Density dependent habitat selection in response to habitat loss in a coral reef fish

Habitat degradation alters the availability and quality of resources, with ramifications for how populations distribute across remnant patches. Decisions about habitat use are often made to optimise fitness by minimising competition for resources. Individuals can sort themselves optimally across patches by selecting habitat based on the density of resident individuals, yet it is unclear whether this mechanism is disrupted by habitat loss. Moreover, density-dependent habitat selection could create a socially reinforced 'bandwagon' effect in species that use adults as a positive settlement cue. Here, we aimed to (1) determine the effect of habitat degradation on density-dependent habitat selection and (2) test whether habitat use by adults influences settlement decisions by juveniles via a bandwagon effect in the coral-associated reef fish, Pomacentrus moluccensis. We combined field surveys with a habitat choice experiment in Pomacentrus moluccensis to assess how fish respond to varying coral quality and conspecfici densities. Field observations revealed that adults only used dead coral on degraded reefs where fish densities on surrounding remnant live colonies were exceptionally high. When presented experimentally with the choice of two colonies, fish were more likely to choose a near empty alternate colony when the other colony was severely crowded with conspecifics. Taken together, these results offer strong support for density-dependent habitat selection during habitat loss. This choice cascades to influence juvenile habitat use: juveniles selected dead corals to a greater extent if there was a conspecific adult present. To our knowledge, this is the first empirical demonstration of how habitat degradation can trigger density-dependent habitat selection, which in turn may shape settlement decisions in the next generation via socially mediated cues.