March 15, 2016
With extended power outages, treacherous roads, and the sanctuaries closed for 2 days, information about how the monarchs were affected by last week's storm has been scarce, and sometimes unreliable or contradictory. Here's a summary of the information we have about key questions and concerns.
1. How many monarchs had already left?
At El Rosario, this picture was taken on March 7th — the day before the storm began — and shows substantial numbers of monarchs still clustering in the colony.
At Sierra Chincua, this picture was taken on March 8th — the afternoon that the storm began. Estela Romero commented that day that the colony had dwindled substantially. When Dr. Isabel Ramirez visited Sierra Chincua after the storm on March 11th, she was told by the guides and staff that most of the colony had gone before the storm. However, although some may have left on migration, most had likely moved down the mountains into lower portions of the watershed as they typically do at this time of March.
We don't have information about the timing of departure from other sanctuaries. A mass exodus had not been reported from the region before the storm, though some signs of migration had been seen. In summary, it seems likely that most of the population was in the overwintering region at the time of the storm.
2. How many of the remaining monarchs survived?
At El Rosairo, Estela Romero heard hopeful news from Reserve Biologist Felipe Martinez:
"I went inside El Rosario colony yesterday afternoon (March 10) and I was impressed to see how many big, medium, and small clusters hanging from many oyamels seemed to be intact. I got so emotional. My concern is now to come to the moment when the ice and snow melt and we start to evaluate the possible perishing of monarchs that will not withstand the low temperatures during the many hours below freezing."
Although encouraging to see, butterflies hanging in clusters can be dead — and butterflies on the ground can be alive, even if buried in snow. It takes about a week before their status is clear. What's more, these observations were made before the butterflies were subjected to the coldest temperatures (see below).
"We need careful objective counts of the dead butterflies on the ground about a week after the coldest morning temperature," says Dr. Lincoln Brower.
3. How low did temperatures fall — and for how long?
Perhaps the most hopeful news is that temperatures appear not to have dropped low enough to cause mass mortality on the scale experienced in the deadly storm of 2002. Dr. Ramirez obtained the minimum temperatures for the morning of Friday, March 11th, believed to be the coldest of the storm.
"We visited Chincua on March 11th. It was so beautiful until we reached the colony. There were fallen trees all the way to the colony. The mortality should have happened the night between Thursday and Friday (March 10 & 11). The minimum temperature at that time was -5.9°C in Chincua Station, after more than 24 hours with temperatures below 0°C and precipitation. This record is the same as the one registered by the Station Monarca I located in Llano de Villalobos. These stations are very important. In Angangueo town there isn't any weather station, at least not public, so all the data we see on the Internet are estimations."
The readings reflect temperatures in an open area of the Sierra Chinua Sanctuary, not inside of the butterfly colony where the forest canopy typically maintains warmer temperatures. With winds gusting to 60 mph during the storm, the microclimate may not have been maintained. Dr. Brower will know more when he can access data from temperature probes inside of the forest.
The freezing point of wet butterflies is of utmost concern. When monarchs are wet, the rate of mortality rises as temperatures drop according to Anderson and Brower, 1996.
4. What's the long-term effect on habitat?
The winds struck the southern face of the mountains, the side where the monarchs form their colonies every winter. Huge oyamel firs in the core of El Rosario and Cerro Pelon Sanctuaries (and presumably other sanctuaries) were toppled by the storm's strong winds. These large trees form the forest canopy and create the microclimate monarchs need. Their role in buffering temperature in the colony is dramatic. According to Brower et al. 2011, on average it is about 5°C (9°F) warmer beneath the forest canopy. The loss of the intact canopy degrades the integrity of the forest into the future.
For now, the immediate and long-term impact of the storm on the monarchs and their habitat remains unclear. However, the storm illustrates the need for a monarch population large enough to be resilient to severe weather events like this.
