The ecological impacts of anthropogenic activity are a focal concern of conservation

1 widespread island endemic, the San Clemente Island night lizard. ABSTRACT 17 Habitat fragmentation and loss are fundamental conservation concerns. Because protected 18 areas are often " habitat islands " isolated by human development, studies of physical islands 19 which document genetic patterns associated with natural versus anthropogenic fragmentation may 20 provide insight into the management of anthropogenic habitat islands. The San Clemente Island 21 night lizard, Xantusia riversiana reticulata, is a highly abundant endemic to two California 22 Channel Islands (San Clemente and Santa Barbara Islands), each with a history of anthropogenic 23. CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not. 2 disturbance, and was recently delisted from the Endangered Species Act. We genotyped 917 24 individuals at 23 microsatellite loci to estimate genetic divergence between collection sites and 25 identify natural and anthropogenic landscape features correlated with genetic divergence for X. r. 26 reticulata throughout its range. Genepool delineation resulted in each well-sampled collection 27 site acting as a distinct population, pairwise Fst values were small but significant, and 28 STRUCTURE detected 2 populations on San Clemente Island and 4 populations on Santa Barbara 29 Island. Active sand dunes on San Clemente Island acted as a barrier whereas cholla phase 30 maritime desert scrub, secondary roadways, and canyons > 500ft in length acted as resistant 31 surfaces. Resistant surfaces on Santa Barbara Island were woolly seablite, crystalline iceplant, 32 barren ground, and herbaceous vegetation. Conductance models included California boxthorn and 33 prickly pear cactus on both islands with additional minor features unique to each island. We 34 recommend the application of conductance and resistance models within " habitat islands " to 35 better inform management for species persistence when the intervening habitat between reserves 36 cannot be improved.