Isolation by disturbance: a pattern of genetic structure of the coral Pocillopora grandis in the Gulf of California

Genetic differentiation within species can be influenced by the distance between populations (isolation by distance, IBD) or by differences in the habitats they occupy (isolation by environment, IBE). Understanding the population structure and dynamics that influence genetic diversity within small (<100 km²) multi-use marine protected areas (MPAs) and varying disturbances or protections is relevant to better designing and managing them. Pocillopora grandis communities were studied at 10 different sites in the Gulf of California, Mexico. The comparison between sites inside and outside an MPA or whether the population structure followed IBD or IBE was investigated with the genetic marker HSP70B. Environmental heterogeneity was used to test for IBE, and was measured by comparing the present benthic communities by site and through analysis of satellite images, while distance was used to test for IBD. Mantel tests found that the population of P. grandis did not follow IBD or IBE. Instead, cluster analysis grouped low (no-take zones) and high (crown-of-thorns starfish outbreaks and highly visited by tourists) disturbance sites together with the remaining sites within and outside of the MPA in another cluster (ΦRT = 0.082, p = 0.007). Furthermore, lower genetic diversity was found at sites with low and high disturbance. Our results suggest that instead of IBE or IBD influencing the population structure, the protection and disturbance levels could have a correlation with genetic patterns (i.e. isolation by disturbance, ID). These findings will be valuable for MPAs in terms of efficient management of genetic diversity objectives by considering the appropriate spacing and protections, depending on how vulnerable a species is to disturbance.