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Master of Science (MS)
Anderson, Roger A. (Roger Allen)
Wallin, David O.
Comparisons among core and peripheral populations of animals for patterns of habitat use may be an important step in an area of ecological research: the quest to understand the mechanisms underlying species range boundaries. My research on the Western Fence Lizard, Sceloporus occidentalis, at the northwestern edge of the species' geographic range, is intended to lay the groundwork for future analyses of peripheral populations. Sceloporus occidentalis is a propitious species for studying peripheral animal populations because (1) relative to dryland habitats more typical of the species nearer the core of its geographic range, the peripheral population resides in a relatively unique beach-edge maritime habitat, and (2) it is found in high local abundance, is easy to observe and capture, and has low vagility relative to birds and mammals. My research addresses the habitat preferences, dispersal tendencies, and spatial distribution among individuals of S. occidentalis in a population at the species geographic extreme in western Washington.
In 2013-2014 I captured, measured, and marked 359 lizards on the central beach in an apparent metapopulation along the Washington coast west of Marysville, WA. I recorded detailed habitat characteristics for every 10-meter stretch of beach using the line-intercept method (transect was set perpendicular to beach edge) and used Multidimensional Scaling analysis to correlate habitat characteristics with lizard sighting locations, thereby determining which sections of coastal beach edge were most heavily used by fence lizards. Similarity Percentage (SIMPER) analysis was used to determine which habitat characteristics were associated with most of the difference between occupied and unoccupied habitat areas. These characteristics and their relative influences on habitat differences were: distance to nearest patch of sun-exposed soil on the slope above the beach (45%), the abundance of nanohabitats (i.e. different substrata or log surfaces) in the log field below the slope (22%), the number of exposed soil patches on the beach slope (16%), the length (between slope base and beach-edge) of the log field on the beach below the slope (10%), and the relative amount of exposed soil in standard plots on the slope above the beach (5%).
Chi-square analysis revealed that lizards were more likely to bask on log surfaces than on sand and leaf litter surfaces, perhaps because (1) body temperature can be more finely regulated on logs by adjusting angle of the body towards the sun, and (2) antipredator cover is nearer when on a log. Hatchling lizards were more likely than adults to be found on sand substratum. Based on proportions of age and sex classes that dispersed and the distances moved, I inferred that younger lizards were more likely to disperse longer distances than adults along Spee-Bi-Dah beach.
Lizard sighting locations were documented in ArcGIS and the degree of overlap in habitat characteristics with lizard locations determined through Analysis of Similarity, which revealed lizards to be clumped around patches of prime habitat. A reasonable inference is that high population density and intraspecific competition in these habitat patches may be pushing lizards to disperse into suboptimal habitat. However, invasive plant species, particularly Himalayan blackberry, rapidly colonize open soil patches on the beach slope, restricting the lizards' access to habitat necessary for nesting and hibernation, and reducing habitat available for population growth or range expansion.
Habitat degradation by invasive species and humans is likely a significant factor in the apparent decline of fence lizard populations along the Washington coast.
Western Washington University
Tulalip Indian Reservation (Wash.)
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Backus, Paul, "Use of Patchy, Early Successional Slope Habitat Along Coastal Sun-Facing Beaches by the Western Fence Lizard Sceloporus occidentalis at the Species' Northern Geographic Extreme" (2016). WWU Masters Thesis Collection. 510.