Perceptions of Species Abundance, Distribution, and Diversity:Lessons from Four Decades of Sampling on a Government-Managed Reserve

DNA metabarcoding reveals consumption of diverse community of amphibians by invasive wild pigs (Sus scrofa) in the southeastern United States

Invasive wild pigs (Sus scrofa) are one of the most widespread, destructive vertebrate species globally. Their success can largely be attributed to their generalist diets, which are dominated by plant material but also include diverse animal taxa. Wild pigs are demonstrated nest predators of ground-nesting birds and reptiles, and likely pose a threat to amphibians given their extensive overlap in wetland use. DNA metabarcoding of fecal samples from 222 adult wild pigs culled monthly from 2017 to 2018 revealed a diverse diet dominated by plant material, with 166 plant genera from 56 families and 18 vertebrate species identified. Diet composition varied seasonally with availability for plants and was consistent between sexes. Amphibians were the most frequent vertebrate group consumed and represented the majority of vertebrate species detected, suggesting amphibians are potentially vulnerable to predation by wild pigs in our study region. Mammal, reptile, and bird species were also detected in pig diets, but infrequently. Our results highlight the need for research on the impacts of wild pigs on amphibians to better inform management and conservation of imperiled species.

Evaluating a herpetofaunal monitoring program in the Rolling Plains of Texas

We evaluated the herpetofaunal monitoring program on a state-owned property in the Rolling Plains ecoregion of Texas to determine whether the current study design and survey effort are meeting the following monitoring objectives: (1) inventory herpetofaunal diversity on the property, (2) improve understanding of herpetofaunal habitat preferences, and (3) track herpetofaunal response to land management activities. We used the iChao1 species richness estimator to evaluate the sample completeness of the current dataset, which was collected intermittently from 2004 to 2016. Species richness was evaluated by site and year as well as pooled across sites and years. Observed species richness across all sites and years was 23 species, and estimated richness was 23.999 (23.090-34.052, 95% CI). Observed species richness for each year was less than the cumulative total, with 20 species observed in 2004, 6 species observed in 2013, 11 species observed in 2014, and 19 species observed in 2016. The estimated species richness for individual years of sampling ranged from 6.250 to 39.320 species. Our analysis suggests that inadequate sampling design and effort prevents all three objectives from being addressed using the existing dataset. Necessary improvements to the monitoring program to meet all objectives include increasing the spatial and temporal scale of the monitoring effort and utilizing a random sampling protocol. Monitoring methods that address variation in detection probability, such as occupancy and mark-recapture, will also need to play a role. Our findings show that monitoring programs require substantial investment in materials and personnel to obtain datasets sufficient for rigorous biodiversity monitoring.

A novel approach for estimating densities of secretive species from road-survey and spatial-movement data

Context Accurate estimates of population density are a critical component of effective wildlife conservation and management. However, many snake species are so secretive that their density cannot be determined using traditional methods such as capture–mark–recapture. Thus, the status of most terrestrial snake populations remains completely unknown. Aim We developed a novel simulation-based technique for estimating density of secretive snakes that combined behavioural observations of snake road-crossing behaviour (crossing speed), effort-corrected road-survey data, and simulations of spatial movement patterns derived from radio-telemetry, without relying on mark–recapture. Methods We used radio-telemetry data to parameterise individual-based movement models that estimate the frequency with which individual snakes cross roads and used information on survey vehicle speed and snake crossing speed to determine the probability of detecting a snake, given that it crosses the road transect during a survey. Snake encounter frequencies during systematic road surveys were then interpreted in light of detection probabilities and simulation model results to estimate snake densities and to assess various factors likely to affect abundance estimates. We demonstrated the broad applicability of this approach through a case study of the imperiled southern hognose snake (Heterodon simus) in the North Carolina (USA) Sandhills. Key results We estimated that H. simus occurs at average densities of 0.17 ha–1 in the North Carolina Sandhills and explored the sensitivity of this estimate to assumptions and variation in model parameters. Conclusions Our novel method allowed us to generate the first abundance estimates for H. simus. We found that H. simus exists at low densities relative to congeners and other mid-sized snake species, raising concern that this species may not only have declined in geographic range, but may also occur at low densities or be declining in their strongholds, such as the North Carolina Sandhills. Implications We present a framework for estimating density of species that have traditionally been considered too secretive to study at the population level. This method will greatly enhance our ability to study and manage a wide variety of snake species and could be applied to other secretive wildlife species that are most frequently encountered during road surveys.

Trap characteristics and species morphology explain size-biased sampling of two salamander species

Demographic studies often depend on sampling techniques providing representative samples from populations. However, the sequence of events leading up to a successful capture or detection is susceptible to biases introduced through individual-level behaviour or physiology. Passive sampling techniques may be especially prone to sampling bias caused by size-related phenomena (e.g., physical limitations on trap entrance). We tested for size-biased sampling among five types of passive traps using a 9-year data set for two species of aquatic salamanders that have a 20 and 61 fold change in length over their ontogeny (Amphiuma means, Siren lacertina). Size-biased trapping was evident for both species, with body size distributions (body length mean and SD) of captured individuals differing among sampling techniques. Because our two species differed in girth at similar lengths, we were able to show that size biases (in length) were most likely caused by girth limitations on trap entry rates, and potentially by differences in retention rates. Accounting for the biases of sampling techniques may be critical when assessing current population status and demographic change.