Supply Side Ecology and External Drivers 19802005

Marine ecologists have known for a long time that success of many intertidal species depend on the supply of pro-pagules (larvae, zygotes, and spores) from the plankton, but it was not until the 1980s that experiments were executed to assess how the supply of propagules influenced the patterns of distribution and abundance of adults in benthic assemblages.

Propagule supply and early post-settlement mortality markedly influence both the strength ofinteractions among established individuals and overall patterns of distribution and abundance on rocky shores. Abundance of established individuals is often directly proportional to the density of settlement and consequently, and strength of adult interactions depends on variation of settlement. In contrast, if settlement is high enough to consistently saturate the system, then local populations tend to be driven by strong interactions among adults regardless of settlement variation. In some cases, heavy early postsettlement mortality

Figure 5 The intertidal zone near Antofagasta in northern Chile, a region with upwelling and abundant seaweeds. Photo by P. S. Petraitis.

can lead to low densities of adults despite an abundance of settlers, and this has been shown for several seaweeds and many invertebrate species. The causes of variation in pro-pagule supply can be classified into two broad categories -oceanographic transport or regional offshore production. Although invertebrate larvae and some macroalgal spores are motile, their movements are most directly important at small spatial scales near the substrate just prior to settlement. By and large, propagules of benthic species are transported at the mercy of currents and other oceanic transport phenomena. For instance, coastal upwelling results in a net offshore transport of propagules and leads to a reduction in settlement along a shoreline. This commonly occurs with invertebrate species that have long residence times in the plankton. In contrast, seaweeds, which have very short planktonic stages, often dominate intertidal sites within regions characterized by seasonal or permanent upwelling (Figure 5).

Regional offshore production influences the supply of larvae to a coastal habitat in two ways. First, phyto-plankton production in nearby waters offshore affects the abundance of planktotrophic larvae that feed for several weeks in the plankton potentially leading to greater larval supply in areas with greater phytoplank-ton production. Second and in opposition, increased production in offshore can generate increased resources and habitat for the associated pelagic community that preys upon larvae and thus leads to a reduced larval supply.

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