Monarch Butterflies and the Plight of Migratory Species

by Caitlin Andrews

Each year, on the last day of October, people in Mexico honor their ancestors and deceased loved ones during the holiday of Day of the Dead. Over three days of celebration, they march in parades wearing colorful masks and costumes, build ornate altars, and decorate gravestones with orange marigolds—gifts to the departed. Everywhere, there is color, and nowhere is this as true as in forests of central Mexico, where, each year, the holiday coincides with the arrival of millions of monarch butterflies. At the end of a two month, nearly 3,000 mile journey taking them from across the United States and Canada to the same patch of Mexican forest each year, the monarchs have always appeared with such regularity that the local villagers have incorporated them into their traditions. These butterflies, or mariposas, the people say, are the souls of departed loved ones descending from the heavens and returning for a visit to Earth (1).

Yet, in 2013, as Day of the Dead came and passed, there was something noticeably missing from the festivities. At first, people assumed that the monarchs’ absence meant that there was simply a delay in their migration cycle, since a cold spring that year had prevented the monarchs from returning to the north on time (2). But, while 33 million monarchs eventually arrived at their winter roosts in the forest, this number paled in comparison with the astounding 60 million butterflies estimated in the 2012 migration (3, 4). It was a record low both in terms of population size and forest coverage, with butterflies spanning only 1.5 acres, or approximately half of the previous year’s coverage. Most troubling of all, these numbers merely lent support to a long-observed pattern of decline in the butterflies’ populations, suggesting that this was not a matter of normal fluctuation but a sign that there were serious threats facing the monarch butterfly—an icon of Mexico’s ecology and culture (2).


In the natural world, there are few sights more breathtaking than the migration: whether one is witnessing flocks of birds navigating over the Appalachians, pods of whales coming up for air on their tireless journeys across oceans, or herds of zebras traversing the African plains. When we witness events like these, it is as if we are temporarily immune to the problems of our world. After all, it can be difficult to associate these throngs of animals with thoughts of endangered species or ecological destruction. However, we may be too quick to assume that animals that come in the thousands or millions, like monarch butterflies, are not worth our time or in need of protection. Recently, a group of conservationists has been calling for a reexamination of the threats to migratory species.

This movement could not have come at a better time, as migratory and non-migratory species, alike, are facing increasing pressures from a multitude of sources, including habitat loss, urbanization, overexploitation, and climate change. In some ways, migratory species are able to reap the benefits of their mobility, especially in light of rising global temperatures. As warm temperatures move northward, it may be easier for migratory species to shift their ranges in response, while non-migratory species might find it difficult to cover the necessary distances. However, migratory species also face unique challenges as a result of their lifestyles. First, climate change can disrupt important signals that animals use to decide when to begin their migrations, such as seasonal light and temperature changes (5). Additionally, because they travel over long distances, migratory animals often pass through many different habitats; thus, they may be more vulnerable to a range of threats at each stop along the way, while species with smaller ranges are likely exposed to a more limited set of dangers (6). Urbanization introduces new obstacles, which can be physical or chemical. On land, migration can be severely impeded by fences and dams, while marine and freshwater animals can face salinity changes and chemical pollutants. These barriers not only act to slow or prevent movement but can also cause serious injury and even death to animals who come across them (7). Also, when animals gather in large herds or flocks during their migratory season, it is an open invitation for hunters and poachers to exploit these large assemblies for mass killings—and quick profits (6).

However, perhaps the greatest challenge facing migratory species is the fact that, unlike humans, animals do not recognize park boundaries or international borders. Even if animals are protected within a park or country, their migrations will likely take them outside of these areas, into territories where they may or may not be safe. For example, American bison frequently stray out of Yellowstone National Park during their winter migrations; beyond the protection of the park, they may be killed by livestock herders, who fear that the bison will transmit diseases to their cattle (5). In some cases, it can be difficult to determine who is responsible for the protection of migratory species. This is especially true for oceanic species, as it can be unclear who “owns” the ocean and has the duty and authority to lead marine conservation efforts. More often than not, however, the greatest issue is coordination. When a species migrates across many borders, it can be difficult—if not impossible—to get every country within its range to cooperate, particularly if there are preexisting political disputes between them. Unfortunately, whenever politics are involved, conservation is bound to be pushed to the side (5).


Fortunately for the monarch butterflies and other migratory animals, a new understanding of the plight of these species has begun to take hold in the conservation movement. These efforts have largely been spearheaded by a group called The Convention on the Conservation of Migratory Species of Wild Animals (“CMS”). Backed by the United Nations, CMS states its mission as bringing together “the States through which migratory animals pass…[and laying] the legal foundation for internationally coordinated conservation measures throughout a migratory range” (8). CMS is the only global convention focusing entirely on the conservation of migratory species, and, since its establishment in 1979, it has made great strides in uniting countries behind conservation efforts. One of its greatest successes came just recently, with the enactment of “The Gorilla Agreement” in 2008. This legally-binding treaty was the first of its kind to focus on issues of gorilla conservation across all ten range nations, most notably the Democratic Republic of Congo and Rwanda. It was unclear whether these two nations would be willing to put aside their long-term political conflicts for the sake of the gorillas, but they ultimately did, and the agreement will ensure that all range nations agree to work toward improving the conservation of these incredible animals going forward (8).

Beyond issues of politics, there is another issue that CMS and other organizations must face, and that is an issue of the image of migratory species. In his article “Animal Migration: A Migratory Phenomenon,” Princeton Professor David Wilcove explains that migratory species, like the monarch butterfly, may be on the decline but are not necessarily “endangered” in terms of their overall numbers; therefore, they are often overlooked by conservationists, who tend to focus on animals facing imminent extinction. However, Wilcove writes, “migration is fundamentally a phenomenon of abundance and must be protected as such” (6). He advocates for a rebranding of “endangered species” to include a separate scale for migratory animals, in which species are ranked based on patterns of population decline and habitat loss, and not simply based on numbers. By describing animal migration as an “endangered phenomenon,” Wilcove believes, conservationists may begin to rethink what it means for a species to be worth conserving today.


While the monarch butterfly is commonly viewed as one of the most iconic migratory species, the US, Canada, and Mexico have failed to launch a unified effort to conserve the habitats along the monarchs’ migratory route. For many years, the majority of the threats facing monarchs appeared to be in Mexico, with illegal logging decimating the pine and fir tree forests where the monarchs roost each winter (2). Tree coverage is not only crucial for providing the monarchs with places to roost, but it also keeps the understory relatively warm and dry, which is essential during winters in the high altitude Mexican forests (1, 2). Fortunately, in the past decade, the Mexican government has recognized the value of the monarch butterfly, both ecologically and economically, as huge numbers of tourists flock to Mexico to witness the arrival of the butterflies each year. Large-scale illegal logging peaked in 2007 and has nearly become a non-issue, so it is with great bewilderment that Mexicans have continued to observe a decline in the number of monarchs returning to the forests each winter (2).

The fact of the matter is that Mexico can do all that it can to conserve the monarchs, but, if the butterflies are not protected along the rest of their 3,000 mile route, then a winter safe haven is just that—a temporary, seasonal sanctuary. All throughout the United States, monarchs are facing a threat arguably more potent than deforestation. In the Midwest in particular, industrial farmers are using herbicides that kill all ground cover except for their crops, which have built-in, engineered immunity. As a result, the abundance of milkweed—the plant that sustains newly-hatched monarch caterpillars—has drastically declined, with some states seeing as much as a 90% reduction in milkweed (3). Since 2000, farming has destroyed approximately 100 million acres of monarch habitat in the Midwest, and this number is only growing (2).

There have been some efforts to replant milkweed in the US, but most have failed. First, replanting is typically a losing battle against the ever-growing industrial farms that just end up spraying more herbicide each growing season. Also, a campaign encouraging residents to plant milkweed in their backyards ended up backfiring when people began to plant tropical varieties of milkweed. Because these varieties persisted through the winter, monarchs failed to recognize when it was time to migrate south, and many populations faced devastating losses as the weather grew too cold (2). Other campaigns have initiated citizen science projects for monitoring monarch populations, but this has only provided estimates of population size—and, as of yet, no solutions (1).

It is unclear what lasting effects decreasing populations will actually have on the monarchs’ migration, but we can only assume that, if current trends continue, the future of these butterflies and the ecosystems they inhabit may be grim. One of the most amazing aspects of the monarch migration is that the northward journey is undertaken by four successive generations of butterflies, each of which completes one leg of the trip. This means that the individuals making the journey south for the winter are not the same butterflies that previously roosted in Mexico. So, how do they find their way to a patch of forest that they have never been to? Some hypothesize that these individuals use chemical signals left behind by their predecessors. But, with fewer butterflies leaving Mexico at the end of the winter, there will be weaker signals left behind, and the monarchs going to Mexico the next year may have trouble finding their way without a strong signal to guide them home (4).


With the passing of another year and another Day of the Dead in Mexico, it remains to be seen whether the monarch butterfly population has continued to dwindle. However, it is important to remember that, even after the monarchs have departed for their winter roosts, our obligation to the monarchs—and to Mexico—does not end. Even though the monarchs only bring color to our backyards for part of the year, it does not mean that our responsibility leaves with them. We must remember to hold up our end of the bargain for the sake of the monarchs, and for the sake of the communities and ecosystems all along their migratory route.

The same holds true for other migratory species, both in the US and around the world. While range countries, in particular, have a duty to protect the species that pass through their land, the threats facing migratory animals are not local issues. Climate change and many other environmental problems are global threats, and, therefore, are the responsibilities of all of us. We have to ask ourselves what the world would look like without great migrations. Would we feel the same sense of awe at the sight of a lone wildebeest crossing the African plains or a single starling flying overhead as we would if an entire herd or flock of these animals passed before us? How colorless would our lives become if schools of salmon never made their upriver journey or the monarchs never returned in springtime? While this might seem a far-off fantasy, we must protect these species now if we wish to preserve these sights—not only for ourselves but for future generations, as well.

Caitlin Andrews ’16 is a junior in Cabot House concentrating in Organismic and Evolutionary Biology.

Works Cited

  1. “Why Fewer Monarch Butterflies Are Surviving Their Winter Migration to Mexico.” PBS, 24 Dec. 2013. Web.
  2. Wade, Lizzie. “Monarch Numbers in Mexico Fall to Record Low.” Science/AAAS, 29 Jan. 2014. Web.
  3. Robbins, Jim. “The Year the Monarch Didn’t Appear.” NY Times 22 Nov. 2013. Web.
  4. Plumer, Brad. “Monarch Butterflies Keep Disappearing. Here’s Why.” Washington Post 29 Jan. 2014. Web.
  5. Wilcove, David S., and Martin Wikelski. “Going, Going, Gone: Is Animal Migration Disappearing.” PLoS Biology 6.7 (2008): E188. Web.
  6. Wilcove, David S. “Animal Migration: An Endangered Phenomenon?” Issues in Science and Technology (Spring 2008). Web.
  7. Wolff, Wim. “The Significance of Artificial Barriers to Migration Across International Borders.” Convention on the Conservation of Migratory Species of Wild Animals: 14th Meeting of the CMS Scientific Council (2007). Web.
  8. Convention on the Conservation of Migratory Species of Wild Animals. Web. <;.

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