The effect of hunting on non-target prey

Blog written by Jessica L. Mohlman & James A. Martin. Read the full paper here.

Predator-prey dynamics have long fascinated ecologists because of their effects on community ecology and behavior. These topics have even spilt over into the entertainment industry with famous cartoons such as “Looney Toons” depicting such relationships. As one of the main archnemeses of Bugs Bunny, Elmer Fudd’s mission was to try and hunt the trickster hare. However, Bugs Bunny was often able to outsmart Elmer Fudd and escape his hunting tactics. Not all prey are as calm and collected around predators as Bugs Bunny. In fact, most predators provoke fear in prey resulting in them changing their behavior. Human hunters, such as Elmer Fudd, may even provoke a more profound threat to prey than natural predators.

            While Bugs Bunny may have seemed calm and collected around Elmer Fudd, he was still altering his habitat use and increasing his alertness as a way to reduce his risk of predation by the hunter. Actively attempting to avoid direct predation, such as the pursuit of a hunter, can have negative indirect effects of its own, such reduced foraging effort, greater loss of energy, and increased vulnerability to other predators.

            Historically, ecologists have studied the direct lethal effects (e.g. prey death) and indirect nonlethal effects (e.g. reduced foraging) of human hunting on prey. However, what about the effects of hunting on prey that are not the target of predation? While Bugs Bunny was the target of Elmer Fudd’s pursuits, the hunter often found himself effecting other non-target characters, such as Daffy Duck, instead.

When it comes to hunting, why would we expect that the effects would be confined to only the species being directly pursued? What about other species that share the same habitat as the target prey, such as Daffy Duck? Additionally, are these indirect effects on non-target prey greater if the prey is also a game species?

 Within our study, we investigated such effects through examining the indirect effects of hunting Eastern Cottontails (Sylvilagus floridanus; hereafter, rabbit) on Northern Bobwhite (Colinus virginianus; hereafter, bobwhite). These species were ideal to answer our question of “how does hunting one species effect another?” as both of these species are commonly hunted within the United States and share similar hunting seasons and habitat. Additionally, both species are hunted on foot with the use of hunting dogs.

We were particularly interested in the possible effect of rabbit hunting on bobwhite because of its designation as a species of conservation priority. Due to the fact that bobwhites are not being actively pursued during rabbit hunts, increased perceived risk of predation by bobwhite may provoke anti-predator behavior. We hypothesized that bobwhite would respond to signals during rabbit hunts as threats and would be unable to discern that they were not the target of predation by the hunters. We tested this hypothesis by subjecting bobwhites to varying levels of rabbit hunting intensity and predicted that their behavior would change across this gradient. Specifically, we predicted that bobwhites may try to reduce this perceived risk by decreasing their movement under greater rabbit hunting pressure or increasing their movement as a result of more frequent encounters with hunters. Moreover, we hypothesized bobwhite would remain closer to escape cover (e.g. scrub/shrubs) and hardwoods where there was hunting pressure.

Rabbit hunting treatments were established on a Wildlife Management Area (WMA) in the state of Georgia. The treatments were assigned across the study site in a randomized complete block design (i.e., three levels of rabbit hunting and two spatial blocks). The treatments included 0 days of hunting per week (“No Rabbit Hunting”), 3 days of hunting per week (“Reduced Hunting”), and 5 days of hunting per week (“Reference”), with duplicates of each treatment.

Prior to hunting seasons, we captured bobwhite and equipped them with radio-tags to allow us to track their movements using telemetry. We tracked coveys (e.g. groups) of bobwhite during both rabbit and bobwhite hunting days across the varying rabbit hunting treatments. While being tracked, locations were taken of the bobwhite coveys every 30 minutes to determine fine-scale movement patterns. The telemetry data were analyzed to determine specific movement patterns such as step-length (i.e., distance moved every 30 minutes), path straightness, and trajectory distance (i.e., total distance moved in a day). To determine bobwhite use of hardwood and scrub/shrub habitats, we measured the distance of each individual location to the nearest example of that habitat feature.  

We found that bobwhite adjusted their movements to the spatial patterns of rabbit hunters to mediate the perceived predation risk. In areas in which rabbit hunting occurred, bobwhite decreased their overall movement, regardless of if hunting occurred for 3 or 5 days. Additionally, bobwhite increased their use of scrub/shrub and hardwoods where rabbit hunting occurred. Nevertheless, the full extent of the effect of rabbit hunting on bobwhite still needs to be investigated. Heightened antipredator behavior through decreased movement may assist with bobwhite predator avoidance by taking advantage of their cryptic coloration in the landscape. However, decreased movement and increased use of poor habitats, such as hardwoods, may also have negative effects as a result of reduced foraging time or increased susceptibility to other predators. Additionally, heightened use of antipredator behavior may reduce survival by means of increased energy use to avoid the perceived predation. While the full extent of rabbit hunting on bobwhite still needs to be investigated, we have shown that there is indeed an effect on behavior. This is critical for management and conservation efforts, as it reinforces the fact that species do not live independently of one another and that hunting and management activities targeted at one species may have negative nonlethal effects on other species.