ANTABUlTU BENTHOS
169
inherent in Southern Hemisphere plants and animals. Given the
periods of geological time involved, the Scotia Arc could supply a
differential dispersal route in both directions between the Antarctic
and South America and thus to the Atlantic and Pacific Oceans.
The other great dispersal mechanism available in the southern part
of the Southern Hemisphere is the West Wind Drift (Fig. 17). This is
also an obviously highly differential migration route, with longer distances between land masses but perhaps a more certain method for
species which can utilize a passive form of dispersal. The West Wind
Drift has also probably been in operation through the Tertiary. The
fact that the latitudinal belt through which the West Wind Drift has
operated may have fluctuated, would only have served to make this
more effective as a dispersal mechanism. Two studies supplying statistical data on the effectiveness of the West Wind Drift have been those of
Holdgate (1960) in relation to the marine fauna of Tristan da Cunha,
and of Fell (1962) in relation to echinoderms.
For such organisms as have been able to utilize the mainly passive
dispersal methods supplied by the West Wind Drift, circum-subantarctic distribution is possible. By reaching southern South America any
such species can be tested by the filter of the Scotia Arc. Similarly any
Antarctic species which have reached southern South America, the
Falklands or South Georgia by a northward migration may be dispersed
through the Subantarctic by the West Wind Drift. Thus any Southern
Hemisphere species with the necessary mechanisms for dispersal by the
West Wind Drift, over the period of time available through the Tertiary, could have been recruited to the Antarctic biota by utilizing only
those dispersal mechanisms which are in operation at present. Similarly
Antarctic organisms which are not strictly stenothermal, and which
have the necessary dispersal mechanisms could also have been distributed to the Subantarctic islands.
Much of the Antarctic fauna seems adapted to live over a wide
depth range, and a high percentage of the fauna is found in depths
considerably deeper than is usual in other parts of the world. Biogeographers have therefore legitimately postulated that some of the
Antarctic fauna may have been derived from adjacent deep-water areas,
or that it could have migrated to Antarctica along the series of roughly
radiating ridges, especially the Kerguelen and Macquarie Ridges, even
though these are too deep to be considered as possible migration routes
for normal shelf animals.
Dawson (1970) has shown that the Macquarie Ridge provides only
the most tenuous link between Macquarie Island and the Antarctic for
echinoderms.
169
inherent in Southern Hemisphere plants and animals. Given the
periods of geological time involved, the Scotia Arc could supply a
differential dispersal route in both directions between the Antarctic
and South America and thus to the Atlantic and Pacific Oceans.
The other great dispersal mechanism available in the southern part
of the Southern Hemisphere is the West Wind Drift (Fig. 17). This is
also an obviously highly differential migration route, with longer distances between land masses but perhaps a more certain method for
species which can utilize a passive form of dispersal. The West Wind
Drift has also probably been in operation through the Tertiary. The
fact that the latitudinal belt through which the West Wind Drift has
operated may have fluctuated, would only have served to make this
more effective as a dispersal mechanism. Two studies supplying statistical data on the effectiveness of the West Wind Drift have been those of
Holdgate (1960) in relation to the marine fauna of Tristan da Cunha,
and of Fell (1962) in relation to echinoderms.
For such organisms as have been able to utilize the mainly passive
dispersal methods supplied by the West Wind Drift, circum-subantarctic distribution is possible. By reaching southern South America any
such species can be tested by the filter of the Scotia Arc. Similarly any
Antarctic species which have reached southern South America, the
Falklands or South Georgia by a northward migration may be dispersed
through the Subantarctic by the West Wind Drift. Thus any Southern
Hemisphere species with the necessary mechanisms for dispersal by the
West Wind Drift, over the period of time available through the Tertiary, could have been recruited to the Antarctic biota by utilizing only
those dispersal mechanisms which are in operation at present. Similarly
Antarctic organisms which are not strictly stenothermal, and which
have the necessary dispersal mechanisms could also have been distributed to the Subantarctic islands.
Much of the Antarctic fauna seems adapted to live over a wide
depth range, and a high percentage of the fauna is found in depths
considerably deeper than is usual in other parts of the world. Biogeographers have therefore legitimately postulated that some of the
Antarctic fauna may have been derived from adjacent deep-water areas,
or that it could have migrated to Antarctica along the series of roughly
radiating ridges, especially the Kerguelen and Macquarie Ridges, even
though these are too deep to be considered as possible migration routes
for normal shelf animals.
Dawson (1970) has shown that the Macquarie Ridge provides only
the most tenuous link between Macquarie Island and the Antarctic for
echinoderms.