Broad-headed snakes (Hoplocephalus bungaroides) are small (< 90 cm snout-vent length), live-bearing, venomous elapid snakes. They are nocturnal, and juveniles feed mostly on lizards that they ambush beneath rocks. Adults have a broader diet that includes lizards, birds, and small mammals29,34. In Morton National Park, the location of an isolated and deeply diverged southern clade of the species31, broad-headed snakes grow slowly, reaching maturity at 5–6 years35. The species is long lived (up to 28 years), and they have long generation lengths (10.4 years28). Mating occurs between autumn and spring, females ovulate in late spring, offspring are born in March and April, and females rarely reproduce annually28,29. During the cooler months (May–October), broad-headed snakes occupy the western edge of exposed sandstone plateaux where they shelter under small, exposed rocks that have exfoliated away from underlying sandstone substratum29. During the warmer months (November–April), broad-headed snakes leave the exposed outcrops for the shelter of old growth eucalypt forests where they use tree hollows as refugia36. It is during the cooler months, when the majority of snakes in the population shelter beneath small, thin, exfoliating rocks on easily accessible ridgelines, that they are most vulnerable to illegal poaching27.
Field sites and methods
We studied a metapopulation of broad-headed snakes located approximately 160 km south of Sydney, New South Wales, Australia. One population was located on a sandstone plateau (roughly 20 km long by 2 km wide) inside Morton National Park (henceforth, ‘gated’). The other population was located 6 km east on a sandstone plateau (roughly 27 km long by 9 km wide) on crown land (i.e. public land without tenure; henceforth, ‘ungated’). The two populations are separated by a steep valley dissected by a river, but there is some gene flow from the gated population to the ungated population37. At the gated population, there are two sets of locked gates at the entry and at access points of the fire trail that traverses the plateau. These gates were installed in 2008 and have since made it more difficult for people to access and disturb rock outcrops, including reptile poachers and rock collectors. By contrast, the ungated population is easily accessible to poachers because there are no locked gates on the fire trails which crisscross the plateau. Many fire trails throughout the ungated population terminate at rock outcrops, so poachers do not have to walk far (often just metres) to find broad-headed snakes. Because previous publications detailing the specific location of these populations have led to increased habitat disturbance and poaching of snakes27, we opted not to include location details or maps.
Since 1992 (gated population) and 2007 (ungated population), one of us (JKW) has carried out annual mark-recapture studies of these populations of broad-headed snakes. Each year during late winter and/or spring, a team of herpetologists (usually 3–5 people) surveyed four study sites at the gated population and three study sites at the ungated population. At each site, the team lifted all sun-exposed rocks that could be lifted without risking a back injury. All reptiles found under rocks were identified and recorded, and any broad-headed snakes or small-eyed snakes (Crytophis nigrescens) were hand captured and briefly held with thick welding gloves. If a broad-headed snake or small-eyed snake was captured, the researcher measured the snout-vent length and tail length (with a ruler, to nearest mm), determined the sex (via tail shape), and weight (with spring balance, to nearest g). We recorded the snakes’ microchip number, and if the snake was unmarked, we injected a miniature transponder (Trovan Midichip 8 mm x 1.4 mm) under the skin. During surveys, the team noted whether rocks had been disturbed by humans, and if so, the nature of the disturbance (i.e. whether rocks were overturned, displaced, or broken). Disturbed rocks were easily identified because aside from being displaced or overturned, they often had the remains of squashed invertebrates or vertebrates (lizards and frogs) beneath them. Any unmarked rocks were given a unique identification number (with a paint pen, underneath the rock) to enable us to assess long-term usage of rocks by snakes, and disturbance to rocks. After processing each snake, the rock was returned to its exact original location, and the snake was released under the rock.
All procedures were approved by the University of Technology Sydney Animal Ethics Committee and were carried out in accordance with relevant guidelines and regulations under licence from state and federal wildlife agencies.
Evidence for removal of snakes from the ungated population
Because sites at the ungated population had higher levels of disturbance than sites at the gated population33, we hypothesised that humans were removing snakes for the illegal pet trade. If poaching has occurred, broad-headed snakes from ungated sites should have lower survival rates than snakes from gated sites. To test this hypothesis, we analysed mark-recapture data collected from 2007–2019 using Cormack-Jolly-Seber (CJS) models in Program MARK v9.038. Because previous studies have shown that survival rates vary with age and size28,35, we allocated each snake to one of two size classes (sub-adults and adults, SVL > 349 mm [henceforth ‘adults’], and juveniles, SVL < 350 mm). To investigate whether survival rates differed among populations, we allocated each snake to one of two populations (gated or ungated). Thus, there were four groups in the input file: gated adults, gated juveniles, ungated adults, and ungated juveniles. Next, we ran a series of models in MARK to test the following a priori hypotheses: (1) survival rates are higher in gated than ungated sites; (2) survival rates of adults and juveniles are higher in gated than ungated sites; (3) survival rates vary through time independent of location; and (4) survival rates are constant independent of location. For these hypotheses, we ran equivalent survival models in which the probabilities of recapture were constant, time-dependent, or group-dependent. We then used the Akaike Information Criterion corrected for small sample size (AICc) to identify the most parsimonious model from the candidate model set39.
Estimation of life history parameters
Life history parameters were either estimated in this study or were taken from previously published studies of these populations (Table 1). Because good estimates of parameter uncertainty are necessary to construct informative stochastic demographic models, we used program MARK to obtain estimates of environmental (process) variation around survival rates. We did this using the variance components subroutine in MARK (appendix D, MARK Book v 19, see 40). For this analysis, we used the mark-recapture data set for the gated population, with two groups (juveniles and adults). We then ran CJS models, and estimated variance components for each age class separately from the model . Initial population size estimates were obtained from previous analyses that extrapolated estimates from study sites to the plateau (Table 128). Although our initial population sizes are extrapolations upon estimates and are potentially imprecise, we accounted for this uncertainty by varying initial population size to see what effect it has on population growth rates (see below). Using previously published literature from these populations and parameters estimated in this study, we obtained the following life history parameters required for population viability analysis: (1) age at maturity for males and females; (2) maximum age of reproduction; (3) maximum litter size; (4) sex ratio at birth; (5) percentage of females breeding annually; (6) mean number of offspring per female; (7) biological survival rates calculated by age class; and (8) dispersal rates (Table 1).
Population growth rate is central to our ability to predict population dynamics41 and is essentially governed by rates of fecundity, survival, immigration and emigration. Our ability to estimate fecundity and survival is relatively robust due to the long-term demographic data available for these populations. Although we cannot differentiate between mortality and permanent emigration, adult broad-headed snakes show site fidelity, and are often recaptured underneath the same rocks where they were originally captured29,42. Furthermore, there is virtually no suitable habitat outside of the national park into which snakes can permanently emigrate31,43. Population genetics for our study area showed that dispersal occurred in the juvenile life stage and was unidirectional, with juvenile dispersal movements occurring only from the gated population into the ungated population37. Thus, we have reasonable justification to treat the metapopulation as closed.
Population trajectories using population viability analysis
To determine the effects of poaching on the ungated population, we used the program Vortex 10.2.5.044 to conduct 100-year baseline scenario population viability analyses (PVA), using 1000 iterations to obtain a mean population growth rate and a probability of population extinction. The baseline scenario (Fig. 1) uses our best estimates for life history parameters, all of which are either estimated in this study (see Table 1 & Results) or are taken from previously published results from these populations of broad-headed snakes (Table 1). Where we were uncertain about any life history parameters, we employed subsequent sensitivity tests that artificially varied the parameters to determine their effect on population growth rate (see below; Table 2).
Using our mark-recapture data we calculated the age-specific survival rates of each population (see Results) and found that the annual survival of our ungated population was substantially lower than that of our gated population for both juveniles and adults (Table 1). These survival rates, however, included the impacts of illegal poaching activities. To investigate how poaching affects population viability, we made the assumption that without this impact both populations would have the same survival rates as the gated population. Therefore, for all subsequent analyses, we assumed all life history parameters to be equal between both ungated and gated populations, with unidirectional dispersal from the gated population into the ungated population.
Whilst useful for predicting population trajectories45, population viability analysis does not provide quantitative information about which parameters most influence population trajectories through time, nor does it account for uncertainties in estimated life history parameters46. To account for these uncertainties and directly assess the influence of different parameters, we artificially varied the values of five parameters for sensitivity testing in Vortex, including: (1) immigration; (2) initial population size (n); (3) carrying capacity (k); (4) harvest rate of adult females; and (5) harvest rate of adult males (Table 2). All parameters of the gated population were maintained at base scenario levels for sensitivity analysis.
Since we were interested in the relative importance of each parameter on the population growth rate, we limited variation around the parameter to values that we as experts considered biologically plausible. Unidirectional dispersal of juvenile snakes (1–3 years) from the gated population to the ungated population was estimated to be at a rate of ~4% of the juvenile population of origin (Table 137), and this value was allowed to vary between 0–8%. The size of our gated population had been previous estimated as 595 (95% CI: 389–808) snakes27, and since the available habitat of both ridges is similar in extent, we allowed initial ungated population to vary between 100–1200 snakes. We assumed the lowest possible carrying capacity for both populations to be close to the estimated initial population size and allowed the carrying capacity of the ungated population to double (n = 600–1200). Since poaching of broad-headed snakes is illegal, we have little knowledge about how many snakes are removed each year. We do, however, know the difference in survival between the gated and ungated populations. Since annual adult survival in the ungated population is about ~22% lower than that of the gated population, we can estimate that about 22% of adults (n ≈ 48) at the initial total population size (n = 600) are being removed from the population, or at least are not persisting in the population, each year. Given this uncertainty, to investigate the effect of poaching on population viability, we varied the rate of harvesting of male and female adults between 1–30 individuals, respectively (Table 2). In each iteration of the sensitivity analyses, stochastic variation was maintained at the same level as the base scenario. To optimise sampling of the available parameter space for each parameter, we ran 250 sensitivity samples in Vortex using the Latin hypercube sampling method.
For analysis, the summary output file from the sensitivity test in Vortex was read into the statistical program R version 4.0.247. To allow for comparisons between the effects of each parameter on the stochastic population growth rate, all predictor variables were scaled and centred. To ensure there was limited correlation between the artificially manipulated predictor variables, we created pairwise scatterplots and calculated correlation coefficients using the pairs.panels function using the ‘psych’ package48.
A generalised linear model was constructed with the stochastic population growth rate as the response variable and all artificially varied parameters as predictor variables. Model predictions for the population growth rate were plotted against the main effects to visualise the relative slopes of the predictor variables. The anova function was used on the model object to produce an analysis of deviance table, and the percentage of deviance explained by each of the predictor variables was recorded.
Effects of poaching on population viability
To investigate and visualise how poaching may be affecting the population viability of the ungated population, we maintained the assumption that without the impact of poaching both populations would have the survival rates of the gated population. To investigate whether this was a sound assumption, we ran a second population viability analysis, where survival rates were kept at the observed levels for the gated population and harvesting of adult females (n = 22) was imposed on the ungated population. We also included unidirectional dispersal from the gated population into the ungated population.