The flutter of a single butterfly's wings may or may not be capable of causing tsunamis, but the loss of millions of butterflies is definitely being felt here in North America. Populations of the iconic and beloved monarch butterfly (Danaus plexippus plexippus) have dropped an astonishing 96.5 percent over the past few decades, from an estimated 1 billion in the mid-1990s to just 35 million in early 2014. Conservation groups have been worrying about this decline for several years, and last week the U.S. Fish and Wildlife Service (FWS) finally took notice. According to the agency, monarch butterflies may deserve protection under the Endangered Species Act.

The FWS decision comes in response to a petition from the Center for Biological Diversity, the Center for Food Safety, the Xerces Society for Invertebrate Conservation, as well as monarch scientist Lincoln Brower, professor emeritus at Sweet Briar University in Virginia.

Monarch butterflies face numerous threats along their migration route between the U.S. and central Mexico, but their decline hinges on one primary factor: the similar decline in milkweed (plants from the genus Asclepias). Monarchs depend on milkweed—it's the only plant they eat, and they lay their eggs on milkweed leaves during the summer breeding season. But milkweed is much less appreciated by farmers, who consider it (as you might guess from its name) a weed and regularly wipe it out from their fields. Research published last June found that milkweed populations in the U.S. dropped 21 percent between 1995 and 2013. The vast majority of that milkweed loss—about 70 percent—overlapped with prime monarch breeding areas.

The study suggested—but didn't set out to prove—that the milkweed decline is tied to the expanded use of herbicide-resistant crops, specifically Monsanto's Roundup Ready plants. These crops can withstand treatment of the herbicide Roundup (glyphosate), but milkweed cannot. According to the petition, this has all-but wiped out milkweed on soybean and corn farms in the Midwest, reducing monarch butterfly habitat by more than 66 million hectares.

A few other factors have affected the monarchs' decline. Logging threatens their wintering grounds in the mountains of Mexico's Oyamel fir forest. Other pesticides kill the butterflies. Weather and climate have also had disastrous effects: a winter storm in 2002 killed 500 million monarchs, an event from which the species has yet to recover.

It's too early to say if FWS will decide to protect monarch butterflies, or what form those protections may take. The public now has 60 days to submit comments and data about monarch butterflies (visit regulations.gov and search for docket number FWS-R3-ES-2014-0056), after which the FWS will begin a 12-month review of the findings. That means we shouldn't expect any news on this until early to mid-2016.

Until then, enjoy every monarch you see, and maybe consider adding a few milkweed plants to your yard or garden. The organization MonarchWatch offers tips on how to create and register a Monarch Waystation habitat at your home, school or business to help the butterflies along their migratory journey. You may not be able to create a tsunami-sized impact, but it certainly won't hurt.

Photo by Joyce Cory via Fickr. Used under Creative Commons license