Abstract

Many analyses of coastal ecological systems hinge on whether a system "retains" the species under study. Often this retention is ascribed to an "eddy" or the influence of some bathymetric feature. However, retention is rarely defmed precisely. This vagueness has led to the incorrect assumption that retention is largely a property of the physical circulation of the system, perhaps as influenced by behaviors that affect an individuals transport by the mean currents. We defme a coastal system as "retentive" ifmost of the adults present in the system are descendants of adults which inhabited the region at an earlier time. The criterion for retention depends on two physical parameters and two biological parameters. The two physical parameters are the mean distance a larvae or adult is moved in a generation by the mean currents (L_adv), and the standard deviation of this distance (L_diff).

The biological parameters are the population growth rate (R) and lifetime (T_g). A system will be retentive iflog(R T_g) > L²_adv/(2L²_diff). Consideration of the physics alone will fail to determine whether a region is retentive. For example, any reduction in the growth rate R, such as caused by competition with an invasive species, will tend to cause a region to be less retentive. The ecological consequences of this retention criterion will be explored. Adaptations that allow a species to be more easily retained are found to be common in benthic species with planktonic larvae in the Northeast Pacific, and it is suggested that a failure to be retained will explain the northward limit of an invasive crab species, H. sanguineus, in the Gulf of Maine.

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