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Non-Overlapping Distributions of Feral Sheep (Ovis aries) and Stout Iguana (Cyclura pinguis) on Guana Island, British Virgin Islands.


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Stout Iguanas (Cyclura pinguis) remain one of the most critically endangered reptiles in the world. Factors contributing to that status include habitat loss, predation by introduced species, and competition with introduced herbivores. On Guana Island, British Virgin Islands, the presence of feral sheep (Ovis aries) has been a hypothesized detriment to iguanas. Using motion sensitive cameras, we documented the distribution of feral sheep on Guana Island in 2010. We also quantified the impact of feral sheep on ground vegetation by comparing plant abundance at long term sheep exclosures and areas where sheep were absent to areas where sheep were present. Finally, we compared sheep distribution to iguana distribution on the island. The co-occurrence of sheep and Stout Iguanas was less than expected, indicating possible competition. Although we detected no difference in vegetative cover between areas where sheep were present and absent, the long-term exclosures showed that the exclusion of sheep allowed the abundance of many plant species to increase. Our data support the hypothesis that feral sheep are altering the abundance of ground-level vegetation and limiting iguana distribution on the island.
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... Regardless of the platform used, or the ecosystem of interest, SampleFreq can be used to measure plant frequency. As an example, vegetation point classification from nadir images using the closely-related program SamplePoint has been completed across many ecosystems with varying plant communities, including grasslands, temperate and tropical forests, alpine tundra, salt marshes and deserts (Guo et al., 2016, Parrish et al., 2017, Skipper et al., 2013, Goonan et al., 2009, Bacopoulos et al., 2018,Tabeni et al., 2014 as well as agricultural settings (Nielsen et al., 2015). We therefore think it reasonable to anticipate successful use of SampleFreq in these ecosystems, with the caveats that image resolution must be adequate for identification of the species of interest, and that the vegetation canopy allows determination of where a plant is rooted. ...
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As our understanding of ecological systems grows, natural resource management becomes ever more dependent on timely, accurate, and inexpensively-collected monitoring data to support management decisions. Vegetation cover, density, and frequency are abundance metrics used in resource management; however, frequency data can be collected more quickly than density data and with more repeatability and less sensitivity to inter- and intra-seasonal variation in plant morphology. Moreover, frequency is perhaps the best method for monitoring invasive species across extensive areas. A limitation to the use of frequency data is that plot size affects frequency. The optimal plot size is one that yields measurements suitably removed from 0 or 100% to allow detection of both upward and downward frequency trends, yet the optimum plot size cannot be known before sampling. We addressed this conundrum by developing SampleFreq software that facilitates frequency measurements from digital nadir images of any scale with up to 10 nested plot sizes within the confines of the image dimensions. We conducted accuracy and agreement tests of the software using both artificial populations and field plots. Using artificial population plots, accuracy across all users was 93.4% with a repeatability coefficient of 1.4%, indicating high precision. In a field test, SampleFreq and standard field data averaged a 3.4% difference, and were within approximately 10.5% of each other 95% of the time. From the same field test, SampleFreq repeatability coefficient was 6.7%, while the field method was 4.3%, illustrating that both methods have relatively high precision. Because SampleFreq has high potential accuracy, high agreement with field data, and high precision across a range of users, we recommend using SampleFreq with nadir digital images as a suitable alternative method for monitoring plant frequency.
The Bridled Quail-Dove (Geotrygon mystacea) is a columbiform resident of the eastern Caribbean. It is a poorly studied species with a lack of quantitative data to assess population status, but perceived population declines have led to it being considered a species of conservation concern on many islands. I attempted to assess population size and survival of Bridled Quail-Doves on Guana Island, British Virgin Islands. Based on detection probability, density estimates for Guana Island ranged from 1.38 to 1.57 individuals/ha in 2014 and 2015, respectively. Densities varied among cover type, with an estimated 1.13 individuals/ha in dry forest (90% of the island) and 4.63 individuals/ha in ghaut forest (5% of the island). These values would translate to a pooled estimate of ∼429 individuals during the survey period. Of 36 Bridled Quail-Doves captured and marked, the naïve estimate of annual survival was 0.813, with 0.36 captured individuals surviving at least 1 year, and the average minimum age at last recapture was 4.9 years old. This report is the first quantitative based estimates of population size and longevity for the species and may serve as a starting point for longer-term studies on Guana Island, but also for comparison to populations on other islands.
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Measuring percent occurrence of objects from digital images can save time and expense relative to conventional field measurements. However, the accuracy of image analysis had, until now, not reached the level of the best conventional field measurements. Additionally, most image-analysis software programs require advanced user training to successfully analyze images. Here we present a new software program, 'SamplePoint,' that provides the user a single-pixel sample point and the ability to view and identify the pixel context. We found SamplePoint to allow accuracy comparable with the most accurate field-methods for ground-cover measurements. Expert use of the program requires minimal training and its ease of use allows rapid measurements from image data. We recommend SamplePoint for calibrating the threshold-detection level of image-analysis software or for making direct measurements of percent occurrence from digital images.
I examined distribution, population density and structure, diet, habitat use, home-range dynamics, and physical condition of the Anegada population of Cyclura pinguis using ground surveys, interviews with residents, mark-and-recapture estimates, fecal analysis, feeding experiments, radiotelemetry, and life-history data. When compared with a study of 1968, the density of the extant population at a 43-ha study site was 0.36/ha, as opposed to 2.03/ha. Leaves represented only 38% of dietary volume, in contrast to 71% in 1968. Fruit comprised 56% of the diet. Well fed individuals of C. pinguis from a relocated population on Guana Island would not eat the leafy components of the Anegada iguanas' diet. Home ranges on Anegada overlapped and were 100 times larger than previously recorded. The sex ratio was two males to one female as opposed to 1:1 in 1968. Iguanas had proportionately lower body mass than animals captured in 1968. Population decline on Anegada seems largely due to increased competition from live stock, managed in 1968 but now feral, eating most palatable vegetation from the understory. Predation by cats and dogs is also a threat. The total remaining population is estimated at <200 individuals. A national park designed to protect the endangered population of C. pinguis has been proposed for Anegada.
Visual obstruction measurements were used to determine height and density of vegetation in a Kansas grassland. These visual obstruction measurements were compared with the weight of vegetation collected from each site. The weight of vegetation collected was significantly correlated with the visual obstruction measurements.
Eight individual rock iguanas (Iguana pinguis) from Anegada Island were relocated to Guana Island by Lazell, 1984–1987, in order to establish a second population reservoir for this endangered species. The species may have originally occupied the entire Puerto Rico Bank. The relocation has been successful and, in the area currently providing the best habitat, we estimate a density of 9 or 12 animals of various age classes per 19 ha. The optimal area contains a sheep exclosure with relatively dense understory vegetation and numerous exotic as well as native species of plants. Iguana activity is concentrated on east facing slopes and ridge-tops that get morning sun. Outside the exclosure most edible ground cover and shrubs have been eaten by sheep, leaving toxic or noxious species, such as Croton or Lantana, in the understory where I. pinguis adults generally forage. Removal of sheep may be critical to continued population growth of these reptiles. Views on relocation or repatriation of other endangered Antillean Iguana species are advanced, with some ideas on minimum viable population sizes and a possible explanation for the extirpation of I. pinguis from much of its former range.
Overview of impacts of feral and introduced ungulates on the environment in the eastern United Stated and Caribbean
  • M S Lowney
  • P Schoenfeld
  • W Haglan
  • G W Witmer
Lowney, M.S., P. Schoenfeld, W. Haglan, and G.W. Witmer. 2005. Overview of impacts of feral and introduced ungulates on the environment in the eastern United Stated and Caribbean, pp. 64-81. In: D.L. Nolte and K.A. Fagerstone (eds.), Proceedings of the 11th Wildlife Damage Management Conference. Wildlife Damage Management, Internet Center for DigitalCommons@ University of Nebraska -Lincoln.
Guana and Necker island population assessments
  • G Perry
  • N Mitchell
Perry, G. and N. Mitchell. 2003. Guana and Necker island population assessments 2002. Iguana 10:49.