Within-site variation of growth rates and terminal sizes in Mytilus californianus along wave exposure and tidal gradients.

Mytilus californianus is a foundation species of rocky shores of western North America. Its dominance depends on rapid growth to large sizes, which confers an advantage in size-dependent species interactions. Initial rates of growth and final (terminal) sizes of the mussels depend on environmental factors. Prior comparisons of growth made over large spatial scales (tens of meters to hundreds of kilometers) indicate that temperature, submergence time, and wave exposure affect growth. However, there are few studies quantifying variation in temperature, wave force, and mussel growth parameters at small scales within local populations-that is, meter-level increments. Such measures are necessary to better understand the consequences of the complex spatial mosaic of physical factors in the intertidal zone. We measured variation in temperature, wave force, size-specific shell growth, and terminal size at 3-4-m intervals along horizontal contours within two mussel beds separated by 15 ds of latitude. Both mussel beds showed the same general trends: growth rates attenuated along gradual clines from low and wave-exposed to high shore and sheltered. For example, young adults from low and wave-exposed microhabitats grew 9- and 6-fold higher than those from high-shore-wave-sheltered points. While higher flow may promote growth by enhancing feeding, it also appears to exert a positive effect by moderating energetically costly temperature stress. Consistent with the growth rate findings, cumulative degree-hours explained 83% and 69% of the variation of terminal sizes in regressions for the two locations.

Mussel bed boundaries as dynamic equilibria: thresholds, phase shifts, and alternative states.

Ecological thresholds are manifested as a sudden shift in state of community composition. Recent reviews emphasize the distinction between thresholds due to phase shifts-a shift in the location of an equilibrium-and those due to alternative states-a switch between two equilibria. Here, we consider the boundary of intertidal mussel beds as an ecological threshold and demonstrate that both types of thresholds may exist simultaneously and in close proximity on the landscape. The discrete lower boundary of intertidal mussel beds was long considered a fixed spatial refuge from sea star predators; that is, the upper limit of sea star predation, determined by desiccation tolerance, fixed the lower boundary of the mussel bed. However, recent field experiments have revealed the operation of equilibrium processes that maintain the vertical position of these boundaries. Here, we cast analytical and simulation models in a landscape framework to show how the discrete lower boundary of the mussel bed is a dynamic predator-prey equilibrium, how the character of that boundary depends on its location in the landscape, and how boundary formation is robust to the scale of local interactions.

Complex equilibria in the maintenance of boundaries: experiments with mussel beds.

Stationary boundaries of sedentary species may belie dynamic processes that form them. Our aim was to test an implication of an evolving body of theory, that such boundaries are manifestations of complex regulatory dynamics. On rocky shores of British Columbia, large-scale field experiments altered the densities of predatory sea stars (Pisaster ochraceus), causing shifts in the location of the lower vertical boundaries of their prey, sea mussels (Mytilus californianus). While control mussel beds remained unchanged, experimental reductions of sea star densities caused the downward extension of the lower boundaries, and experimental increases in sea stars densities caused the upward recession of the lower boundary well into the zone presumed to be a spatial refuge from predation. Cleared plots prepared within the initial boundaries were recolonized to varying degrees, depending on predator densities. After 30 months, plots on sea star removal sites showed high densities of adult mussels, control plots showed intermediate densities, and sea star addition plots showed only a sparse cover of alternative prey. Observations by divers at high tide showed that as small prey were depleted progressively from removal, to control, to addition sites, correspondingly larger mussels were attacked, including very large individuals comprising the lower boundary of addition sites. The findings contradict classic theory of zonation based on static prey refuges and support an alternative theory in which boundaries are maintained by complex, spatially structured equilibria.

The shifting balance of littoral predator-prey interaction in regimes of hydrodynamic stress.

Above lowshore levels of wave-beaten rocky shores, desiccation from tidal exposure and hydrodynamics stresses from wave action are thought to create refuges from predation, allowing concentrations of sedentary prey such as mussel beds. Underwater time-lapse photography on rocky shores in Southern California revealed that dense aggregations of spiny lobsters prey on mussels during nocturnal high tides. In contradiction of the refuge hypothesis, the densest aggregations occurred on midshore levels of the most wave-exposed site, a semi-protected site showed intermediate densities, and a protected site showed only sparse numbers of lobsters. On wave-beaten shores, the lobsters' high mobility and rapid prey handling allowed them to exploit intertidal prey in the brief period at extreme high tide, when both desiccation and hydrodynamic stresses were at a minimum. The spatial differences in lobster densities were, however, positively related to the recruitment rates of juvenile mussels, the preferred prey. A field experiment demonstrated that predation by lobsters within a mussel bed affects the age/size structure of the bed without changing primary percent coverage. Therefore, concentrations of adult prey on some wave-swept sites appear to result from elevated rates of prey recruitment that surpass rates of predation, rather than absolute refuges from predation.