To cross or not to cross: modeling wildlife road crossings as a binary response variable with contextual predictors

Abstract

Citation: Siers, S. R., R. N. Reed, and J. A. Savidge. 2016. To cross or not to cross: modeling wildlife road crossings as a binary response variable with contextual predictors. Ecosphere 7(5):e01292. 10. 1002/ecs2.1292 Abstract. Roads are significant barriers to landscape-scale movements of individuals or populations of many wildlife taxa. The decision by an animal near a road to either cross or not cross may be influenced by characteristics of the road, environmental conditions, traits of the individual animal, and other aspects of the context within which the decision is made. We considered such factors in a mixed-effects logistic regression model describing the nightly road crossing probabilities of invasive nocturnal Brown Treesnakes (Boiga irregularis) through short-term radiotracking of 691 snakes within close proximity to 50 road segments across the island of Guam. All measures of road magnitude (traffic volume, gap width, surface type, etc.) were significantly negatively correlated with crossing probabilities. Snake body size was the only intrinsic factor associated with crossing rates, with larger snakes crossing roads more frequently. Humidity was the only environmental variable affecting crossing rate. The distance of the snake from the road at the start of nightly movement trials was the most significant predictor of crossings. The presence of snake traps with live mouse lures during a portion of the trials indicated that localized prey cues reduced the probability of a snake crossing the road away from the traps, suggesting that a snake's decision to cross roads is influenced by local foraging opportunities. Per capita road crossing rates of Brown Treesnakes were very low, and comparisons to historical records suggest that crossing rates have declined in the 60+ yr since introduction to Guam. We report a simplified model that will allow managers to predict road crossing rates based on snake, road, and contextual characteristics. Road crossing simulations based on actual snake size distributions demonstrate that populations with size distributions skewed toward larger snakes will result in a higher number of road crossings. Our method of modeling per capita road crossing probabilities as a binary response variable, influenced by contextual factors, may be useful for describing or predicting road crossings by individuals of other taxa provided that appropriate spatial and temporal resolution can be achieved and that potentially influential covariate data can be obtained.

An objective road risk assessment method for multiple species: ranking 166 reptiles and amphibians in California

“…In addition to being attracted to paved road surfaces for thermoregulation, many large snakes have wide homeranges or may move large distances between winter hibernacula and summer foraging areas. In contrast to smaller species, larger snakes are also less likely to avoid roads (Rosen and Lowe 1994;Andrews and Gibbons 2005;Andrews et al 2008;Siers et al 2016). High road mortality (e.g., Klauber 1931;Rosen and Lowe 1994;Jones et al 2011), reduced abundance near roads (Rudolph et al 1999;Jones et al 2011), increased extinction risk (Row et al 2007), and decreased genetic diversity (Clark et al 2010;Hermann et al 2017) have been documented for numerous snake species; as have positive responses to barriers and underpasses (Dodd et al 2004;Colley et al 2017).…”

Behavioral differences following ingestion of large meals and consequences for management of a harmful invasive snake: A field experiment

“…We obtained estimated location coordinates with handheld global positioning system (GPS) units. The Euclidean distance between successive daily refugia locations was calculated as the daily relocation distance, a standard daily movement metric of brown treesnake telemetry studies (e.g., Anderson et al., ; Christy et al., ; Santana‐Bendix, ; Siers et al., ; Tobin et al., ), although it is well understood that the actual foraging/movement path can far exceed this distance (Christy et al., ; Clark, ; Lardner, Savidge, Reed, & Rodda, ; Tobin et al., ).…”

“…Snakes assigned to the unfed control group were fed a transmitter which was manually massaged down the esophagus into the stomach, aided by application of a water-based lubricant. Force feeding of transmitters alone is a lower stress procedure relative to surgical implantation and has been successfully employed with no detected behavioral artefacts in a previous study (Siers et al, 2016).…”

“…Effectiveness of visual surveys, trapping, and toxic bait tools for brown treesnake control has been demonstrated to be influenced by internal factors (sex, size, and body condition) and external factors (availability of alternative prey; Gragg et al., ; Rodda, Savidge, Tyrrell, Christy, & Ellingson, ; Tyrell et al., ; Christy, Yackel Adams, Rodda, Savidge, & Tyrrell, ; Lardner et al., ; Christy, Savidge, Yackel Adams, Gragg, & Rodda, ; Siers, Savidge, & Reed, ). These factors have also been indicated to influence brown treesnake movement characteristics (Santana‐Bendix, ; Tobin, Sugihara, Pochop, & Linnell, ; Siers, Reed, & Savidge, ; Christy et al., ). If brown treesnakes decrease movement and foraging during digestion, this will have implications for the effectiveness of various control tools that typically rely either on visual detection by human searchers or response of foraging snakes to lures or baits (e.g., Christy et al., ; Clark et al., ; Engeman & Vice, ; Lardner et al., ; Tyrell et al., ).…”

Burmese Pythons

“…A more extensive analysis showed that traf c volume, gap width, and surface type all were negatively related to the probability of a snake crossing a road, and those snakes that did cross tended to be larger. Proximity to traps along a perimeter also decreased the probability of a snake crossing a road (i.e., possible demonstration of an intercepting effect) (Siers et al 2014(Siers et al , 2016.…”