Benthic Community and Climate Change - Marine Ecology in a Changing World - page 129

Chthamalus that reach their northern limits in Scotland and dominate shores
in southern Europe and North Africa (Hawkins et al . 2003). Observations
since 1985 indicate that warm-water barnacle species have become more
abundant (Southward 1991) (Fig. 1) and that there is a strong correlation
between barnacle abundance and sea surface temperature: the abundance
of the warm-water species ( Chthamalus spp.) is positively correlated with
inshore temperature whereas the abundance of the cold-water species
( Semibalanus balanoides ) is negatively correlated. Abundances of other
southern intertidal species such as Patella depressa and B. perforatus have
increased in the region of the biogeographic boundary in southwest England
since the 1980s (Herbert et al . 2003, Mieszkowska et al . 2005) and those of
many northern species have decreased (e.g., Semibalanus balanoides and
Patella vulgata ). It is likely that these increases/decreases in abundance are
associated with increasing sea temperatures (Harris et al . 1998, Helmuth
et al . 2006a, Hawkins et al. 2009).
Increasing sea temperatures have facilitated the range extension of
warm-water species and numerous studies have focused on the role of water
temperature in setting species distributional limits. In order to effectively
determine the overall impact of climate change it is considered necessary to
carry out studies throughout species ranges rather than just at the margins
of species range (Helmuth et al . 2002, Sagarin and Somero 2006). Changes in
water temperature can have the effect of eliminating species from intertidal
regions, resulting in contractions in species geographic distribution as
climatic conditions exceed the species' physiological threshold of tolerance.
Conversely, range expansion occurs when new individuals colonize at sites
that become physiologically tolerable for the fi rst time as environmental
Fig. 1. Abundance of Chthamalus spp. and Semibalanus balanoides at Cellar Beach, Yealm estuary,
Devon. Points represent average abundance during each autumn. Taken from: Hawkins et al .
2003 (Copyright Clearance Center: License Number: 3087770882741).