time average of 7.44 hectares.

Given that we have yet to see a 10-fold increase in monarch numbers in one year, my thinking now is that it will take monarchs at least two, and perhaps more, years to recover from the effects of the last breeding season and the winter of 2009-2010. All of us interested in monarchs are going to have to work especially hard to keep this migration going.