Many populations are spatially-structured and exist as metapopulations, with distinct patches connected through movement of individuals. Understanding metapopulation dynamics – how patches are connected and whether they persist – is particularly challenging in the marine environment, where much of the connectivity takes place through larvae too tiny to track and experiencing high mortality. A metapopulation can persist either by individual patches retaining enough of their own offspring to exist in isolation – termed self-persistence – or through exchange among patches that provides sufficient recruits for a patch to persist within the network – called network persistence. Though there has been much theoretical development of persistence mechanisms, they have not been well tested in empirical systems.
We use a model system of yellowtail clownfish (Amphiprion clarkii) in the Philippines to understand connectivity and persistence in an empirical marine metapopulation. Clownfish are particularly well-suited to metapopulation studies, with limited movement as adults and a relatively short pelagic larval duration. They also have distinct habitat patches at a range of scales, from coral reef patches to individual anemones, allowing us to compare different ways of assessing metapopulation dynamics. Using mark-recapture and genetic parentage methods, we estimate survival, reproduction, dispersal, and anemone occupancy to understand and characterize persistence mechanisms.
Results/Conclusions
We find that the 17 subpopulations at our study site have persisted over seven sampling years with stable abundance and similar size distributions throughout that time, suggesting the overall metapopulation is persistent. When we calculate persistence metrics for the network of 17 subpopulations, however, we estimate that each recruit will produce only 0.20 recruits over its lifetime, lower than the threshold of one recruit produced per recruit necessary for replacement. We do not find self-persistence in any of the subpopulations, which is unsurprising given our low estimate of lifetime recruit production. We also do not find evidence of network persistence among our 17 sites; the dominant eigenvalue of the realized connectivity matrix is much lower than one, the threshold for network persistence. These findings suggest that we are sampling only a portion of a full metapopulation and a larger area is required for persistence of the metapopulation in isolation. The abundances at the sites seem to be stable but appear to require recruitment from sites outsides those we sample.