Cameras, Coyotes, and the Assumption of Equal Detectability

Abstract

: Remote cameras are an increasingly important tool in management and wildlife studies. However, we often do not know if they provide an unbiased sample of populations. Using a marked, radiocollared population of coyotes (Canis latrans) of known social status, we evaluated the influence of temporal (daily and seasonal) and spatial (distance between units, habitat, and proximity to human structures) factors on vulnerability to photo‐captures. During 8 unbaited camera sessions of 6 weeks each, we obtained 158 coyote photographs at a photo‐capture success rate of 1.6%. We were able to identify not only marked individuals, but also a number of uncollared adults through variation in their pelage. Photo‐capture of adults peaked 2 weeks after we established camera stations. Annual success for photographing adult coyotes was greatest during March and April, which corresponded with the dispersal season. The majority of photo‐captures occurred at night, and adult photo‐captures peaked around midnight, with smaller peaks at dawn and dusk. Rather than reflecting a circadian activity pattern, nighttime captures seemed to reflect when adult coyotes were most vulnerable to photo‐capture. Characteristics of camera locations, such as amount of human activity, being on roads versus trails, and habitat type, also influenced the number of photo‐captures. We conclude that remote cameras do not always provide an unbiased sample of populations and that animal behavior is important to consider when using these systems. Researchers using camera techniques need to carefully consider when, where, and how cameras are placed to reduce this bias.

Influence of camera-trap sampling design on mammal species capture rates and community structures in southeastern Brazil

“…Although camera traps are well suited to standardization when compared to other sampling methods for mammals (Ahumada et al 2011), the places where the data are collected influences the efficiency in obtaining adequate measurements of species studied Engeman (2005), and the choices about where and when the camera traps will be placed can influence the sampling process. These choices can lead to biased results because species use the environment differently and researchers use different criteria to define the best sampling locations (Larrucea et al 2007). …”

“…Despite the set of factors influencing the detection probability of species (e.g., Yasuda 2004, Larrucea et al 2007, Guil et al 2010, Harmsen et al 2010, there is a growing demand for the use of camera traps as a tool for estimating population parameters or monitoring the population tendencies of threatened species. When the same http://www.biotaneotropica.org.br http://www.biotaneotropica.org.br/v13n2/en/abstract?article+bn02013022013…”

“…The influence of habitat use on the detection probabilities and capture rates of mammals (e.g., Srbek-Araujo & Chiarello 2005, Larrucea et al 2007, Tobler et al 2008, Harmsen et al 2010) is one of the main sources of variation in camera-trap data, reinforcing the fact that comparisons of relative abundance indices between species should always be made with caution (Harmsen et al 2010). The same caution should be applied to compare the same species in the same site or among areas based on similar or different sampling surveys.…”

“…The detection of mammals may vary depending on the animal's sex, age, social status (alpha or beta and resident or transient) and territoriality (Larrucea et al 2007), the distribution of reproductive females, the local abundance of prey (Guil et al 2010), intraspecific and interspecific interactions (Harmsen et al 2010), human pressure and the physical environment (Guil et al 2010). Furthermore, the seasonality of the species can change the intensity with which a given species is recorded (Yasuda 2004), and the capture rate is influenced by the temporal and seasonal activity patterns of the species (Larrucea et al 2007). Differences in detectability among species also can be caused by relatively subtle differences in behavior (Harmsen et al 2010).…”

“…the use of camera-trap records as surrogates for abundance and population density of species whose individuals cannot be identified (Carbone et al 2001, Jennelle et al 2002, Yasuda 2004, Larrucea et al 2007, Srbek-Araujo & Chiarello 2007, Rowcliffe et al 2008, Rovero & Marshall 2009, Harmsen et al 2010. Despite these limitations and recommendations, the index of relative abundance, without calibration, obtained from the rate or frequency of capture has been widely used in camera-trap studies (e.g., Jácomo et al 2004, O'Brien et al 2003, Silveira et al 2003, Trolle & Kéry 2005, Weckel et al 2006, Kasper et al 2007, Rowcliffe et al 2008, Rovero & Marshall 2009, without consideration of the variations in species capture probability (Gu & Swihart 2004).…”