Walk the shoulder of any Canadian highway in summer and you’ll likely notice an unusual number of ants underfoot. While these warm, exposed areas have the kind of easily excavated substrate conducive to colony building, there’s a more compelling reason for their abundance: roadsides offer a 24-7, all-you-can-eat buffet for ants. So concluded W. Pickles in the Journal of Animal Ecology back in 1942, the first empirical reference to insect mortality via motor vehicles. The surprisingly few published studies since then support his observation, including one by a team from Sudbury’s Laurentian University led by James Baxter-Gilbert. Although he was investigating road mortality in reptiles on sections of Ontario’s Highway 400 that traverse the amphibious landscapes east of Georgian Bay, his study design presented opportunity to look at road effects on other organisms — including insects.
Over the course of two summers, Baxter-Gilbert and co-workers collected 117,000 road-killed insects during daily surveys of a two-kilometre stretch of divided highway south of Sudbury. Though large, this number represented only a fraction of what was actually killed: countless individuals were stuck to vehicles, obliterated on the pavement, too small to see or scavenged by small animals, including, as Pickles suggested, ants. The collection was dominated by three insect orders: Hymenoptera (bees, wasps, etc.), Lepidoptera (butterflies, moths) and Diptera (flies, mosquitoes, etc.). Fully a third of the total were pollinators. “That might seem disproportionate,” notes Baxter-Gilbert, “but makes sense when you think about who’s flying around at car level.”
That metric alone serves to illustrate why transportation corridors should also be conservation tools, a line of thought that has given rise to its own burgeoning science: road ecology. Overpasses, underpasses, tunnels, fences — all are employed with varying degrees of success with larger animals. The scarce attempts to lower insect mortality, however, have seen little success. An example: the alkali bee is a solitary ground-nesting species and the best pollinator of economically important alfalfa, increasing yields some 50 per cent. Looking to mitigate the impact of a major highway that bisects an alfalfa-growing area of Washington state, a fine-mesh roadside fence was erected in the area bees nested with the hope they’d fly up and over, avoiding traffic at a safe height. After clearing the barrier, however, 99 per cent of bees immediately descended to their normal two-metre flight level with predictable outcomes — the same documented in spades by the Baxter-Gilbert study. “Ultimately, we conclude insects are dying on roads in astronomical numbers,” he says, “especially those species we know to be super-important to ecosystems.”
What does astronomical mean? Even the most conservative estimates seem insane. Ontario’s Highway 400 is a medium-use road of 10,000 vehicles per day in an area of medium insect diversity. Extrapolating their estimate to the entire length of similar roads in Canada and the continental U.S., Baxter-Gilbert’s group came up with 10 billion Lepidoptera, 25 billion Hymenoptera, and a staggering 60 to 190 billion Diptera deaths per year.
For high-use roads in areas of higher insect diversity and abundance, you’d expect the numbers to skyrocket. And they do. In a study of Lepidoptera mortality in central Illinois, weekly collection along 13 roadside areas showed, unsurprisingly, more animals killed per 100 metres at higher traffic rates, and that the peak in monarch butterfly mortality coincided with their migration through the area. The data suggested more than 20 million Lepidoptera were killed on state roads during a single week — monarchs alone possibly exceeding 500,000. “We don’t yet know what these kind of numbers mean,” notes Baxter-Gilbert. “But we should watch them into the future as roads grow.”
Context is indeed elusive when you can’t monitor populations by reliably counting individuals as, say, you might do with caribou. “There are so many unknowns when you’re dealing with animals that only live [as adults] a few days or weeks,” says Baxter-Gilbert. “For instance, if a high proportion are killed before they can mate, that seems like a problem. If every bee that should live 15 days and collect pollen for its colony dies on the highway after three, that also seems like a problem,” he continues. “Of course, we’re assuming the loss of 10 or 25 billion individuals is significant; however, we have no clue what carrying capacity for a species would be before it crashes, or how much loss an ecosystem can sustain — especially with pollinators.”