Dr. Brower is a world-renowned monarch scientist who has studied overwintering biology in Mexico since the 1970s. He shares his observations and concerns after his trip to see the colonies in late February.
What did you notice at Sierra Chincua?
Chincua is just a remnant of what has been there over the years. The colony measured only .05 hectares in December 2013, practically nothing. There are probably only 20 trees with butterflies, and the trees are very sparsely covered.
I don't know why Sierra Chincua—and Cerro Pelon—don't have large colonies because they always did in the past. At Chincua, I suspect Lllanos de Toros has been degraded by logging, for one thing.
How did El Rosario compare?
Rosario has a relatively big colony, about .5 hectares when measured in December. The butterflies seemed far more active than they should be at this time of year. We saw a spectacular display of sheeting behavior. Huge sheets of monarchs were flying off the steep slopes. In the distance, the whole background was filled with flying butterflies. It was just beautiful.
There was a lot of evidence of bird predation on the ground. In our early research we found that the smaller the colony, the higher the rate of predation. If you think about it, a smaller colony has a greater surface area, and the birds are attacking from the edges. Also, since you have the same number of predators, predation rates should be higher in a smaller colony.
Ecotourism is another concern. I suspect it's having a major negative impact. People are getting too close and disturbing the butterflies.
Are the butterflies in good condition?
The monarchs at El Rosario looked normal to me, from what I casually observed.
How is this year's dry season affecting them?
Fortunately, drought has not been much of a factor this year. It wasn't excessively dry. It's relatively green compared to what it was last year at this time.
