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shallowest areas are also dominated by D. antarctica . Xiphophora is common at 2 m
depth and Halopteris funicularis around 2-5 m. Lessonia brevifolia forms beds
around 4-12 m depth, M. pyrifera being abundant in deep waters. The laminarian
Ecklonia radiata, the most common kelp of New Zealand, has not been recorded
neither at the Chatham nor the Auckland Islands (Schiel 1990 ; Schiel et al. 1995 ).
The ecology of the seaweed communities from the sub-Antarctic islands region is
poorly known. The harsh environmental conditions as well as the remoteness and
inaccessibility of many islands impede experimental studies. The scarce studies
characterizing the structure and functions of seaweeds of intertidal zones reveal that
physical environmental factors exert a strong control on these assemblages, probably
regulating also the action by herbivores (Smith and Simpson 2002 ; Freeman et al. 2011 ).
On the other hand, the importance of the habitat-forming species Durvillaea antarctica
on the ecology of the littoral system has been emphasized (Klemm and Hallam 1988 ;
Smith and Simpson 2002 ). In subtidal areas of these islands, the biological processes
occurring within the Macrocystis forests have been also studied (Dayton 1985 ;Graham
et al. 2007 ), including aspects related with their trophic role in the food web (Kaehler
et al. 2006 ), aspects of primary production, and geochemical cycles (Delille et al. 2009 ).
14.3 Biogeographical Processes
The development of the ACC as a result of the opening of the Tasmanian Gateway
between Australia and Antarctica (~33 Ma) and the Drake Passage between South
America and Antarctica (~23 Ma) permitted the circumpolar distribution of biota in
the southern hemisphere (Clayton 1994 ). Many cool water taxa evolved in the early
Tertiary at high latitudes (the so-called Weddellian species) and colonized lower
latitudes, e.g., Tierra del Fuego in southern South America (Luning 1990 ).
A relatively recent long-range dispersal across the Southern Ocean is reflected in
many species reported in South America, the Falkland Islands, and various sub-
Antarctic islands (Clayton 1994 ; see also Chap. 17 by Rothausler et al.).
In order to interpret the present distribution patterns and origin of biota in the
southern hemisphere, different theories dealing with separation through plate tec-
tonics, long-distance dispersal during different periods, and the role of the forma-
tion of the ACC linking the land masses have been proposed over the last decades.
Models of the species distribution involving ACC dispersal were widely replaced
by vicariance and the role of past geological events (plate tectonic) (Cheshire et al.
1995 ; Waters 2008a ). However, recent evidence based on new molecular
techniques emphasizes the role of dispersal in distribution of marine biota in the
southern hemisphere and relates some evidence of vicariance in southern waters
with more localized and recent vicariant models, e.g., the isolation by the Bassian
Isthmus that connected Tasmania with Australia during Pleistocene low sea-level,
rather than with plate tectonics (Waters 2008a ; Fraser et al. 2009a ). Recent ecolog-
ical, phylogenetic, and phylogeographic studies suggest also that rafting cannot be
ignored as an important mechanism of long-distance dispersal in southern waters
(Waters 2008a ; see also Chap. 17 by Roth
ausler et al.).
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