Integrating telemetry data at several scales with spatial capture–recapture to improve density estimates

Abstract

Accurate population estimates are essential for monitoring and managing wildlife populations. Mark-recapture sampling methods have regularly been used to estimate population parameters for rare and cryptic species, including the federally listed Mojave desert tortoise (Gopherus agassizii); however, the methods employed are often plagued by violations of statistical assumptions, which have the potential to bias density estimates. By incorporating spatial information into conventional density estimation models, spatial capture-recapture (SCR) models can account for common assumption violations such as spatially heterogeneous detection probabilities and temporary emigration when animals leave plots during a survey. We conducted mark-recapture surveys at 10 1-km 2 plots in and adjacent to the Ivanpah Valley of California and Nevada from 2015 to 2019. Locality data were collected concurrently using radio-telemetry and GPS data loggers. GPS data demonstrated that desert tortoises frequently exhibited temporary emigration outside a plot during the survey periods, thereby complicating standard approaches for closed-model density estimation. We integrated mark-recapture survey data for subadults and adults at each plot with corresponding spatial capture locations and supplementary spatial data using a modified SCR model fitted in a Bayesian framework. We compared density estimates modeled with conventional non-spatial methods, as well as three SCR models based on symmetrical usage areas described by various levels and types of supplementary spatial data. The conventional model consistently resulted in inflated estimates of density while the SCR models allowed us to generate spatially corrected estimates for a species where detectability and densities are low. However, we found that if not properly specified, the temporal scale of supplementary data may result in an unintended source of bias in parameter estimates. Integrating spatial data over a larger temporal scale than mark-recapture surveys were conducted resulted in higher detection probabilities and lower density estimates, due to an overestimation of space use. Our results not only demonstrate the importance of accounting for spatial information but also the value of understanding the potential for bias when integrating multiple data sets at different temporal resolutions. The methods presented can be used to enhance monitoring efforts for the Mojave desert tortoise and other species where mark-recapture methods are used.

Integrated animal movement and spatial capture–recapture models: Simulation, implementation, and inference

“…More common in SCR studies is that the recapture rate is lower, for example, L opez-Bao et al ( 2018) detected 65 individual wolves from scat surveys, but the recapture rate was only 1.46. Similarly, Mitchell et al (2021) found 143 individual tortoises in their study but had very few recaptures. However, both studies collected concurrent telemetry data that were used to improve the fit of standard SCR models and, with the models presented here, these types of studies could be expanded to examine resource use and the potential effects on the demographic rates.…”

“…Taking advantage of advancements in bio‐logging data, we can fully capitalize on integrated SCR movement models and provide a means for a better understanding of the ecological processes driving movement and demography. However, this integration presents some challenges, including spatial or temporal mismatch between the SCR data and the location data (Mitchell et al, 2021). In cases of spatial mismatch, auxiliary location data could be used as the basis for an informative prior on the movement model parameters (e.g., σ 2 , γ) or included in the movement model likelihood, but they should not be used to inform the detection parameters (e.g., $$$ {\upsigma}_{\mathrm{det}}^2 $$$, λ 0 ) or population size ( N ).…”

A spatially explicit model for density that accounts for availability: a case study withMojavedesert tortoises

“…Previous studies have identified environmental features associated with the presence of tortoises throughout their geographic range (Nussear et al, 2009), but less effort has been directed toward identifying environmental features that explain variation in density. Although other studies have provided local estimates of density (e.g., Berry et al, 2020; Mitchell et al, 2021), data limitations and computational hurdles have made it challenging to generate rigorous estimates that are comparable across the species' range. By leveraging data from the range‐wide survey effort led by USFWS and using recent modeling and software advancements, we were able to estimate local densities and regional trends across much of the range of the Mojave desert tortoise.…”

“…Estimates of density across the survey region were low (<10 tortoises/km 2 ) compared with historical estimates of some local populations that ranged as high as 50–100 adult tortoises/km 2 (Berry & Medica, 1995; Berry et al, 2020). These examples are exceptional and not representative of tortoise populations range‐wide, however, as they are likely biased high due to violations of analytical assumptions (Mitchell et al, 2021; USFWS, 2011). On average, our density estimates were also lower than many contemporary estimates (e.g., Mitchell et al, 2021).…”

“…These examples are exceptional and not representative of tortoise populations range‐wide, however, as they are likely biased high due to violations of analytical assumptions (Mitchell et al, 2021; USFWS, 2011). On average, our density estimates were also lower than many contemporary estimates (e.g., Mitchell et al, 2021). Although variation in estimates may result from different analytical approaches (e.g., incorporating trends, estimating density of different size classes), it also likely reflects that, unlike many long‐term monitoring studies, our distance‐sampling transects were located randomly, which reduced bias, and encompassed areas of low‐quality habitat that cannot support high densities of adult tortoises.…”