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218 FACILITATION MARK D. BERTNESS Brown University Facilitation occurs when an organism benefits from the presence of another organism that is not itself negatively impacted. Two types of facilitation, or positive interactions, are common in rocky intertidal communities : habitat-ameliorating positive interactions and associational defenses. TYPES OF FACILITATION Habitat amelioration and associational defenses occur under predictable conditions. Habitat amelioration occurs when an organism reduces potentially limiting physical stresses, such as heat, desiccation, and wave forces, on other organisms. Habitat-ameliorating positive interactions are common in rocky intertidal habitats because () they are physically stressful habitats for most of the organisms that live in these habitats, and () many of the key stresses, such as heat, desiccation, and waves, can be ameliorated by sessile intertidal neighbors. Associational defenses, a second common type of positive interaction in intertidal communities, occur when organisms are protected from their enemies when living in association with other organisms. They are common in intertidal communities because () consumer pressure is intense in many shoreline habitats, and () common sessile organisms are often chemically or physically defended from consumers and provide refuge for less-defended neighbors. Facilitations can be facultative, meaning that although beneficial they are not necessary for the survival and reproduction of facilitated species, or obligatory, meaning that they are absolutely necessary for facilitated species’ survival and persistence. Facilitation can also occur intraspeci fically, when individuals of the same species benefit one another (also often called group benefits), or interspeci fically, when a facilitator species positively impacts other species. Mutualisms, in which two species benefit each other, are a specific type of reciprocal facilitation or positive interaction that can be either facultative or obligatory and highly evolved. FOUNDATION SPECIES Facilitation is a particularly important process in rocky intertidal communities because they are commonly dominated by foundation species. Foundation species are common and abundant species in a community that create and maintain habitats that other community members are dependent on. Most common rocky intertidal foundation species, such as canopy-forming seaweeds and bed-forming mussels, live in dense groups that occur because of strong intraspecific facilitation or group benefits of physical stress amelioration or associational defense, and the same amelioration of physical and biotic conditions that drive intraspecific group benefits typically facilitate other species. Species that modify environmental conditions and provide suitable habitats for other organisms have also often been recently referred to as ecosystems engineers. High-intertidal algal canopies are a good example of an intertidal foundation species or ecosystem engineer. On New England rocky shores, Ascophyllum nodosum forms a dense algal canopy on wave-protected rocky shores (Fig. ). At high-intertidal F elevations virtually all of the mobile and sessile organisms that live in the understory are dependent on habitat amelioration by the algal canopy to live in the intertidal habitat. Other common foundation species that are responsible for providing habitats for rocky-intertidal organisms around the world include other seaweed canopies , mussel beds in many temperate-zone rocky intertidal habitats, and tunicate beds on the Pacific coast of South America (Fig. ). stressful habitats, while physical stress ameliorating facilitations are common in physically stressful habitats. Conversely, in physically benign habitats, physical stress ameliorating facilitations are unimportant, whereas consumer pressures are elevated and associational defenses are of increased importance. These general patterns lead to predictable variation in the occurrence and importance of facilitation in rocky intertidal communities at local, regional, and global spatial scales (Fig. ). WHERE FACILITATIONS ARE FOUND Although facilitation is a common process in many rocky intertidal communities, the types of facilitation common in specific communities vary predictably as a function of the limiting stresses in a given community. In particular, consumers generally are not important in physically Heat- and desiccation-ameliorating positive interactions are one of the most common facilitations in rocky-intertidal communities. They occur when algal canopies, mussel beds, tunicate beds, algal turfs, and dense aggregations of barnacles and other sessile organisms in the intertidal provide relief from high temperatures and desiccation to organisms that live under their cover. Because the importance of this type of interaction in rocky-intertidal communities varies as a function of physical stress, these types of facilitation are more important at high-intertidal heights than at lowintertidal heights and more important at lower latitudes than higher latitudes. At high-intertidal heights many intertidal organisms are dependent on living under algal canopies or in mussel beds to limit physical stresses, but at lower tidal heights intertidal organisms are less...
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