“The evolution of marine larval dispersal kernels in spatially structured habitats: analytical models, individual-based simulations, and comparisons with empirical estimates”
Allison K. Shaw, Cassidy C. D’Aloia, and Peter M. Buston (Mar 2019)
How far do baby fish travel away from their parents? Answering this simple question is fairly challenging, and has long been a goal of marine ecology. These ‘dispersal distances’ determine how fish stocks and marine reserves should be managed. Shorter distances would mean that marine reserves should be closer together (so fish can easily move between them) and that fish stocks in distant locations should be managed separately (because fish don’t readily move between them). Longer distances would mean that marine reserves can be farther apart and fish stock populations managed together. Here, researchers Allison Shaw, Cassidy D’Aloia, and Peter Buston take a fresh look at this question, asking what distribution of dispersal distances is expected, based on natural selection in spatially structured habitats. The researchers developed a series of models in different types of environments with edges (where dispersing too far means death), akin to many natural environments such as coral reefs. In models with environments most similar to an actual reef (the Belizean Barrier Reef), they found that most fish disperse short distances and only some travel long distances from their parents. Finally, the researchers compared model results to empirically measured dispersal distances of a coral reef fish from the same region. Model results and empirical results weren’t a perfect match, but they were remarkably similar with both indicating that the majority of fish disperse no more than a kilometer or two from where they were born. If these results withstand scrutiny, then they provide another line of evidence in support of the idea that baby fish may not be dispersing far from their parents. If that’s the case we might need to rethink the spatial scale at which some marine populations are connected and managed.
Understanding the causes of larval dispersal is a major goal of marine ecology, yet most research focuses on proximate causes. Here, we ask how ultimate, evolutionary causes affect dispersal. Building on Hamilton and May’s 1977 classic paper (“Dispersal in stable habitats”), we develop analytic and simulation models for the evolution of dispersal kernels in spatially structured habitats. First, we investigate dispersal in a world without edges and find that most offspring disperse as far as possible, opposite the pattern of empirical data. Adding edges to our model world leads to nearly all offspring dispersing short distances, again a mismatch with empirical data. Adding resource heterogeneity improves our results: most offspring disperse short distances with some dispersing longer distances. Finally, we simulate dispersal evolution in a real seascape in Belize and find that the simulated dispersal kernel and an empirical dispersal kernel from that seascape both have the same shape, with a high level of short-distance dispersal and a low level of long-distance dispersal. The novel contributions of this work are to provide a spatially explicit analytic extension of Hamilton and May 1977, to demonstrate that our spatially explicit simulations and analytic models provide equivalent results, and to use simulation approaches to investigate the evolution of dispersal kernel shape in spatially complex habitats. Our model could be modified in various ways to investigate dispersal evolution in other species and seascapes, providing new insights into patterns of marine larval dispersal.