On Friday, the New York Times published an article titled, “The Year the Monarch Didn’t Appear.” It’s an OpEd piece, so maybe it can be forgiven, but I feel that it is misleading in several ways.
The fundamental problem that the article addresses does exist…monarch populations are crashing. The the ultimate and proximate causes are much more complex than those laid out in the article.
First, let me explain the life cycle of the monarch, because it is a bit different than most insects. Monarchs are famous for their migration, which takes them from southern Canada to Mexico and back. Though the monarchs travel south in the autumn and north in the spring, no single individual is capable of making the whole trip. It can take 3 or 4 generations of monarchs. No monarch has ever seen both Canada and Mexico and no one knows how the overwintering sites are conserved across several generations.
The population on the western side of the Rocky Mountains overwinters in southern California, but the populations east of the Rockies overwinter in a select few sites in Mexico. These sites are very limited in distribution. In fact, there are only twelve known locations (Slayback and Brower 2007) (you can see the small blue circle on the map above); only the peaks of a few mountains provide the right combination of moisture and cool temperatures. In recent years, these sites have been threatened by logging and human development (Brower et al 2008).
If the temperatures get too high, the metabolism of the Monarchs speeds up and they essentially starve to death (Masters et al 1988). If there is too much or too little precipitation, the Monarchs cannot survive in their overwintering habitat (Oberhauser and Peterson 2003). This is an important point that the article neglects. We have already seen temperature increases in these areas, which are threatened not only by human development, but also by a changing climate.
The NY times article makes a lot of good points. It is true that there has been a dramatic reduction in the abundance of Asclepias syriaca (common milkweed) in some areas. Common milkweed has historically been the most important host plant of the monarch (Hartzler 2010), and its decline is due mostly to agriculture and development. (And while I agree that this is also a threat to the bees, I do resent them being called “flying dust mops.”)
But if the overwintering sites disappear in Mexico, there is not a great hope of saving the migration, no matter how much milkweed is planted in northeastern North American gardens. Last summer, I saw great fields full of the common milkweed…and not a single caterpillar. That is an anecdote, but it helps to explain my point. And it is the extinction of the migration we are talking about here, not the butterfly. The butterfly persists in its western range, as well as in Australia and New Zealand (it is known as the wanderer there). Even if the great migration is lost on the east coast, the species will persist, albeit in much lower numbers.
It is interesting to think of this case as one of an extinction (or attempted conservation) of a natural phenomenon rather than a species.