This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
The decline of monarch butterflies is leaving an indelible mark. For those of us who grew up catching them, rearing them in deli cups, and releasing them, perhaps we feel a loss, not only for ourselves, but also for the future generations of backyard biodiversity lovers. Will this iconic symbol of nature, the Bambi of the insect world, disappear? The sharp decline of monarchs, especially in eastern North America, has left scientists non-scientists befuddled and worried. From populations of several hundred million butterflies in the 1990s, to annual losses driving the population incrementally down by 60-80 percent, it is clear that something isn’t right.
Our changing climate is certainly affecting monarch butterflies, although we are just beginning to understand the severity of these effects. Late in February, it was nearly 70°F in Ithaca, NY. Buds were breaking, as were temperature records. I heard a lecture at Cornell University that projected that this spring would come 2-3 weeks early compared to 2012, which previously held the record as the earliest spring in recent decades. In 2012, I watched a monarch butterfly lay an egg on a milkweed in Ithaca on May 5th—a full five weeks earlier than is typical. This year I began looking for them in April.
Agribusiness is another factor. The designer insecticides they are deploying are spectacular as pest killers, but how are we to know their side effects? Can we trust industry or the EPA to police the unknown? University research is beginning to emerge showing the sub-lethal (read: causing sickness, but not death) effects of designer pesticides like neonicitinoids on non-pests. It is hard not to think about DDT, which killed insect pests but also had rippling effects through to the tops of food chains, where they had an impact on peregrine falcons. For an animal like the monarch butterfly, which travels thousands of miles to complete its annual cycle, small amounts of toxins in the environment can have major impacts.
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But insects are also famous for their booms and busts, ups and downs in their populations, and scientists are challenged with separating typical fluctuations from long-term declines. It was fortunate that in the early 1990s, scientists and a newfound army of citizen scientists got serious about collecting data on monarch butterflies across their whole life cycle. The first synthesis of two hundred or so citizen scientists’ observations in 1955 already gave important information about the monarch’s population fluctuations. In 1953, monarchs were exceedingly rare, almost absent throughout the breeding grounds in mid-western and eastern North America, whereas they had been plentiful in the previous two years. Three years ago monarchs were at an all-time low.
Then, numbers sextupled over the next two years. This winter, they were down by some 27 percent Ups and downs are normal.
In 2012, two conflicting peer-reviewed studies were published, one finding a decline in monarch population overwintering in Mexico and the other finding no decline in monarchs flying south at the beginning of the fall migration. This piqued my interest. Was one right and one wrong, or were both right, and do we simply did not understand what happened between these two stages? When I brought this discrepancy up for discussion at the Monarch Biology and Conservation Conference in June 2012, much to my surprise, some monarch biologists thought it was inappropriate to discuss the issue, because, to them, there was no ambiguity in the decline. One senior monarch researcher grabbed my hand, and chastised me for speaking about variation in monarch declines.
Conventional wisdom has pointed to the decline in milkweeds as the main culprit, and a tremendous effort is currently being placed on planting milkweed (which is, after all, the only food for monarch caterpillars). My own research, however, has suggested that habitat destruction in the USA, lack of flower resources, and logging at the overwintering sites in central Mexico are all contributing to the decline of monarch butterflies. Lack of milkweed does not seem to be the main cause.
Monarchs travel through vast expanses from Mexico to Canada, tasting their way as they go. Nearly all mating, egg laying, and milkweed eating occurs in the United States and Canada, yet each autumn monarchs travel back to Mexico. As monarchs transition to adulthood in summer, their dependency on milkweed ceases, and they start the cycle with their southward journey, fueled only by water and nectar from other plants. As they migrate across thousands of miles, poisons in plants or otherwise degraded habitats lacking flowers impact their populations. Healthy habitats mean healthy monarch populations. The cues monarchs use to migrate are couple to climate, and sustained changes in the pace of monarch migration have been observed over the past decade. A polluted landscape in a changing climate is disastrous for migratory species.
If monarchs are environmental sentinels that travel the continent, then much more than the sustainability of this butterfly is at stake, and careful study and action is in order. Migratory species like the monarch face distinct requirements during the summer breeding season, during their migrations, and at their overwintering sites. Recent scientific evidence points to each essential requirement being challenged for monarchs, and no one government, team of citizen scientists, or group of researchers will be able to crack the code of what should be done to save the species. Migratory species like the monarch are indeed royal representatives of the wilderness that we all wish to preserve for the many services they provide. Planting native milkweeds won’t hurt monarchs (it can only help!), but please don’t expect such plantings to serve as a quick and sure fix for the long-term decline in monarch butterfly populations.