Spatial dynamics of habitat use informs reintroduction efforts in the presence of an invasive predator.
Abstract
Islands experience major impacts from introduced species, especially nocturnal predators. The brown tree snake (Boiga irregularis) extirpated almost the entire native avifauna on Guam after introduction. Reintroductions from neighbouring islands can restore bird communities but will take place in heavily managed units where snake populations can be controlled. Yet, reintroductions often proceed without relevant biological information such as habitat and space use when nocturnal predators are present. To guide efforts, we studied the avian frugivore community's diurnal and nocturnal habitat and space use on the island of Saipan, where the snake is absent. Using radiotelemetry and a Bayesian framework, we compared data typically accessible to resource managers (diurnal home range [DHR] and habitat selection), which often form the basis of reintroduction plans. We contrasted this with data on nocturnal habitat use that is often not available but relevant when nocturnal predation threat is high. Diurnal home range size varied within and among species with Micronesian Starlings (Aplonis opaca) having DHRs at least 45 ha larger than other species. For all species, night roost locations were generally spatially clustered within a DHR and covered a smaller spatial extent. Two species had higher probabilities of roosting outside DHR boundaries but, when outside, roosts were often within 200 m of the DHR boundary. All species selected forested habitats during day and night, with some species choosing native forest over non-native habitat. Synthesis and applications. If individuals roosted randomly throughout diurnal home ranges (DHRs), landscape-level suppression of the brown tree snake on Guam might be the only viable option for reintroductions. However, we found that all species showed clustered roosting and high site fidelity. If site fidelity is common, then individuals that roost within fenced areas where snake populations are severely reduced will experience high survival even if diurnal home ranges extend into the surrounding unprotected matrix. There was a large overlap in habitat selection during day and night, indicating managed areas should be composed of native forest with non-native forests of secondary importance. In systems with nocturnal predators, understanding variability in diurnal and nocturnal habitat use could lead to better informed management decisions.