Rocky Intertidal Habitats
III. Distribution Patterns
A. Zonation
For the MBNMS in general, four zones of rocky intertidal organisms associated with different tidal heights have traditionally been distinguished (Table 1). The splash zone is almost always exposed to air, and has relatively few species. The periwinkle, Littorina keenae, is used in some cases as an indicator of this zone, and microscopic algae are common in winter months when large waves produce consistent spray on the upper portions of the rocky shore. The high intertidal zone is exposed to air for a long periods twice a day. The barnacle, Balanus glandula, and red algae, Endocladia muricata and Mastocarpus papillatus, are used as indicators of this zone, but these species are also found in other areas of the rocky shore.
The mid-intertidal zone is exposed to air briefly once or twice a day, and has many common organisms. At wave-exposed sites, the mussel, Mytilus californianus, can dominate the available attachment substratum. The low intertidal zone is exposed only during the lowest tides, and the presence of the seagrass, Phyllospadix, is a good indicator of the mean lower low water tide level (0.0 m). This zone is also where sponges and tunicates are most common. Zones will form at different distances from the sea when there is no tidal height difference (Marsh and Hodgkin 1962, Lebednik et al. 1971, Kinnetic Labs. 1985), zones will form within zones (De Vogelaere 1991) , zones will expand with increasing wave exposure (Ricketts et al. 1985) and, while dramatic and extensively referred to, zonation patterns are highly variable (Foster et al. 1988, Foster 1990).The mechanisms that determine zonation patterns are often broken down into the categories of physical and biological factors, and it is a combination of these that determines each site's biological characteristics. The mixed semidiurnal tides in central California result in drying and wetting cycles that vary greatly over small vertical distances of shoreline. Some of these sharp break points are associated with well defined boundaries between zones, and have been termed critical tide levels (Doty 1946). Tides influence temperature, light, nutrient availability and salinity in addition to simple desiccation, and all of these factors have been shown to affect growth and reproduction of intertidal species (Connell 1972, Ricketts et al. 1985, Foster et al. 1988, 1991). However, with the increased use of field experiments, it is now clear that zonation of organisms at many sites is also caused by biological factors. For example predation by sea stars in the low zone may set the lower limit of the mussels (Paine 1974). It has also been demonstrated that competition for space (e.g., Connell 1961, Lubchenco 1980, Foster 1982), grazing (e.g. Robles and Cubit 1981, Underwood and Jernakoff 1984), settlement and recruitment (e.g. Grosberg 1982, Underwood and Denley 1984, Connell 1985), associations between species (Dayton 1975) and movement behavior (e.g. Frank 1965, Wolcott 1973, Phillips 1976) can influence zonation patterns.
B. Patchiness within zones
Examples of patchily distributed organisms are common, though these patterns are rarely quantified or mapped (De Vogelaere 1993). Indeed, rocky shores are sometimes referred to as mosaics of patches undergoing succession after a variety of possible disturbance events and times (Paine and Levin 1981; Sousa 1984, 1985). Disturbances that open up space for colonization are caused by hydrodynamic forces (e.g., Denny 1987, Gaylord et al. 1994), predation (VanBlaricom 1988), wave-tossed rocks and logs (Dayton 1971, Shanks and Wright 1986), and substratum weathering and exfoliation (De Vogelaere, unpublished data). Disturbances are common enough that some species persist as fugitives, dispersing from one patch to another, as the dominant competitors crowd them out (Sousa 1985, but see De Vogelaere 1991 for a recent review of the concept). Size and severity of the disturbance, as well as position within the patch (border or center) will also effect which species are present (Foster et al., 1988). One of the clearest examples of how disturbances can influence species diversity is from intertidal boulder fields (Sousa 1979a, b). Small boulders that are frequently disturbed will be occupied by ephemeral species such as the green alga Ulva spp., intermediate sized boulders are occupied by a variety of species undergoing the successional process, and the largest, rarely turned boulders are occupied by a few competitively dominant species, such as Gigartina canaliculata.
Other, more stable microhabitats also cause patchiness within zones. For example, species such as crabs are better adapted to living in cracks and crevices (Ricketts et al. 1985). Tide pools also have a unique assemblage of species, often including fish that have the ability to home to specific pools (e.g. Yoshiyama et al. 1992). As with disturbance events, the size, shape and location of the tidepool will influence the associated assemblage of species (Metaxas and Scheibling 1993). Substratum type, relief and aspect are also important contributors to the patchy patterns we observe within intertidal zones (Foster et al. 1988).
Direct biological factors such as grazing, predation, dispersal limits, and larval recruitment behavior cause patchy patterns (Foster et al. 1988); moreover, there is increasing evidence of the importance of complex interactions between these factors, as well as indirect interactions (e.g. Jernakoff 1983, Dungan 1986, Wootton 1993). For example, the large territorial limpet, Lottia gigantea, creates a garden of microscopic algae by bulldozing away other species (Stimpson 1970) while its mucus trail enhances growth of the algae it feeds upon (Connor and Quinn 1984). Algal abundance can also be enhanced by the presence of barnacles that form barriers to most grazing limpets (Jernakoff 1983). If the additional factor of bird predation on limpets is considered (Hahn and Denny 1989), then it becomes clear that seemingly simple explanations for distribution patterns are not complete without assessing the indirect role of the other species in the local rocky shore assemblage (Wootton 1993).
C. Differences between sites and biogeography
One of the goals of ecology is to find unifying principles that explain patterns across all areas of investigation. As experiments are replicated across several intertidal sites (e.g. Kinnetic Laboratories Inc. 1985, 1992), numerous differences in the abundances of organisms and how they interact have been found, making generalization difficult (Underwood and Denley 1984, Foster 1990, Menge 1994). Rocky shores 50 meters apart may have different assemblages because of differences in wave exposure (Ricketts et al. 1986), seasonal inundation by sand (Littler et al. 1983, Stewart 1983), local currents that influence dispersal of spores and larvae (Kanter 1980, Ebert 1982, Roughgarden et al. 1988), and other factors already mentioned above that cause variability within zones.
If sites are examined within a range of latitudes, biogeographic patterns become apparent. Recognized biogeographic breaks in California occur at Cape Mendicino, Monterey Bay and Point Conception, and species are characterized as having northern and southern affinities relative to these breaks (Foster et al. 1988, 1991, Barry et al. 1995). Biogeographic breaks are presumably caused by temperature and other factors associated with water masses of major current systems (Hayden and Dolan 1976, Foster et al. 1988). Large expanses of sand habitats, such as the beaches of Monterey Bay, may also create dispersal barriers for rocky shore species. However, in the case of the Monterey Bay break, the patterns may also be an artifact caused by the intense collecting for taxonomic studies associated with the Hopkins Marine Station (see Biological Diversity) and the paucity of collections from the relatively inaccessible Big Sur coastline.
Nevertheless, there are clear biogeographic patterns in species distribution and population regulation phenomena, such as in the black abalone (Haliotis cracherodii) population collapse in the southeastern Channel Islands (Richards and Davis 1993) and the influence of upwelling on intertidal fish biogeographic provinces (Stepien et al. 1991).
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