Added Value: Gopher Tortoise Surveys Provide Estimates of Gopher Frog Abundance in Tortoise Burrows

The gopher frog Lithobates capito is one of the most terrestrial frogs in the southeastern U.S. and often inhabits gopher tortoise burrows Gopherus polyphemus outside of the breeding season. Gopher frog populations have declined, and the species is under review for listing as threatened or endangered under the U.S. Endangered Species Act. Much of our knowledge on the status of gopher frogs is based on detections of larvae at breeding wetlands, which can be challenging due to environmental variability and provides no information on the terrestrial life stages of the species. Therefore, an alternative method is called for. We recorded observations of gopher frogs during gopher tortoise surveys at four conservation lands in Florida and used line transect distance sampling to estimate frog abundance. We also recorded burrow size, incidence of frog co-occupancy with tortoises, and distance from frog-occupied burrows to breeding wetlands. We observed 274 gopher frogs in 1,097 tortoise burrows at the four sites. The proportion of burrows occupied by gopher frogs among sites ranged from 0.17 to 0.25. Gopher frog abundance in tortoise burrows was 742 (512–1,076 95% CL) at Etoniah Creek State Forest, 465 (352–615) at Ft. White Wildlife Environmental Area, 411(283–595) at Mike Roess Gold Head Branch State Park, and 134 (97–186) at Watermelon Pond Wildlife Environmental Area. We observed up to four frogs in a single burrow. The proportion of frogs detected in burrows occupied by a gopher tortoise ranged from 0.46 to 0.79 among sites, and overall, gopher frogs preferred burrows occupied by tortoises over unoccupied burrows (χ 2 = 15.875, df=3, p = 0.001). Gopher frogs used burrows from 7 to 43 cm in width; mean width of frog-occupied burrows did not differ from that of unoccupied burrows ( F 1,3 = 0.0492, p = 0.8245). Distance from frog-occupied tortoise burrows to the nearest breeding wetland ranged from 141 to 3,402 m. Our data on gopher frogs collected in conjunction with gopher tortoise monitoring efforts using line transect distance sampling and burrow cameras provided novel information on frog abundance in their terrestrial habitat and required no additional effort. However, the extent to which frogs use tortoise burrows relative to other available refuges (small mammal burrows, stumps, or other structures) is unknown, thus our estimates should be considered conservative. We suggest that terrestrial abundance estimates for gopher frogs can complement efforts to monitor breeding activity to provide a more comprehensive means of monitoring population trends in this cryptic species.

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Techniques for Estimating the Size of Low-Density Gopher Tortoise Populations

Journal of Fish and Wildlife Management ◽

10.3996/012017-jfwm-005 ◽

2017 ◽

Vol 8 (2) ◽

pp. 377-386 ◽

Cited By ~ 2

Author(s):

Jonathan M. Stober ◽

Rocio Prieto-Gonzalez ◽

Lora L. Smith ◽

Tiago A. Marques ◽

Len Thomas

Keyword(s):

Habitat Management ◽

Distance Sampling ◽

Probability Of Detection ◽

Line Transect ◽

Efficient Design ◽

Gopher Tortoise ◽

Gopherus Polyphemus ◽

Line Transect Survey ◽

Estimate Population Size ◽

Survey Designs

Abstract Gopher tortoises (Gopherus polyphemus) are candidates for range-wide listing as threatened under the U.S. Endangered Species Act. Reliable population estimates are important to inform policy and management for recovery of the species. Line transect distance sampling has been adopted as the preferred method to estimate population size. However, when tortoise density is low, it can be challenging to obtain enough tortoise observations to reliably estimate the probability of detection, a vital component of the method. We suggest a modification to the method based on counting usable tortoise burrows (more abundant than tortoises) and separately accounting for the proportion of burrows occupied by tortoises. The increased sample size of burrows can outweigh the additional uncertainty induced by the need to account for the proportion of burrows occupied. We demonstrate the method using surveys conducted within a 13,118-ha portion of the Gopher Tortoise Habitat Management Unit at Fort Gordon Army Installation, Georgia. We used a systematic random design to obtain more precise estimates, using a newly developed systematic variance estimator. Individual transects had a spatially efficient design (pseudocircuits), which greatly improved sampling efficiency on this large site. Estimated burrow density was 0.091 ± 0.011 burrows/ha (CV = 12.6%, 95% CI = 0.071–0.116), with 25% of burrows occupied by a tortoise (CV = 14.4%), yielding a tortoise density of 0.023 ± 0.004 tortoise/ha (CV = 19.0%, 95% CI = 0.016–0.033) and a population estimate of 297 tortoises (95% CI = 210–433). These techniques are applicable to other studies and species. Surveying burrows or nests, rather than animals, can produce more reliable estimates when it leads to a significantly larger sample of detections and when the occupancy status can reliably be ascertained. Systematic line transect survey designs give better precision and are practical to implement and analyze.

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Precision, accuracy and bias of walked line-transect distance sampling to estimate eastern grey kangaroo population size

Wildlife Research ◽

10.1071/wr15029 ◽

2015 ◽

Vol 42 (8) ◽

pp. 633 ◽

Cited By ~ 5

Author(s):

Ruth Glass ◽

David M. Forsyth ◽

Graeme Coulson ◽

Marco Festa-Bianchet

Keyword(s):

Population Size ◽

Distance Sampling ◽

Accurate Method ◽

Line Transect ◽

Good Precision ◽

Design Elements ◽

Abundance Estimates ◽

Grey Kangaroo ◽

Eastern Australia ◽

Free Ranging

Context Distance sampling is widely used to estimate the size of wildlife populations, including kangaroos. However, the performance of distance-sampling abundance estimates has seldom been evaluated for wild mammal populations of known size. Aims We evaluated the precision, accuracy, bias and interval coverage of abundance estimates from walked line-transect sampling, a commonly used distance-sampling method, for a marked free-ranging population of eastern grey kangaroos (Macropus giganteus) at Yanakie Isthmus, Wilsons Promontory National Park, south-eastern Australia. Methods In each of two study periods (November 2012 and May 2013) we first determined the true size of the uniquely marked kangaroo population by conducting 10 intensive searches of the study area. We then conducted distance sampling along six systematically spaced line transects. We walked each transect four times in November 2012 and seven times in May 2013. Data were analysed using Program DISTANCE. Key results Our intensive searches revealed that 141 and 124 collared kangaroos were present in the study area in November 2012 and May 2013, respectively. When transects were walked four or more times (i.e. ≥400 observations), maximum precision (coefficient of variation; CV of ~13%) was achieved in both survey periods. Walking transects twice (i.e. ~200 observations) produced abundance estimates with CVs of <20% in each study period. The accuracy (root mean square error) of abundance estimates varied from 1 to 13 (November 2012) and from 3 to 28 (May 2013). Bias ranged from −9% to +23%, but stabilised at between −1% and −9% when transects were walked four or more times in each study period. The 95% confidence intervals for the abundance estimates always included the true population size. Conclusions Our results indicated that walked line-transect distance sampling is a precise and accurate method for estimating eastern grey kangaroo abundance. The small negative biases that occurred when sample sizes were large were likely to be due to some animals moving outside the study area. Implications Provided that the key design elements and assumptions are met, estimates of kangaroo abundance from walked line-transect distance sampling should have good precision (CV < 20%) and minimal (<10%) bias.

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A comparison of line-transect distance sampling methods for estimating gopher tortoise population densities

Wildlife Society Bulletin ◽

10.1002/wsb.605 ◽

2015 ◽

Vol 39 (4) ◽

pp. 804-812 ◽

Cited By ~ 4

Author(s):

Traci D. Castellón ◽

Betsie B. Rothermel ◽

Saif Z. Nomani

Keyword(s):

Sampling Methods ◽

Distance Sampling ◽

Line Transect ◽

Population Densities ◽

Gopher Tortoise

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Mongolian Gobi supports the world's largest populations of khulan Equus hemionus and goitered gazelles Gazella subgutturosa

Oryx ◽

10.1017/s0030605316000417 ◽

2016 ◽

Vol 51 (4) ◽

pp. 639-647 ◽

Cited By ~ 7

Author(s):

B. Buuveibaatar ◽

S. Strindberg ◽

P. Kaczensky ◽

J. Payne ◽

B. Chimeddorj ◽

...

Keyword(s):

Distance Sampling ◽

Aerial Survey ◽

Desert Ecosystem ◽

Gobi Desert ◽

Anthropogenic Pressure ◽

Abundance Estimates ◽

Equus Hemionus ◽

The Status ◽

Wild Ass ◽

Gazella Subgutturosa

AbstractMongolia's Gobi Desert ecosystem, a stronghold for populations of the Asiatic wild ass (khulan) Equus hemionus and the goitered gazelle Gazella subgutturosa, faces conservation challenges as a result of rapid economic development, including mining-related infrastructure projects. There is a paucity of reliable data on population abundance for these ungulates in the region, which makes it difficult to assess how they are responding to increasing anthropogenic pressure. Our aim was to obtain abundance estimates for khulan and goitered gazelles to inform their management and form the basis of a long-term monitoring programme. Each year during 2012–2015 we surveyed a total of 64 line transects spaced 20 km apart, with a total of 3,464 km of survey effort across 78,717 km2. Distance sampling analysis provided annual estimates of density and abundance, which were cross-referenced with the results of an aerial survey conducted in 2013. Overall, we observed 784 groups (14,608 individuals) of khulan and 1,033 groups (3,955 individuals) of goitered gazelles during the four surveys. The abundance estimates for 2013 were 35,899 (95% CI 22,680–40,537) khulan and 28,462 (95% CI 21,326–37,987) goitered gazelles. These estimates were congruent with the results from the aerial survey, which overlapped spatially and temporally with our ground-based survey. Our findings confirm that Mongolia's Gobi Desert supports the largest population of khulan and goitered gazelles in the world, and we provide a critical update on the status of the two species.

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Use of line transect methods to estimate abundance of benthic stream fishes

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f95-021 ◽

1995 ◽

Vol 52 (1) ◽

pp. 213-222 ◽

Cited By ~ 36

Author(s):

William E. Ensign ◽

Paul L. Angermeier ◽

C. Andrew Dolloff

Keyword(s):

Distance Sampling ◽

Line Transect ◽

Strong Positive Correlation ◽

Stream Fishes ◽

Visual Estimation ◽

Abundance Estimates ◽

Benthic Stream ◽

Roanoke Logperch ◽

Strip Transect ◽

Stream Salmonid

Visual estimation of stream salmonid abundance using strip transect sighting models has become commonplace. Application of visual estimation to other stream fishes, particularly benthic forms, has been limited. Examination of the distribution of sighting distances for the Roanoke darter (Percina roanoka), Roanoke logperch (P. rex), and black jumprock (Moxostoma cervinum) indicates that strip transect sighting models that assume probability of sighting remaining constant out to the limits of observer visibility are not appropriate for these benthic species. Our datasets indicate that distance sampling models that assume decreased sighting probability with increasing distance of the target from the observer provide a reasonable alternative to strip transect sighting models. There was a strong positive correlation between abundance estimates calculated using two alternative distance sampling models, as well as between the distance sampling model estimates and an estimate of abundance obtained with a backpack electroshocker.

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Detection of Gopher Tortoise Burrows Before and After a Prescribed Fire: Implications for Surveys

Journal of Fish and Wildlife Management ◽

10.3996/052018-jfwm-045 ◽

2018 ◽

Vol 10 (1) ◽

pp. 62-68 ◽

Cited By ~ 2

Author(s):

Jennifer M. Howze ◽

Lora L. Smith

Keyword(s):

Habitat Structure ◽

Survey Methodology ◽

Distance Sampling ◽

Population Estimates ◽

Line Transect ◽

Past Century ◽

Prescribed Burn ◽

Gopher Tortoise ◽

Imperfect Detection ◽

Abundance Estimate

Abstract Gopher tortoise Gopherus polyphemus populations have declined by as much as 80% over the past century, primarily as a result of habitat loss. In 2006, the eastern population of the gopher tortoise was petitioned for federal listing as threatened. In response, a Candidate Conservation Agreement was developed for the gopher tortoise. A Candidate Conservation Agreement is a voluntary agreement between the U.S. Fish and Wildlife Service and other interested parties to address the conservation needs of a species before it becomes federally listed and to enact measures to preclude the need to list the species. The gopher tortoise Candidate Conservation Agreement identified an assessment of the status of populations on protected lands as a priority and line transect distance sampling (LTDS) was adopted as the standardized survey methodology. Surveys with LTDS rely on detection of gopher tortoise burrows because tortoises are fossorial. However, gopher tortoise burrows vary greatly in size and small burrows of juveniles are rarely detected. Although LTDS is statistically robust and allows for imperfect detection, few studies have examined how detection varies with tortoise burrow size and whether habitat structure may influence detection of gopher tortoise burrows. Both factors could affect the accuracy of population estimates using LTDS and interpretation of demographic parameters needed for the Candidate Conservation Agreement. Therefore, we conducted surveys for burrows using LTDS before (28 March–13 April 2016) and after (9–18 May 2016) a prescribed burn, which reduced vegetation cover. We detected significantly more burrows (P < 0.001, n = 651) of all sizes after the burn, and the burrow abundance estimate was 64% higher postburn. Our study showed that conducting gopher tortoise surveys after a prescribed burn increased detections and provided a more accurate population estimate. We therefore recommend conducting surveys immediately after a burn. However, varying burn cycles on large sites may make it difficult to survey following a prescribed burn and because the effects of a burn on habitat structure may vary within a site, methods to account for variation in detection due to habitat structure are needed. Population estimates for gopher tortoises using LTDS that do not account for variation in detection due to habitat structure likely underestimate population size.

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Abundance estimation for line transect sampling: A comparison of distance sampling and spatial capture-recapture models

PLoS ONE ◽

10.1371/journal.pone.0252231 ◽

2021 ◽

Vol 16 (5) ◽

pp. e0252231

Author(s):

Nathan J. Crum ◽

Lisa C. Neyman ◽

Timothy A. Gowan

Keyword(s):

North Atlantic ◽

Distance Sampling ◽

Model Specification ◽

Abundance Estimation ◽

Line Transect ◽

Abundance Estimates ◽

Credible Intervals ◽

Capture Recapture ◽

Line Transects ◽

Nominal Coverage

Accurate and precise abundance estimation is vital for informed wildlife conservation and management decision-making. Line transect surveys are a common sampling approach for abundance estimation. Distance sampling is often used to estimate abundance from line transect survey data; however, search encounter spatial capture-recapture can also be used when individuals in the population of interest are identifiable. The search encounter spatial capture-recapture model has rarely been applied, and its performance has not been compared to that of distance sampling. We analyzed simulated datasets to compare the performance of distance sampling and spatial capture-recapture abundance estimators. Additionally, we estimated the abundance of North Atlantic right whales in the southeastern United States with two formulations of each model and compared the estimates. Spatial capture-recapture abundance estimates had lower root mean squared error than distance sampling estimates. Spatial capture-recapture 95% credible intervals for abundance had nominal coverage, i.e., contained the simulating value for abundance in 95% of simulations, whereas distance sampling credible intervals had below nominal coverage. Moreover, North Atlantic right whale abundance estimates from distance sampling models were more sensitive to model specification compared to spatial capture-recapture estimates. When estimating abundance from line transect data, researchers should consider using search encounter spatial capture-recapture when individuals in the population of interest are identifiable, when line transects are surveyed over multiple occasions, when there is imperfect detection of individuals located on the line transect, and when it is safe to assume the population of interest is closed demographically. When line transects are surveyed over multiple occasions, researchers should be aware that individual space use may induce spatial autocorrelation in counts across transects. This is not accounted for in common distance sampling estimators and leads to overly precise abundance estimates.

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Getting the Most From Surveys: How Method Selection and Method Modification Impact Butterfly Survey Data

Annals of the Entomological Society of America ◽

10.1093/aesa/saab004 ◽

2021 ◽

Author(s):

Katherine C Kral-O’Brien ◽

Adrienne K Antonsen ◽

Torre J Hovick ◽

Ryan F Limb ◽

Jason P Harmon

Keyword(s):

Species Richness ◽

New Species ◽

Count Data ◽

Distance Sampling ◽

Line Transect ◽

Similar Species ◽

Method Selection ◽

M 10 ◽

Sampling Procedures

Abstract Many methods are used to survey butterfly populations, with line transect and area surveys being prominent. Observers are typically limited to search within 5 or 10 m from the line, while observers are unrestricted in larger specified search regions in area surveys. Although methods differ slightly, the selection is often based on producing defendable data for conservation, maximizing data quality, and minimizing effort. To guide method selection, we compared butterfly surveys using 1) line versus area methods and 2) varying width transects (5 m, 10 m, or unrestricted) using count data from surveys in North Dakota from 2015 to 2018. Between line and area surveys, we detected more individuals with area surveys, even when accounting for effort. However, both methods accumulated new species at similar rates. When comparing transect methodology, we detected nearly 60% more individuals and nine more species when transect width increased from 5 m to unrestricted, despite similar effort across methodology. Overall, we found line surveys slightly less efficient at detecting individuals, but they collected similar species richness to area surveys when accounting for effort. Additionally, line surveys allow the use of unrestricted-width transects with distance sampling procedures, which were more effective at detecting species and individuals while providing a means to correct count data over the same transect length. Methods that reduce effort and accurately depict communities are especially important for conservation when long-term datasets are unavailable.

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Comprehensive health assessment and blood analyte reference intervals of gopher tortoises (Gopherus polyphemus) in southeastern FL, USA

Conservation Physiology ◽

10.1093/conphys/coab015 ◽

2021 ◽

Vol 9 (1) ◽

Author(s):

Annie Page-Karjian ◽

Kathleen Rafferty ◽

Clerson Xavier ◽

Nicole I Stacy ◽

Jon A Moore ◽

...

Keyword(s):

Plasma Protein ◽

Health Assessment ◽

Clinical Signs ◽

Keystone Species ◽

Reference Intervals ◽

Upper Respiratory Tract ◽

Gopher Tortoise ◽

Gopherus Polyphemus ◽

Baseline Information ◽

Mycoplasma Spp

Abstract The gopher tortoise (Gopherus polyphemus), a keystone species, is declining throughout its geographic range. Lack of knowledge with respect to the potential infectious diseases present within wild populations creates a dilemma for wildlife biologists, conservationists and public policy makers. The objective of this study was to conduct a health assessment of two previously unstudied gopher tortoise aggregations located at two sites in southeastern FL. Samples were collected from 91 tortoises (48 adults, 35 juveniles, 8 hatchlings) captured at Florida Atlantic University’s Harbor Branch Oceanographic Institute, in Fort Pierce, FL, USA in 2019, and Loggerhead Park in Juno Beach, FL, USA, during 2018–2019. Samples of blood, nasal swabs and oral/cloacal swabs were analyzed for hematology, plasma protein electrophoretic profiles and infectious disease testing including Mycoplasma spp. serology and polymerase chain reaction (PCR) assays for Ranavirus, Herpesvirus and Anaplasma spp. Hematological and plasma protein electrophoresis reference intervals are presented for adult and juvenile tortoises from both sites combined. Clinical signs consistent with upper respiratory tract disease (URTD) were observed in 18/91 (20%) tortoises, and antibodies to Mycoplasma agassizii were detected in 33/77 (42.9%) tortoises. Adult tortoises were significantly more likely than juveniles to have URTD clinical signs, and statistically significant, positive relationships were observed between the presence of antibodies to Mycoplasma spp. and carapace length, packed cell volume and plasma globulin concentrations. Anaplasma spp. inclusions were observed in 8/82 (10%) tortoises, but PCR detected Anaplasma sp. in 21/83 (25%) tortoises. Herpesvirus and Ranavirus were not detected in any blood or swab samples. This work contributes important baseline information on the health of gopher tortoises toward the southern end of the species’ range.

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Bridled Quail-dove Geotrygon mystacea population assessment after hurricanes Irma and Maria, St. Eustatius, Caribbean Netherlands

Bird Conservation International ◽

10.1017/s0959270920000647 ◽

2021 ◽

pp. 1-12

Author(s):

FRANK F. RIVERA-MILÁN ◽

HANNAH MADDEN ◽

KEVIN VERDEL

Keyword(s):

National Park ◽

Food Limitation ◽

Distance Sampling ◽

Population Status ◽

Management Actions ◽

Abundance Estimates ◽

Detection Mode ◽

Limited Food ◽

Seed Eating ◽

Free Ranging

Summary Structural vegetation damage and food limitation are important effects of major hurricanes, particularly for fruit/seed-eating, forest-dependent Caribbean birds with restricted distributions and small populations, such as the Bridled Quail-dove Geotrygon mystacea. Motivated by the lack of abundance estimates, corrected for detection probability, we conducted distance-sampling surveys inside and outside the Quill National Park each May in 2016-2019. Detection mode was the most important covariate, with others receiving no support from the data. Detectability of available single individuals and clusters of individuals within 60 m of transect centrelines averaged 0.957 ± 0.114 standard error for audio detections, 0.434 ± 0.052 for visual detections, and 0.693 ± 0.064 for detection modes combined. Availability averaged 0.475 ± 0.138 and the product of detectability and availability averaged 0.329 ± 0.098. Density averaged 1.459 ± 0.277 individuals ha-1 and population size averaged 642 ± 122 individuals in 440 ha. Density did not differ along and away from forest trails, but was higher inside than outside the park and at elevations within 201-400 m than 100-200 m and 401-600 m. Density declined by 76% after hurricanes Irma and Maria in 2017. We suggest that major hurricanes together with free-ranging livestock overgrazing degraded foraging habitats, limited food supply, and caused a population bottleneck. Our methodology can be implemented across the distribution range to assess population status and trends and evaluate the result of management actions at key conservation sites. Bridled Quail-dove populations probably were declining on most islands before the 2017 hurricanes and population status warrants revision.

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