Flying insect abundance declines with increasing road traffic

Abstract

1. One potentially important but underappreciated threat to insects is road mortality. Road kill studies clearly show that insects are killed on roads, leading to the hypothesis that road mortality causes declines in local insect population sizes.2. In this study we used custom-made sticky traps attached to a vehicle to target diurnal flying insects that interact with roads, sampling along 10 hightraffic and 10 low-traffic rural roads in southeastern Ontario, Canada. We used a paired sampling design to control for potentially confounding differences in the road characteristics (e.g. road width) and surrounding land covers (e.g. housing density) between high-traffic and low-traffic roads. We then used these data to test the prediction that fewer flying insects collide with vehicles, per vehicle (i.e. insect abundance is lower), on high-traffic than low-traffic roads.3. We found significantly fewer insects at the high-traffic roads than at the low-traffic roads as predicted. There was a 23.5% decline in the number of insects/km/vehicle on high-traffic relative to low-traffic roads. 4. Given the high rates of insect mortality observed in previous studies, it is likely that road mortality contributes to these observed negative effects of traffic intensity. Thus the growing global road network is a concern for conservationists and land managers, not only because insect population declines contribute to the ongoing global losses of biodiversity but also because insects play a vital role in food webs and provide important ecosystem services.

Little evidence of a road‐effect zone for nocturnal, flying insects

“…It is important to see how the road-effect zone changes with the size of roads and the volume of traffic traversing the road (e.g., Martin et al, 2018), the features of the road such as finishing (i.e., asphalt road, dirt road) and lighting, the type of habitat the roads transect, and the current state of a population's success (i.e., if they are already in decline or if they are thriving in the area), in order to target appropriate mitigation measures. Future research should quantify the size of the road-effect zone for different taxa, and also target the causal mechanism of the road-effect zone.…”

“…Future research should quantify the size of the road-effect zone for different taxa, and also target the causal mechanism of the road-effect zone. It is important to see how the road-effect zone changes with the size of roads and the volume of traffic traversing the road (e.g., Martin et al, 2018), the features of the road such as finishing (i.e., asphalt road, dirt road) and lighting, the type of habitat the roads transect, and the current state of a population's success (i.e., if they are already in decline or if they are thriving in the area), in order to target appropriate mitigation measures. The more information we have, the better we can anticipate and mitigate the broad-scale, ecosystem-level impacts of roads.…”

“…Roads can have detrimental impacts on invertebrate species, including barrier effects (Knapp et al, 2013;Koivula & Vermeulen, 2005), and road mortality (Baxter-Gilbert et al, 2015;Keilsohn, Narango, & Tallamy, 2018;Martin et al, 2018;Rao & Girish, 2007;Seibert & Conover, 1991). Roads can have detrimental impacts on invertebrate species, including barrier effects (Knapp et al, 2013;Koivula & Vermeulen, 2005), and road mortality (Baxter-Gilbert et al, 2015;Keilsohn, Narango, & Tallamy, 2018;Martin et al, 2018;Rao & Girish, 2007;Seibert & Conover, 1991).…”

“…There are more than 64 million kilometers of roads fragmenting landscapes across the globe. Roads can have detrimental impacts on invertebrate species, including barrier effects (Knapp et al, 2013;Koivula & Vermeulen, 2005), and road mortality (Baxter-Gilbert et al, 2015;Keilsohn, Narango, & Tallamy, 2018;Martin et al, 2018;Rao & Girish, 2007;Seibert & Conover, 1991). However, the ecological impacts of roads are not restricted to the site of the road itself, and they often extend into the surrounding landscape, over a distance known as a "road-effect zone" (Forman & Alexander, 1998).…”

Flower power in the city: Replacing roadside shrubs by wildflower meadows increases insect numbers and reduces maintenance costs

“…Although habitat size and isolation can influence Orthoptera abundance and species richness in urban areas [105], we found the highest density of Orthoptera in both study years in one of the smallest plots, a meadow area of only 3.3 m 2 in the pedestrian zone of a residential area: eight individual Caelifera/m 2 of suction sample, which is also a high density for extensively mown meadows in ecological compensation areas [46] or natural grasslands [104]. It is possible that the lack of car traffic in the immediate vicinity has reduced the mortality rate of these mobile insects, which may otherwise suffer marked road deaths [31,106]. High Orthoptera densities in some of our study plots support the idea that even small urban green spaces can be of value to wildlife if basic habitat requirements of species are taken into account [80].…”