Chapter 2
Biogeographic Partition
of the Ocean
A
s suggested in the previous chapter, ecological geography is a special case of
the wider subject of biogeography—the latter being a discipline that, ideally,
should tell us not only how species are distributed but also how they aggregate to
form characteristic ecosystems, sustaining optimum biomass under characteristic regional
conditions of turbulence, temperature, irradiance, and nutrients. Unfortunately, classical
biogeography has made only limited progress in even the first of these tasks and since
ecological geography cannot proceed without some understanding of how and why
individual species are distributed as we observe them to be, it will be useful—before
proceeding any further—to consider the present status of the biogeography of the sea.
How far can our present knowledge of species distributions, or biogeography, satisfy
the needs of ecological geographers? In this chapter, I propose to explore the problems
associated with the use of biogeographic data for our purpose and to suggest how we may
profit from some earlier analyses of patterns of distribution of organisms at the regional
and ocean-basin scale.
Of course, oceanic biogeography has inherent difficulties compared with terrestrial
biogeography, as was discussed long ago by de Beaufort (1951). The relatively high cost
of collecting samples at sea, the high levels of expatriation of plankton species, the relative
lack of isolation between natural regions, and the problem of observing three-dimensional
distributions that vary in both space and time are among the most serious difficulties.
From the start, therefore, progress was slow compared with terrestrial biogeography,
which (for example) by about 1870 had already accurately located “Wallace’s line” in the
Indo-Pacific archipelago, separating the Australian from the Oriental faunas. But, at sea,
progress was slower and only the biogeography of the benthic fauna of shallow seas was
equally well developed by mid-20th century, as described in the classical work of Sven
Ekman (1953).
Modern marine biogeography may be said to have taken its origin in the suggestion
by Mary Somerville, in the 1862–70 editions of her Physical Geography, that the global
distribution of the marine fauna was best described by a series of nine latitudinal “homozoic” zones. These were “neither parallel with one another nor do they coincide with
lines of latitude but (respond) to the effect of warm and cold currents.” In the North
Atlantic, then, the Arctic zone has its “greatest breadth between the pole and the Gulf of
St Lawrence, and its least extension is (to) the extreme north of Scandinavia.” This is far
more sensitive to reality than some partitions of the global ocean that have been proposed
in recent years. She also described a progressive faunistic change from the intertidal zone
to the shelf edge and inferred the tropical submergence of high latitude, cold-water fish.
Although only a small part of the total effort of oceanography has ever been devoted
to biogeography, even as late as the 1960s strong teams, expressly devoted to this task,
19
Biogeographic Partition
of the Ocean
A
s suggested in the previous chapter, ecological geography is a special case of
the wider subject of biogeography—the latter being a discipline that, ideally,
should tell us not only how species are distributed but also how they aggregate to
form characteristic ecosystems, sustaining optimum biomass under characteristic regional
conditions of turbulence, temperature, irradiance, and nutrients. Unfortunately, classical
biogeography has made only limited progress in even the first of these tasks and since
ecological geography cannot proceed without some understanding of how and why
individual species are distributed as we observe them to be, it will be useful—before
proceeding any further—to consider the present status of the biogeography of the sea.
How far can our present knowledge of species distributions, or biogeography, satisfy
the needs of ecological geographers? In this chapter, I propose to explore the problems
associated with the use of biogeographic data for our purpose and to suggest how we may
profit from some earlier analyses of patterns of distribution of organisms at the regional
and ocean-basin scale.
Of course, oceanic biogeography has inherent difficulties compared with terrestrial
biogeography, as was discussed long ago by de Beaufort (1951). The relatively high cost
of collecting samples at sea, the high levels of expatriation of plankton species, the relative
lack of isolation between natural regions, and the problem of observing three-dimensional
distributions that vary in both space and time are among the most serious difficulties.
From the start, therefore, progress was slow compared with terrestrial biogeography,
which (for example) by about 1870 had already accurately located “Wallace’s line” in the
Indo-Pacific archipelago, separating the Australian from the Oriental faunas. But, at sea,
progress was slower and only the biogeography of the benthic fauna of shallow seas was
equally well developed by mid-20th century, as described in the classical work of Sven
Ekman (1953).
Modern marine biogeography may be said to have taken its origin in the suggestion
by Mary Somerville, in the 1862–70 editions of her Physical Geography, that the global
distribution of the marine fauna was best described by a series of nine latitudinal “homozoic” zones. These were “neither parallel with one another nor do they coincide with
lines of latitude but (respond) to the effect of warm and cold currents.” In the North
Atlantic, then, the Arctic zone has its “greatest breadth between the pole and the Gulf of
St Lawrence, and its least extension is (to) the extreme north of Scandinavia.” This is far
more sensitive to reality than some partitions of the global ocean that have been proposed
in recent years. She also described a progressive faunistic change from the intertidal zone
to the shelf edge and inferred the tropical submergence of high latitude, cold-water fish.
Although only a small part of the total effort of oceanography has ever been devoted
to biogeography, even as late as the 1960s strong teams, expressly devoted to this task,
19