Marginal sinks or potential refuges? Costs and benefits for coral-obligate reef fishes at deep range margins

Escalating climate-related disturbances and asymmetric habitat losses will increasingly result in species living in more marginal habitats. Marginal habitats may represent important refuges if individuals can acquire adequate resources to survive and reproduce. However, resources at range margins are often distributed more sparsely; therefore increased effort to acquire resources can result in sub-optimal performance and lead to marginal populations becoming non-self-sustaining sink-populations. Shifting resource availability is likely to be particularly problematic for dietary specialists. Here, we use extensive in-situ behavioural observations and physiological condition measurements to examine the costs and benefits of resource-acquisition along a depth-gradient in two obligate corallivore reef fishes with contrasting levels of dietary specialisiation. As expected, the space utilised to secure coral-resources increased towards the lower depth margin. However, increased territory size...