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The lower limit in mussel beds is typically controlled by the interaction between recruitment and sea star predation, and the upper limit by desiccation (Robles & Desharnais 2002). But within the mussel bed itself, the ability of mussels to dominate space is limited primarily by the dislodgement of individuals by waves (see earlier). Dislodgment opens patches (gaps) of bare substratum in the bed, temporarily providing space for fugitive species, which are eventually snuffed out by reinvasion of the bed (Denny & Gaylord 2010). This process, referred to as ‘disturbance’, has been defined as ‘the displacement, damage or death of organisms caused by an external physical force or condition or incidentally by a biological entity’ (Sousa 2007, p. 186). Most of the information on the effects of, and recovery from, disturbance comes from studies of Mytilus beds and their associated flora and fauna on exposed shore sites on the Pacific and NE Atlantic coasts of North America (Seed & Suchanek 1992; Svane & Ompi 1993; Wootton 1993; Beukema & Cadee 1996; Carroll & Highsmith 1996; Hunt & Scheibling 1998, 2001; Bertness et al. 2002; Guichard et al. 2003; Sousa 2007, 2012; Calcagno et al. 2012).

Apart from wave action, common physical agents that initiate gap formation include impact or abrasion by waveborne objects such as cobbles, logs or ice; extremes of air or water temperatures; desiccation; fouling by brown algae and barnacles; hummocking; abrasion by suspended sand; and burial under deposited sand (Sousa 2007). Wave and log damage occurs mostly during the winter and is typically responsible for removing 1–5% of M. californianus cover per month on exposed shores (Paine & Levin 1981). The initial size of disturbance gaps can range from single mussel size to areas as large as 60 m². Subsequent enlargement of the gap (as much as 5000%) may occur, especially during winter months, primarily due to weaker byssal thread attachments (Witman & Suchanek 1984). While some mussel beds remain uniform and flat, others form elevated hummocks consisting of small groups of 10–20 mussels, which become detached from the rock and are forced upward above the bed surface. Mussels (Brachidontes rodriguezii) in hummocks show lower attachment strength than those in the single‐layered matrix (Gutiérrez et al. 2015). Accordingly, wave conditions associated with the passage of cold fronts (i.e. transition zones from warm air to cold air accompanied by moderate to strong winds and wave action, with seven‐day average recurrence times based on historical weather data) caused detectable mussel dislodgment in a high proportion of hummocks but have virtually no impact on single‐layered areas. Biological disturbances that disrupt the matrix of Mytilus beds are predators such as crabs and sea stars and epizoism by algal fronds; these usually occur on the scale of individual or a few mussels, but the sea star Picaster forms larger gaps in M. californianus beds (Seed & Suchanek 1992).

Patch structure and dynamics will probably differ among habitat types because of environmental differences influencing the intensity and outcome of biological interactions. Hunt & Scheibling (2001) compared the dynamics of natural and experimentally constructed mussel patches (M. trossulus and M. edulis) in two intertidal habitats, tidepools and emergent rock, over a time series (5, 10, 15 months) and among seasons (three successive five‐month intervals). In tidepools, mussels naturally occurred in small patches (median <25 cm²), while mussels on emergent rock formed extensive beds with decimetre‐scale gaps, but these beds started as small patches following disturbance such as ice scour. For their experimental patches (15 cm²) in both habitats, the authors assessed the relative importance of physical (wave disturbance) and biological (predation, growth, recruitment and immigration) processes in determining patch size and structure. Individual experimental and natural patches varied greatly in size over time, but mean patch area remained relatively constant. Mean size of individuals in experimental patches decreased due to the loss of larger mussels, while numbers increased due to recruitment. Wave disturbance appeared to be more important than predation in determining patch structure and dynamics, although losses due to either process did not differ consistently between habitats over time. Growth rates were low (≤0.4mm per month), but were greater in tidepools than on emergent rock, whereas recruitment and immigration rates generally did not differ between habitats. Although each process contributed to changes in patch size and structure, overall they did not result in a marked divergence in mean patch area or biomass between tidepools and emergent rock over the 15‐month experiment. This study highlights how integrative approaches, with monitoring of patches and of individuals within patches, can provide detailed mechanistic understanding of patch structure and dynamics.

Minchinton et al. (1997) monitored the recovery of intertidal algae and sessile macrofauna after a rare occurrence of scouring sea ice denuded the intertidal area of an exposed rocky shore in Nova Scotia, Canada. They found that barnacle cover was restored soon after the ice scour and macroalgae cover in about two years, but that it took considerably longer (four to six years) for the mussel beds (M. edulis and/or M. trossulus) to recover. Similar results have been reported by Brosnan & Crumrine (1994), who trampled 250 experimental plots in the intertidal zone for a period of one year and then monitored recovery. The algal–barnacle community recovered in the year following trampling, but mussel beds (M. alifornianus) and their associated species had not recovered within two years following cessation of trampling, and over this period no mussel recruitment was recorded. Two trampling studies (Beauchamp & Gowing 1982; Goldstein 1992) were performed 14 years apart at the same intertidal site, Santa Cruz on the west coast of California, with areas differing in intensity of trampling. There was little evidence of impact or change in either studies, although similar studies elsewhere have documented clear trampling effects (Ghazanshahi et al. 1983; Durán & Castilla 1989; Underwood & Kennelly 1990; Keough et al. 1993; Keough & Quinn 1998; Clark et al. 2002), as have others on the California coast (Beauchamp & Gowing 1982; Ghazanshahi et al. 1983; Goldstein 1992; Addessi 1994). In 1995, Van De Werfhorst & Pearse (2007) carried out a new study at the same Santa Cruz sites as Beauchamp & Gowing (1982) and Goldstein (1992), using a different strategy sampling regime, and were able to demonstrate trampling effects. Mussel bed cover, with the associated higher number of species within it, decreased with increased human trampling, while the frequency of bare rock areas increased. Consistent with the previous studies, the two brown algal species that were abundant on the less trampled sites were completely absent at the most trampled. These results, in conjunction with those of the earlier studies, indicate that rocky intertidal assemblages can be resilient to human trampling, and that sampling scale and design are important in evaluating and monitoring trampling impacts (Van De Werfhorst & Pearse 2007). Micheli et al. (2016) evaluated the separate and combined influences of disturbance from storm waves and disturbance associated with human trampling of rocky shores by conducting an experiment mimicking controlled levels of trampling at sites with different wave exposures and before and after a major storm event in central California, United States. Their results show that trampling and storm waves affected the same taxa and had comparable and additive effects on rocky shore assemblages. Both disturbance types caused significant reduction in percent cover of mussels (M. californianus) and erect macroalgae, and resulted in significant reorganisation of assemblages associated with these habitat‐forming taxa. A single extreme storm event caused similar per cent cover losses of mussels and erect macroalgae as 6–12 months of trampling, which combined additively rather than synergistically. Mussel beds in wave‐exposed sites were more vulnerable to trampling impacts than algal beds at protected sites. Mussels and erect macroalgae recovered within five years after trampling stopped. These results suggest that impacts from local human use can be reversed in relatively short periods. See Mendez et al. (2019) for a similar study on Brachidontes spp. in Patagonia, Argentina.

Waves are not only associated with hydrodynamic stress but can also carry heavy loads of sand, periodically disturbing intertidal shores through sand burial or sand scour (reviewed in McQuaid et al. 2015). Acting as an agent of disturbance, sand removes plant tissue, epiphytes or invertebrates with poor attachment to the rock surface through scouring and decreases light, oxygen and substratum available to organisms through burial. It can lead to temporary species impoverishment by selective elimination of maladapted species, although in the longer term it may also enhance species richness by increasing habitat heterogeneity, allowing within‐shore coexistence of sand‐intolerant species and those associated with sand deposits (McQuaid & Dower 1990).

Unexpectedly, sand stress strongly affects the survival of M. galloprovincialis and P. perna individuals but is not related to their physiological tolerances and does not explain their vertical zonation. When buried under sand, P. perna mortality rates are higher than those of M. galloprovincialis in both laboratory and field experiments, yet it is P. perna that dominates the low shore where sand inundation is recurrent (Zardi et al. 2006). Although both species accumulate sediments within the shell valves while still alive and sand buried, the quantities are much greater for P. perna, causing intense visible damage and clogging of the gills, which explains its higher mortality rates. Presumably, the accumulation of sand within the shell of P. perna is linked to its gaping behaviour. Wave and sand stress vary also in time, altering the timing and mortality rates of the two mussel species (Zardi et al. 2008). During periods of high sand accumulation in mussel beds, the indigenous species has increased mortality rates that are higher than those of M. galloprovincialis, while the pattern is reversed during winter, when wave action is high (Zardi et al. 2008). When sand stress is high, the less stable secondary substratum of sand and shell fragments weakens the attachment strength of mussels living within a bed. Consequently, the indigenous species loses its advantage in attachment strength over the invasive species, and this results in a seasonal shift in the competitive balance between the two.

There has been considerable interest in the ecology of patches, especially concerning the successional⁶ events leading to their recolonisation. In the case of M. californianus, the patches are initially colonised by diatoms, then by ephemeral algae such as Ulva and Porphyra spp. and then by perennial algae. These successional stages are followed by acorn barnacles, Balanus glandula and Semibalanus cariosus, then goose barnacles, Pollicipes polymerus and sometimes M. trossulus, and then by M. californianus. The latter needs secondary space (as opposed to primary space, i.e. rock surface), such as substrates like algae, barnacles and byssus threads, for larval settlement, but later becomes competitively dominant over all other sessile species. These events take about five years, depending upon degree of wave exposure (Dayton 1971; Paine & Levin 1981; Wootton 2002).

The mechanism driving these successional events is generally thought to be competition with larger, later‐colonising species assuming competitive dominance over smaller, early‐succession species, until finally sea mussels dominate. M. californianus exhibits competitive dominance: small barnacles are smothered, larger barnacles such as Semibalanus cariosus are overgrown and abraded and goose‐neck barnacles, Pollicipes polymerus, are slowly crushed to death. Both M. californianus and Semibalanus cariosus have refuge in size from predation by whelks Nucella spp., and thus could potentially monopolise all space were it not for predation by sea stars, Pisaster ochraceus, from lower positions on the shore and for creation of new colonisable spaces by log damage. Studies on the Olympic Peninsula, Washington, United States also show that by consuming the early successional stages, predators such as whelks and birds and herbivores such as chitons and limpets actually accelerate succession (Suchanek 1981; Paine 1984).

The importance of large‐scale ocean currents in the global distribution of bivalves has already been dealt with in this chapter. Locally, areas with strong currents usually provide favourable feeding conditions for bivalves. However, very strong currents can have an inhibitory effect on feeding and consequently growth. Also, strong currents may prevent larval settlement and byssal attachment of spat, ultimately resulting in local variability in recruitment.

Fishing methods can affect bivalve abundance directly by causing significant mortality and indirectly by causing shell damage. In Spain, mussel seed from intertidal exposed rocky shores is the method most used by farmers to seed ropes in mussel culture areas (Peteiro et al. 2007). This practice, while legal, must have a detrimental effect on mussel beds and their community structure, although to date there is no documented evidence of damage.