Response to Bode and colleagues: ‘Resilient reefs may exist, but can larval dispersal models find them?’
Bode and colleagues’ response to our paper posits that the identification of specific reefs that drive recovery is unsafe because models of larval dispersal (‘connectivity’) are insufficiently accurate or consistent to make useful predictions at this scale [1]. They go on to argue that managers should not base decisions on reef-level predictions of connectivity. The evidence they provide for these assertions is that the results of their own modelling—with a different model but run with a ‘comparable parameterisation’—diverge considerably from ours. The legitimacy of Bode and colleagues’ concern rests on the apparent disparity in behaviour between connectivity models. Unfortunately, Bode and colleagues’ study differs from ours in many more ways than simply the modelling of oceanography. In short, (1) their representation of the Great Barrier Reef (GBR) excludes approximately half the reefs we recognise (2,175 versus 3,806), so the network metrics of those reefs ‘in common’ are incomparable; (2) they represent different species by ignoring larvae with short settlement times (<7–14 days) and allowing a continuous release of larvae, whereas we considered a wider range of competencies beginning at 12 hours and simulated punctuated spawning events that are more relevant to invertebrates; (3) their model was parameterised two decades ago (1996–2002), so their assessment makes the unlikely assumption that conditions had not changed by the recent period we represented (2008–2013); (4) their model is unable to capture an important dispersal process on the GBR because it takes a two-dimensional depth-integrated approach that ignores the shear flow captured in our 3D Connie model; and (5) they were unable to implement all of the connectivity criteria we applied. Specifically, their finding of a strongly connected network precluded use of graph theory to identify regionally connected sources as we did. The causes of such strong connections in Bode and colleagues’ analysis are unclear but likely include use of higher rates of larval release and a continuous release period of 156 days, though we consider this to be an inappropriate representation of coral spawning, and we elected to retain the temporal dynamics of multiple punctuated spawning events. Given these five fundamental differences, it is not surprising that their assessment of connectivity patterns differed from ours. However, this discrepancy does not imply that choice of hydrodynamic model alone is the cause. Nor does it constitute evidence that our identification of key source reefs is too uncertain to be useful. The eReefs/Connie hydrodynamic model we adopted has been calibrated against data from tide gauges, wave-rider buoys, an extensive array of moorings and temperature loggers across the GBR, and Argo floats [2] as well as formal assimilation of satellite data. An additional, and