Invasive brown treesnakes (Boiga irregularis) move short distances and have small activity areas in a high prey environment

Abstract

Animal movements reflect temporal and spatial availability of resources as well as when, where, and how individuals access such resources. To test these relationships for a predatory reptile, we quantified the effects of prey abundance on the spatial ecology of invasive brown treesnakes (Boiga irregularis) on Guam. Five months after toxicant-mediated suppression of a brown treesnake population, we simultaneously used visual encounter surveys to generate relative rodent abundance and radiotelemetry of snakes to document movements of surviving snakes. After snake suppression, encounter rates for small mammals increased 22-fold and brown treesnakes had smaller mean daily movement distances (24 ± 13 m/day, [Formula: see text] ± SD) and activity areas (5.47 ± 5 ha) than all previous observations. Additionally, snakes frequenting forest edges, where our small mammal encounters were the highest, had smaller mean daily movement distances and three-dimensional activity volumes compared to those within the forest interior. Collectively, these results suggest that reduced movements by snakes were in part a response to increased prey availability. The impact of prey availability on snake movement may be a management consideration when attempting to control cryptic invasive species using tools that rely on movement of the target species to be effective.

Figure 1

Movement patterns (average linear distance moved per day) on Guam of four radiotracked BTS in this study. Within each plot, we have indicated times when a prey bulge (indicative of prey consumption) was observed (red arrows) and, for females, estimated onset of vitellogenesis (blue arrows, estimated using follicular growth data obtained from Mathies et al.). At the top of each movement plot is each snake’s % change in body mass (relative to mass at the time of transmitter implantation, precise value indicated as white text within bar), their activity area (minimum convex polygon [MCP] method) within the Habitat Management Unit (HMU, perimeter of HMU indicated as black outline), and their size in snout-vent length (SVL, relative size indicated by black snake silhouette with precise numerical value in mm above).

Figure 2

Frequency distribution of average distance moved (straight-line distance between successive locations/number of intervening days) per day for radiotracked brown treesnakes (BTS) in this study. The distribution was significantly right-skewed (Skewness = 2.55, SE of Skewness = 0.08).

Figure 3

Three-dimensional activity volumes (3D Kernel Utilization Distributions, KUDs) of radiotracked brown treesnakes (BTS) frequenting the forest edge (A blue 75% isopleths surrounded by grey 95% isopleths), the forest interior (B green 75% isopleths surrounded by grey 95% isopleths), and combined (C) on Guam. Edge snakes (those with > 30% [mean = 63 ± 22%] of observations within 50 m of forest edge) had significantly smaller 3D activity volumes compared to interior snakes (those with < 30% [mean = 13 ± 7%] of observations within 50 m of forest edge; df = 18, T = − 2.29, p = 0.04). Outer black line in each panel represents the Habitat Management Unit (HMU, 13.596 N, 144.865 E) boundary fence. Inset shows the relative position of the HMU (indicated by the star) on the north end of Guam.

Figure 4

Examples of large (18.53 ha³) and small (3.06 ha³) 3D activity volumes from two male brown treesnakes (Boiga irregularis) with 1820 mm snout-vent length (SVL) (A, blue 75% isopleths surrounded by grey 95% isopleths) and a 982 mm SVL (B, green 75% isopleths surrounded by grey 95% isopleths). Two plots are shown in each panel, one from the lateral perspective as if you were standing on the ground (height = 0) and the other from approximately 45° angle from above. Note how both 3D activity volumes approximate a series of oblate spheroid shapes.

Figure 5

Frequency distribution of perch heights (m) for radiotelemetered male (A, n = 9) and female (B, n = 11) brown treesnakes (Boiga irregularis) after snake suppression within the Habitat Management Unit (HMU) on Guam. Notice the difference in the frequency of heights < 0 (underground) between the sexes reflecting an increased use of underground retreats by females.