Near-Infrared Spectroscopy (NIRS) as a Method for Biological Sex Discrimination in the Endangered Houston Toad (Anaxyrus houstonensis)

Biological sex is one of the more critically important physiological parameters needed for managing threatened animal species because it is crucial for informing several of the management decisions surrounding conservation breeding programs. Near-infrared spectroscopy (NIRS) is a non-invasive technology that has been recently applied in the field of wildlife science to evaluate various aspects of animal physiology and may have potential as an in vivo technique for determining biological sex in live amphibian species. This study investigated whether NIRS could be used as a rapid and non-invasive method for discriminating biological sex in the endangered Houston toad (Anaxyrus houstonensis). NIR spectra (N = 396) were collected from live A. houstonensis individuals (N = 132), and distinct spectral patterns between males and females were identified using chemometrics. Linear discriminant analysis (PCA-LDA) classified the spectra from each biological sex with accuracy ≥ 98% in the calibration and internal validation datasets and 94% in the external validation process. Through the use of NIRS, we have determined that unique spectral signatures can be holistically captured in the skin of male and female anurans, bringing to light the possibility of further application of this technique for juveniles and sexually monomorphic species, whose sex designation is important for breeding-related decisions.

Optimizing the power of human performed audio surveys for monitoring the endangered Houston toad using automated recording devices

Knowledge regarding the locations of populations of endangered species is a critical part of recovery and facilitates land use planning that avoids unnecessary impacts. Regulatory agencies often support the development of survey guidelines designed to standardize the methods and maximize the probability of detection, thereby avoiding incorrectly concluding a species is absent from a site. Here, using simulations with data collected using automated recording devices (ARDs) we evaluated the efficacy of the existing U.S. Fish and Wildlife Service’s survey requirements for the endangered Houston Toad (Anaxyrus houstonensis). We explored the effect of (1) increasing survey duration, (2) increasing the number of surveys, and (3) combinations of environmental conditions (e.g., temperature, humidity, rainfall) on the detection probability and the number of surveys needed to be 95% confident of absence. We found that increases in both the duration of the survey and the number of surveys conducted decreased the likelihood of incorrectly concluding the species was absent from the site, and that the number of surveys required to be 95% confident greatly exceeded the existing survey requirements. Targeting specific environmental conditions was also an effective way to decrease the number of surveys required but the infrequency in which these conditions occurred might make application difficult in some years. Overall, we suggest that the survey effort necessary to achieve confidence in the absence of Houston Toads at a site is more practically achievable with the use of ARDs, but this may not be suitable in all monitoring scenarios.

Evaluating the effects of red imported fire ants (Solenopsis invicta) on juvenile Houston Toads (Bufo [=Anaxyrus] houstonensis) in Colorado County, TX

The spread of invasive species is considered a major threat to biodiversity, second only to habitat loss. Red Imported Fire Ants (Solenopsis invicta) are a globally invasive species with negative impacts reported on native invertebrate and vertebrate species. Federally endangered Houston Toads (Bufo [=Anaxyrus] houstonensis), endemic to Texas, are among the vertebrates reportedly negatively impacted by Red Imported Fire Ants (RIFA). Threats posed by RIFA to Houston Toads needed to be explicitly characterized. Large-scale chemical treatments to suppress RIFA and facilitate brood survival in Attwater’s prairie-chickens (Tympanuchus cupido attwateri) at the Attwater Prairie Chicken National Wildlife Refuge (APCNWR) afforded us an opportunity to experimentally examine the influence of RIFA abundance on juvenile Houston Toad growth and survival. We also sought to examine whether juvenile Houston Toads could grow and survive in a vegetation type similar to a historic species locality. We conducted a terrestrial mesocosm experiment to test whether the application of bait-driven suppressant decreased counts of RIFA relative to untreated sites. We examined whether counts of native ant and non-ant native invertebrates were higher in response to potential decreases in RIFA. We compared growth and survival rates in juvenile Houston Toads among treated and untreated sites, expecting juvenile growth and survival to be higher in response to potentially decreased RIFA counts and increased native invertebrate counts. We saw lower counts of RIFA in treated prairies, but we also observed a decrease in native ant counts possibly due to chemical treatment. Therefore, the application of bait-driven suppressant may not affect RIFA alone. We saw no difference in counts of non-ant invertebrates among treated and untreated sites. Juvenile Houston Toads did not differ in growth and survival among treated and untreated sites. We recognize that the lack of a relationship between juvenile growth and survival with a treatment effect, and therefore RIFA abundance, may be limited to APCNWR. We encourage additional experimental studies to elucidate RIFA impacts at other sites. We extrapolated apparent survival estimates from our study to one year. These appear comparable to juvenile survivorship required in simulations for Houston Toad population persistence and on this basis, we recommend that APCNWR be re-evaluated as a reintroduction site for Houston Toads. We also recommend further studies to potentially broaden the regulatory definition of Houston Toad habitat beyond the current restrictive view of canopied forest alone. Such studies would need to examine the utility of native grasslands as dispersal corridors/upland habitat for juvenile Houston Toads. Our findings emphasize the utility of experimental studies in directly examining the influence of perceived threats to imperiled species and the role of such clarifications in adapting recovery efforts.