Activity of tjakura (great desert skinks) at burrows in relation to plant cover and predators
Data files
Jul 28, 2023 version files 32.39 KB
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Activity_All_Burrows.csv
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Activity_Totals_All.csv
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Dawn_Dusk_All_Burrows.csv
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General_Activity_2014.csv
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General_Activity_2015.csv
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Predator_Details_All_Burrows.csv
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Predator_Visits_All_Burrows.csv
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README.md
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Substudy_All_Burrows.csv
Abstract
Increased predation where ground cover is reduced after severe wildfire is increasingly implicated as a factor causing decline of vulnerable prey populations. In arid central Australia, one species detrimentally affected by repeated wildfire is the great desert skink or tjakura (Liopholis kintorei), a distinctive lizard of the central Australian arid zone that constructs and inhabits multi-entranced communal burrows. We aimed to test whether tjakura or predator activity at burrow entrances varied with cover and how tjakura respond to predator presence. Using time-lapse photography, we monitored tjakura and predator activity at the largest entrance of 12 burrows ranging from high (>70 %) to low (<50 %) cover and at multiple entrances of two other burrows. Overall activity did not vary between burrows with high and low cover. Within burrow systems, tjakura were more active at sparsely vegetated entrances, often sitting wholly or partly inside the burrow. However, consistent between and within burrow systems, skinks spent proportionally more time fully outside where cover was higher. Predators – mostly native – were detected at most burrows, with no apparent relationship between predator activity and cover. Skinks also did not appear to modify their activity in response to predator visits. Our results indicate that tjakura may spend more time outside burrow entrances when cover is higher, but there was no direct evidence that this related to perceived or real predation risk. Differences in food availability, thermoregulatory opportunities, and opportunities for ambush foraging associated with differences in vegetation cover or composition are other factors likely to be important in determining the activity of tjakura at burrows. Our research demonstrates the usefulness of camera traps for behavioural studies of ectothermic burrowing animals. The complex relationships between tjakura activity and vegetation cover were thereby revealed, suggesting outcomes of fire-mediated habitat change on predator-prey interactions are not easily predictable.
Methods
Data was collected via camera trapping.
Data Analysis
The images were initially sorted into three categories: one or more tjakura visible, other vertebrate species visible, or no vertebrate visible. The images with skinks present were then divided into three position categories: fully inside the burrow entrance, half emerged with front legs visible, or fully emerged from the burrow entrance, with hind legs visible outside (Fig. 2). There were three clearly distinct size classes of tjakura in the images including very small, medium, and large or adult size, so we were able to assign individuals into three approximate age classes; juvenile (current year hatchlings), sub-adult, or adult. Where there were two or more tjakura in the image, each was classified separately for size and position.
We were not always able to distinguish among different individuals from the same burrow. However, obvious differences in size, colour, the presence of distinguishing features (e.g. part of the tail missing), or the presence of multiple individuals in single images allowed us to determine a minimum number of individuals present at each burrow during each recording period.
Activity at burrows in relation to burrow characteristics and vegetation
Repeated measures analysis of variance was used to determine whether the minimum number of individuals varied between the two recording periods (within-subjects effect) or between burrows with high and low vegetation cover. We then used Pearson correlations to test how tjakura activity at the main burrow entrance related to the minimum number of lizards occupying the burrow, the number of entrances in the burrow system, and vegetation cover at the burrow.
Response by tjakura to predator activity at burrows
All vertebrates known or suspected to be predators of tjakura were included in analyses. The number of discrete ‘visits’ by predators to burrows was determined based on the sequence of images. If the same predator was visible in consecutive images, this was counted as a single visit. We then tested for correlations between vegetation cover at burrows and the number of predator visits.
To determine whether tjakura reduced their activity in response to predator visits the relationship between active bouts of skinks and predator visits was first interpreted graphically. The activity of skinks before and after predator visits was compared based on (a) the time interval (min) between a tjakura being visible and the arrival of a predator and the interval (min) between a predator leaving and a tjakura becoming visible again, and (b) the number of active bouts by tjakura in the 24 hours prior to predator arrival and post predator departure. The null hypothesis (no effect of predators) was that there would be no difference in the 24 hours before and after a predator visit in the time interval between tjakura activity and predator activity or the number of tjakura active bouts. Time data were log-transformed prior to analyses.
Daily activity patterns of tjakura
To investigate patterns in daily activity we plotted the time tjakura spent outside the burrow against the average activity of tjakura in the early and late active seasons. The early active season was considered as the time between tjakura first emerging during spring until end of December and the late active season as roughly January to March. We then split the data into dawn, day, dusk and night. Dawn and dusk were defined as an hour-long period during which sunrise and sunset occurred. In late 2014 dawn fell approximately between 0500 and 0600 (sunrise range 5:47am – 5:49am) and in early 2015 dawn fell between 0600 and 0700 (sunrise range: 6:15am – 6:58am), and dusk in both years was approximately between 1900 and 2000 (sunset Nov/Dec range: 7:19pm – 7:32pm, Jan/Feb: 7:39pm – 7:31pm) (Australian Government Bureau of Meteorology, http://www.bom.gov.au, accessed July 2021). We used a 3-factor repeated measures ANOVA with time (dawn/day/dusk/night) and vegetation (low/high) as the fixed factors and burrow site as a random factor nested within vegetation and Tukey’s post hoc test was used for pair-wise comparisons. To do this, we converted number of images to images per hour to account for the different lengths of each period.
Tjakura activity at multiple entrances within a burrow system
Two additional burrows were selected from the remaining eight of the 20 active burrows; one (Burrow A) with four entrances and one (Burrow B) with seven entrances. Reconyx cameras were set up at every burrow entrance in the same way as described previously. Width, height, opening, diameter, and height (cm) of the mound, as well as percentage cover of vegetation in a 1m2 quadrat centred over each entrance were measured and the main entrance was identified, according to previous protocols.
Activity at burrow entrances was recorded at all entrances simultaneously for 4 days in early February 2015. Cameras were set to time-lapse taking 1 picture per minute. One camera (at Burrow A, Entrance 1) malfunctioned, and that entrance was excluded from all analyses. We used correlations to test whether the level of activity at burrow entrances was associated with burrow size, minimum number of individuals using the system, overall activity, and vegetation cover at each entrance.
Usage notes
All data files are in CSV format.