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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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