Landscape dominance of introduced herpetofauna on an oceanic island

Habitat loss and fragmentation can negatively impact native wildlife and facilitate establishment
of introduced species. On islands, introduced species are a primary cause of
extinction and can alter community membership through predation or competition for
resources. Consequently, elucidating the distribution of introduced and native species can
improve understanding of the potential synergistic effects of land use and introduced
species on native island species. The island of Saipan in the Commonwealth of the
Northern Mariana Islands has a long history of herpetofaunal introductions. However, the
distribution of native and introduced herpetofauna on Saipan is poorly understood, and
the effects of land use and introduced species on the native herpetofauna are largely
unknown. In 2018, we randomly selected sites on Saipan from established island-wide
transects and used multiple detection methods, repeated surveys, and single-season occupancy models to account for imperfect detection and construct species distribution
models. We investigated the role of biotic and abiotic factors in explaining occupancy and
detection of Saipan’s native and introduced herpetofauna. We recorded 2568 observations
representing 1 amphibian and 11 reptile species. Introduced species were encountered 3.8
times more frequently and were 33% more broadly distributed than native species. Occupancy
for the native forest dwelling species Emoia caeruleocauda and Perochirus ateles
was positively associated with elevation and enhanced vegetation index, suggesting that
human habitat modification at lower elevations may be reducing their suitable habitat.We
found evidence of a range contraction for the imperiled P. ateles and suggest potential
avenues for implementing targeted conservation of native herpetofauna.