2019). Under this scenario, Brante et al. (2019) evaluated the dispersal behaviour of juveniles and adults of S. algosus as a potential response to competition with P. purpuratus. They also measured the attachment strength of S. algosus in the presence of its competitor as a measure of its escape response ability. Their results show that the presence of P. purpuratus increased the movement activity of juveniles and adults of S. algosus and decreased their attachment strength. Field experiments carried out with marked individuals on a Chilean rocky shore showed that S. algosus exhibits higher local dispersion in the zone where P. purpuratus is present. Mussels' high dispersal ability throughout the whole benthic phase may serve not only to reach the optimal physiological position in the intertidal, but also to reduce interspecific competition.
Figure 3.13 Performance of three indigenous South African mussels, Aulacomya atra, Perna perna and Choromytilus meridionalis, relative to Mytilus galloprovincialis. (a) Growth rate (mm in the first four years). (b) Total annual reproductive output as a percentage of body mass. (c) Survival rate after 24 months at a shore height experiencing 50% exposure to air per tidal cycle.
Source: From Branch & Steffani (2004). Reproduced with permission from Elsevier.
Another example of interspecific competition is the contribution of M. galloprovincialis to the displacement of M. trossulus along much of its historic range in southern California (Shinen & Morgan 2009). Other examples are those between M. californianus and the sea palm, Postelsia palmaeformis, on the Pacific coast (Dayton 1973; Blanchette 1996). The sea palm grows quickly and may overgrow M. californianus, eventually causing mussels to be torn free in the waves. This creates the space necessary for settlement of spores and recolonisation by Postelsia. Then there is the competition between M. galloprovincialis and the large indigenous limpet, Scutellastra argenvillei, on the west coast of South Africa, where the mussel is capable of forming dense, almost monospecific stands low on the shore. A survey indicated that at wave‐exposed locations, the abundance of M. galloprovincialis changes with exposure (Steffani & Branch 2003). At such locations, the mussel covered up to 90% of the primary substratum, whereas in semi‐exposed situations it was never abundant. As the cover of M. galloprovincialis increased, the abundance and size of S. argenvillei on rock declined, becoming confined to patches within a matrix of mussel beds. Both species were absent from sheltered shores and diminished where wave action was extreme. Comparisons with previous surveys indicated that exposed sites now largely covered by the alien mussel were once dominated by dense populations of the limpet. Therefore, the results of this survey provide circumstantial correlative evidence of a competitive interaction between M. galloprovincialis and S. argenvillei, and suggest that wave action mediates the strength of this interaction. The presence of mussel beds provides a novel settlement and living substratum for recruits and juveniles of S. argenvillei, albeit at much lower densities than in limpet patches. Adult limpets are virtually excluded from the mussel beds owing to their large size, which indicates the unsuitability of this habitat as a replacement substratum after competitive exclusion from primary rock space.
The American slipper limpet, Crepidula fornicate, was unintentionally introduced to Europe in the 1870s with oysters imported for farming purposes. Since the limpet is a filter feeder, trophic competition and associated negative effects when epizootic on bivalves have been assumed (Figure 3.14). Thielges et al. (2005) were the first to experimentally test the effects of C. fornicata on survival and growth of its major basibiont, M. edulis. In two field experiments (each lasting two weeks), epigrowth by C. fornicata resulted in a four‐ to eightfold reduction in survival of mussels, equivalent to a mortality of 28 and 30%, respectively. Shell growth in surviving mussels with attached C. fornicata was three to five times lower compared to unfouled mussels, but similar to that with artificial limpets. As a causative agent, interference competition in the form of changes in small‐scale hydrodynamics due to C. fornicata stacks was suggested. This could result in a high energy expenditure for byssus production of mussels. In general, interference and not exploitation competition is suggested to be the major impact of epizootic C. fornicata on its basibionts (organisms that are host to epibionts) in Europe.
Figure 3.14 Stack of four American slipper limpets, Crepidula fornicata, attached to the mussel Mytilus edulis.
Source: From Thieltges (2005). Reproduced with permission from Inter‐Research.
Typically, however, intraspecific competition for space is a more serious problem than interspecific competition, in that heavy spat fall of mussels on to adult beds can cause the underlying mussels to suffocate, thus loosening the entire population from the rock surface (Seed 1976). See Chapter 5 for information on interspecific and intraspecific gamete competition in Mytilus taxa.
Climate Change and Potential and Observed Impacts on Marine Mussels
Climate in a narrow sense is usually defined as the average weather, or more rigorously as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. Climate encompasses patterns of temperature, precipitation, humidity, wind and seasons. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization (https://public.wmo.int/en). Our climate is now changing at a rate faster than any seen in the last 2000 years (www.ecology.wa.gov). This is due to rising levels of carbon dioxide and other heat‐trapping (greenhouse) gases in the atmosphere. The primary greenhouse gases in Earth’s atmosphere are water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3). These act like a blanket around Earth, causing it to warm – a phenomenon referred to as ‘global warming’. According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by about 0.8 °C since 1880. Two‐thirds of the warming has occurred since 1975, at a rate of roughly 0.15–0.20 °C per decade (Figure 3.15). Wide‐ranging impacts of rising temperatures include rising sea levels, melting snow and ice, more extreme heat events, fires and drought and more extreme storms, rainfall and floods. This rise in global average temperature is attributed to an increase in greenhouse gas emissions. The link between global temperatures and greenhouse gas concentrations, especially of CO2, has held throughout Earth’s history (Lacis et al. 2010). CO2 concentrations in the atmosphere are now well over 400 ppm, their highest levels in over 800 000 years (Figure 3.16). Globally, we emit over 36 billion tonnes of CO2 per year, and this continues to increase. There are large differences – more than 100‐fold – in per capita CO2 emissions between countries (Richie & Roser 2020). For example, the United States has contributed most to global CO2 emissions to date, accounting for 25% of cumulative emissions. This is followed by the EU‐28 (22%), China (13%), Russia
