The extraordinary migration of monarch butterflies impassions people in ways few insects do — and so too their tragic decline, with populations plummeting by more than 80 percent in the last few decades, and accounts of sky-darkening orange-and-black swarms in danger of becoming Anthropocene folk tales.
Their demise is largely attributed to an agriculture-induced collapse of the milkweeds upon which they rely for egg-laying and caterpillar-nourishing. Conservation efforts have rightfully focused on replenishing these plants. Yet another, little-appreciated problem is also desperate need of attention: monarch butterfly roadkill.
In a study published in the journal Biological Conservation, biologists led by Tuula Kantola of the University of Helsinki and Robert Coulson at Texas A&M University describe their roadside counts of dead monarchs in southwestern Texas in autumn — smack in the middle of the “Central Funnel,” the region through which most of North America’s eastern monarch population passes on their many-thousand-mile journey to Mexican overwintering sites.
Kantola’s team found an average of 3.4 dead monarchs per 100 meters of road. In certain places, where weather and other, as-yet-unidentified factors conspired to concentrate the butterflies’ passage, that average swelled to 66 car-struck monarchs. Extrapolated to the entire Central Funnel, some 4.7 million monarchs met this ugly fate in 2016 and 2017, when the researchers conducted their counts. That number represents some 3 percent of the entire overwintering population.
While a smattering of citizen science reports and one previously published study, which put the vehicular toll at more than 500,000 monarchs along Illinois interstates during six autumnal weeks in 1999, have hinted at this problem, the new accounting is the most comprehensive yet—and, note the researchers, the difficulty of finding monarch carcasses in tall vegetation and the tendency of their bodies to remain stuck on cars means their estimates are likely conservative.
While roadkill is likely a lesser threat than milkweed loss and the destruction of their winter groves, that 3 percent is still a troubling figure. With so few monarchs remaining, every fractional push towards an extinction threshold is magnified. “Reducing roadkill rate over the Central Funnel,” write Kantola and colleagues, is “an important step towards reversing the continuing decline of this iconic butterfly.” Shaving just half a percent off current roadkill numbers could make a big difference.
What, then, can be done? More remains to be learned about how topography and vegetation and local land use affects monarch mortality on roads, and hotspots need to be identified — and when they are, solutions are close to hand. Conservationists in northern Mexico have erected signs urging drivers to reduce their speed along highways in northern Mexico. The same could be done elsewhere in the Central Funnel.
Kantola’s team also points to Taiwan, where nets erected along a crucial stretch of highway direct migrating purple crow butterflies above traffic. Mortality at that site has dropped by roughly 80 percent. When especially large numbers of butterflies are crossing, one lane of traffic is closed.
These are not difficult solutions to implement. They also apply to more than monarchs. Around the world, “researchers have suggested that butterflies are one of the most common insect orders killed by vehicles,” write Kantola and colleagues. Reducing that toll, perhaps dramatically, is within our power.
Source: Kantola et al. “Spatial risk assessment of eastern monarch butterfly road mortality during autumn migration within the southern corridor.” Biological Conservation, 2019.
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.