Evaluating species-specific responses to camera-trap survey designs

Submitted by editor on 28 January 2021. Get the paper!
A coyote (Canis lupus) is photographed while looking directly in the direction of one of the cameras deployed during fall 2017. The photo belongs to the Minnesota Department of Natural Resources. 

By Fabiola Iannarilli on behalf of all co-authors

Many carnivores are rare, secretive, and patrol large areas, making them particularly challenging to monitor. Camera trapping offers great potential, but simultaneously monitoring multiple species creates a unique set of challenges for wildlife biologists. Should cameras be deployed at random locations, or should biologists devote additional time trying to place cameras at locations where carnivore detections seem more likely? Can biologists be ’lazy’ and take advantage of existing trail networks to more rapidly deploy cameras, because many carnivores are also lazy and take advantage of these same trail systems as they move across the landscape? Should investigators use attractants such as lures or baits to try to attract carnivores to the camera site, and if so, which ones?

A female American black bear (Ursus americanus) and her two cubs checking the lure trees in front of a camera trap. Sites were randomly assigned to one of two lure treatments (salmon oil vs fatty acid scented oil) and the lure was applied to a tree in front of the camera. The photo belongs to the Minnesota Department of Natural Resources.

In a recent study in collaboration with the Minnesota Department of Natural Resources, we evaluated how different survey-design features influenced encounter rates of ten North-American carnivore species. We compared two different survey-design frameworks for selecting where to deploy the cameras (along secondary forested roads or at random locations) and two different placement strategies for randomly-selected sites (with or without looking for local features, such as game trails, that might increase carnivore encounter rates). We also compared responses of each species to two lure types commonly used in carnivore studies, salmon oil and fatty acid scent oil, compared encounter rates in spring versus fall, and quantified the change in daily encounter rates within each sampling session. Between 2016-2018, we collected more than 2.4 million pictures from 23 000 trap-days spread across 100 locations in northern Minnesota, USA.

 A wildlife biologist deploying a camera at one of the 100 locations sampled during each sampling session. When possible, cameras were placed at about 75 cm above the ground and aimed north (ranging from northeast to northwest) to reduce the number of false triggers and blurred photos from direct sunlight.

Different survey-design strategies elicited very different responses across the suite of carnivore species targeted, with the choice of survey-design framework (forested roads versus random locations) having the greatest effect on encounter rates.  Wolves, coyotes, red and gray foxes, striped skunks and bobcats were up to 106-fold more likely to be encountered at sites along secondary roads, such as ATV and snowmobile trails. By contrast, American martens, fishers, and American black bears were up to 3600-fold more likely to be encountered at cameras deployed at randomly selected, forested locations. The type of lure applied also played an important role; wolves, coyotes, gray foxes, martens, fishers and striped skunks (six out of 10 species) were 2- to 4-fold more likely to be encountered at camera sites lured with salmon oil than at camera sites lured with fatty acid scent oil.

 A wildlife biologist pouring lure on a tree located in front of a camera trap. Cameras placed at randomly selected sites were randomly assigned to one of two lure treatments: salmon oil or fatty acid scent oil.

These results, along with others from our study, demonstrate that even species with similar morphology and phylogeny respond differently to survey-design choices. Thus, it is important to carefully consider these differential responses when monitoring several species simultaneously. To maximize encounter frequencies in multi-species camera-trap studies, we recommend applying different survey-design strategies within the same data collection effort (e.g., deploying some cameras along roads and some at randomly-selected locations) and accounting for these survey design choices during data analysis.

A red fox (Vulpes vulpes) passing by in front of one of the cameras placed along secondary roads. In each sampling session, 20 out of 100 cameras were placed along secondary roads within forested areas to assess differences in species encounter rates between cameras deployed along roads and cameras deployed at random locations. The photo belongs to the Minnesota Department of Natural Resources.