Seaweeds Ecology and Climate Change - Marine Ecology in a Changing World - page 170

in poleward shifts of the range. Therefore, a northward range shift occurs
when there is net extinction at the southern boundary or net colonization
at the northern while a southward range shift occurs when there is a net
extinction at the northern boundary or a net colonization at the southern.
Wernberg et al. (2011c) found that, on both the Indian and Pacifi c Oceans
sides on the Australian continent, seaweed communities in the southern
(poleward) part of the tropical-temperate transition progressively came to
resemble past macroalgal communities farther north, the change was of
similar magnitude on both coasts showing a rearrangement of entire local
communities rather than mere shifts of a few individual species. In Sogn
og Fjordane, Norway, a study of the macroalgal community at 22 sites
between 1994 and 2004 showed a signifi cant increase in the abundance of
southern species (Husa 2007).
Temperature and invaders
Non-native marine species are considered a threat to biodiversity because of
their potential to compete with native species and preempt their resources,
to alter energy fl ow through communities, to facilitate the introduction
of other non-native species, and to homogenize regional biotic diversity
(McKinney and Lockwood 1999, Grosholz 2005, Bulleri et al. 2008). While the
threat from invasive species is not directly climate related, climate has often
been proposed to facilitate the establishment, further spread and impact
of invasive species on temperate reefs (Thresher et al. 2003, Scheibling
and Gagnon 2009) since warming can allow warm water species to extend
to or invade previously nonhospitable regions altering the competitive
interactions between introduced and native species (Occhipinti-Ambrogi
2007). In Australia, for example, it was suggested that the spread of the
shore crab Carcinus maenas from Victoria into Tasmania was facilitated by
increasing ocean temperatures in response to a strengthening of the EAC
(Thresher et al. 2003), C. maenas is voracious predator in intertidal and
shallow subtidal reef habitants and exerts strong top down control of reef
communities (Bertness et al. 2002). In Nova Scotia (Canada) warmer water
increased fouling of kelp fronds by an invasive bryozoa, causing reduced
reproductive output and defoliation of the kelps, which led to a switch in
reef community structure (Scheibling and Gagnon 2009).
The rate of marine introductions, including introductions of seaweeds,
has increased over the last 20 years. Marine macroalgae are also a signifi cant
component of marine alien taxa (Schaffelke et al. 2006) with current global
estimates of introduced species ranging from 163 to 260 species (Ribera
Siguan 2002). The invasion of the green algae Caulerpa in the Mediterranean
has profound effects both on the habitat and on ecosystem functioning.
Caulerpa taxifolia originated from aquarium release on the coast of France
(Jousson et al. 1998) and has expanded to very large areas of the Ligurian