Integrating population and genetic monitoring to understand changes in the abundance of a threatened seabird

Population monitoring programs for threatened species are rarely designed to disentangle the effects of movements from changes in birth and death rates on estimated trends in abundance. Here, we illustrate how population and genetic monitoring can be integrated to understand the cause of large changes in the abundance of a threatened species of seabird, the Marbled Murrelet (Brachyramphus marmoratus). Specifically, we used genetic data to test whether a large apparent population decline and subsequent recovery in central California was due to a population collapse followed by dispersal from larger populations to the north (the “rescue hypothesis”), or whether it was due to local changes in distribution where resident individuals dispersed from and then returned to surveyed areas (the “distribution hypothesis”). An alternative explanation, that rapid population recovery was the result of positive local population growth, was not supported by demographic data. The post-recovery population was more similar genetically to the pre-decline population than it was to northern populations. In addition, both pre-decline and post-recovery populations had a similar proportion and number of individuals of migrant ancestry, as determined with genetic assignment tests. Thus, pre-decline and post-recovery samples appear to come from the same population and our results support the distribution hypothesis. By conducting concurrent genetic and population monitoring, we were able to understand observed changes in the abundance in a threatened species, which would not have been possible by only monitoring abundance.