Anyone who has washed a car after a road trip knows that insects often don't survive their encounters with traffic. For many flying invertebrate populations, this is not a problem (at least in the conservation sense), since adults breed shortly after emerging from their larval form and have therefore already reproduced by the time they collide with vehicles. This is not true for longer-lived species such as beetles and dragonflies, however, which breed over an extended period of time. For populations of these animals, which may encounter many roads during their travels through the habitat, traffic could have a serious impact by reducing the number of individuals available to breed.
Since 277 of 441 dragonfly species in the U.S. have been categorized as "species of greatest conservation need" in at least part of their range, there has been increasing interest in documenting all of the environmental factors that may be contributing to these animals' declines. This is important not just for preserving or restoring dragonfly populations, but also for maintaining the health of wetland ecosystems, where dragonflies can be a top predator in aquatic food webs. Previous studies have reported some intriguing trends for dragonfly-vehicle encounters. One survey in India found that dragonflies made up 61% of insect deaths along roadways, while research in Michigan indicated that up to 256 dragonflies are killed per kilometer of driving.
(A common whitetail, Plathemis lydia)
In order to characterize traffic-dragonfly interactions more fully, collaborators from The University of South Dakota, The Nature Conservancy, and Iowa State University teamed up to perform a combination of observations and experiments along roadways in the Des Plaines River Valley near Joilet, Illinois. They selected 4 stretches of road that each differed in the amount of traffic they saw each day (ranging from <10 vehicles to 25,500 vehicles per day). The researchers conducted roadside surveys in order to count how many dragonfly carcasses accumulated over a 24-hour period; this allowed them to calculate mean daily mortality rate per kilometer for each site. Since dragonfly bodies are both light-weight and attractive to hungry scavengers, the scientists also performed an experiment in which they counted how many deliberately-planted bodies disappeared over a 24-hour period. This allowed them to estimate whether their survey data were underrepresenting actual dragonfly deaths along the roadways.
In addition to conducting body counts, the researchers also performed almost 2,000 observations of dragonflies near roadways. This allowed them to assess relative abundances of different species and record dragonfly behavior. In particular, the scientists were interested in evaluating how often the dragonflies flew across the road and, when they did, how high they flew. By surveying cars in a large parking lot, the researchers determined that the average height of vehicles is 1.8 m; thus, dragonflies that fly above this height are less likely to be killed during a road crossing than dragonflies that fly below it.
(A widow skimmer, Libellula luctuosa)
A total of 49 dragonfly bodies were collected during 36 roadway surveys. However, the carcass removal experiment indicated that about two-thirds of dead dragonflies disappeared from the roadside within a 24-hour period. Thus, the researchers multiplied their results by 3.12 in order to determine the real number of bodies that likely accumulated during a day's worth of traffic. This revealed that daily mortality per kilometer was anything from 2.1 to 34.6, depending on the roadway. Nine species of dragonflies were identified from the carcass sampling; the three most common species (making up 71% of all samples collected) were the common whitetail (Plathemis lydia), the widow skimmer (Libellula luctuosa), and the green darner (Anax junius). Interestingly, none of these was the species most commonly seen during the behavioral observations; that was the black saddlebags (Tramea lacerata), which accounted for almost a quarter of all identified dragonflies.
During the behavioral observations, over half (~58%) of all flying dragonflies were seen over the roadway; just under one half (~47%) were seen crossing to the other side. Species at the busiest site were significantly less likely to cross the road, indicating that the dragonflies were aware that commuting conditions were less than ideal for them at that particular roadway. Species identity also influenced the likelihood of road crossings; widow skimmers and black saddlebags crossed the road more and less frequently, respectively, than expected by chance. Further, while widow skimmers, Eastern pondhawks (Erythemis simplicicollis), and blue dashers (Pachydiplax longipennis) spent 79% of their flight time in the danger zone below 1.8 m, the Eastern amberwing (Perithemis tenera) and green darner spent the majority of their time above this height.
(Mating green darners, Anax junius)
Cumulatively, these data allowed the researchers to construct a model comparing predicted and expected mortality for different species; this could then be used to determine which factors were most closely associated with dragonfly deaths along roadways. The models were not strongly associated with relative abundance of different species, minimum flight height, or percent observed crossing the road. However, when models were explored on a site-specific basis--with busier roads considered separately from those with less traffic--the number of dragonflies observed over the road was highly correlated with both death rates and volume of traffic. Thus, unsurprisingly, dragonflies are more likely to end up as roadkill if they live near busy roads and position themselves near passing vehicles.
These findings indicate that the presence of roads can have significant impacts on dragonfly mortality. Thus, it will be important to consider the potential impacts of these anthropogenic corridors when deciding whether one should be installed in a nature preserve or an area where threatened dragonfly species are known to live. Roads may have different impacts on different species, since not all dragonflies flew at the same height or ventured over or across busy roadways. Clearly, more work is needed to provide additional details of how the dragonflies perceive the traffic and adjust (or fail to adjust) their behavior accordingly.
More research is also needed to understand which roadway attributes are most dangerous--is it purely number of vehicles, or do other characteristics such as number of lanes, speed of traffic, and type of vehicle also have an impact on dragonfly mortality? It will also be important to investigate whether these individual dragonfly deaths have a longer-term, broader-scale impact. For instance, does the death of potential breeders reduce population size and act as a constraint on conservation efforts to preserve or restore dragonflies? Further, how does the reduction of top predator numbers impact food web dynamics and ecosystem function in wetlands? These are questions that will need to be answered in order to develop appropriate conservation plans for species like the endangered Hine's emerald dragonfly (Somatochlora hineana) and the habitats in which it lives.
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Soluk, D.A., Zercher, D.S., Worthington, A.M. 2011. Influence of roadways on patterns of mortality and flight behavior of adult dragonflies near wetland areas. Biological Conservation 144:1638-1643.
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