Chapter 8
Longer Term Responses: From
Seasons to Centuries
S
o far, I have discussed a partition of marine ecosystems into biomes and provinces as
if these were permanent entities, fixed in space and invariant in their characteristics
at time scales beyond the seasonal, but—as we all know—that is very far from
their real condition. This is not the place for a full review of how life in the ocean
responds to changes in Earth’s physical systems, but it will be useful briefly to consider the
problems raised for us by the impermanence of conditions in the sea. For an ecological
geographer, the core of the problem arising from this impermanence is the extent to
which the boundaries set between natural regions of the ocean shift in response to
changing conditions; for an ecologist, it is rather the changing conditions within each
region that are more important.
Marine ecosystems certainly have less permanence than terrestrial ecosystems. Ashore,
ecologists are not confronted with shifting ecological discontinuities, or with changes in
the characteristic conditions of individual ecosystems, because, unless man intervenes,
the tree line on a mountain or the passage between grassland and savannah remains
approximately static over a human lifetime. It is only on the millennial scale that such
boundaries migrate significantly, or that characteristic regional ecosystems disappear.
Urban sprawl, deforestation, overgrazing, and intensive agriculture are accomplishing in
a few decades what nature can only do in centuries, but that sad fact does not alter
the argument. Although the human population explosion can produce pressures that
rapidly shift ecological boundaries and modify ecosystems ashore, it is paradoxically more
difficult directly to modify the average locations of the ephemeral and shifting ecological
boundaries of the seas that are the subject of this study. We can accomplish this only
indirectly by atmospheric modification, resulting in a changed global climate and a shifted
ocean circulation.
Indeed, if we are agreed that the regional characteristics of marine ecosystems are
consequent on the characteristics of the physical environment, then we must assume
that ecological conditions are as impermanent as the physical conditions themselves. And
these, it is now well understood, are in continual flux and state of change at all scales
of variability, both spatial and temporal. Although for most practical purposes we regard
the circulation of ocean and atmosphere as having a “normal” behavior (we call it the
“average climate”), we really know that this is a moving average and capable of important
excursions. At least some of these must be accommodated in any model of how biota
are distributed and interact in the oceans. And so they must be accommodated in any
account of how marine ecosystems are distributed and function in today’s oceans.
115
Longer Term Responses: From
Seasons to Centuries
S
o far, I have discussed a partition of marine ecosystems into biomes and provinces as
if these were permanent entities, fixed in space and invariant in their characteristics
at time scales beyond the seasonal, but—as we all know—that is very far from
their real condition. This is not the place for a full review of how life in the ocean
responds to changes in Earth’s physical systems, but it will be useful briefly to consider the
problems raised for us by the impermanence of conditions in the sea. For an ecological
geographer, the core of the problem arising from this impermanence is the extent to
which the boundaries set between natural regions of the ocean shift in response to
changing conditions; for an ecologist, it is rather the changing conditions within each
region that are more important.
Marine ecosystems certainly have less permanence than terrestrial ecosystems. Ashore,
ecologists are not confronted with shifting ecological discontinuities, or with changes in
the characteristic conditions of individual ecosystems, because, unless man intervenes,
the tree line on a mountain or the passage between grassland and savannah remains
approximately static over a human lifetime. It is only on the millennial scale that such
boundaries migrate significantly, or that characteristic regional ecosystems disappear.
Urban sprawl, deforestation, overgrazing, and intensive agriculture are accomplishing in
a few decades what nature can only do in centuries, but that sad fact does not alter
the argument. Although the human population explosion can produce pressures that
rapidly shift ecological boundaries and modify ecosystems ashore, it is paradoxically more
difficult directly to modify the average locations of the ephemeral and shifting ecological
boundaries of the seas that are the subject of this study. We can accomplish this only
indirectly by atmospheric modification, resulting in a changed global climate and a shifted
ocean circulation.
Indeed, if we are agreed that the regional characteristics of marine ecosystems are
consequent on the characteristics of the physical environment, then we must assume
that ecological conditions are as impermanent as the physical conditions themselves. And
these, it is now well understood, are in continual flux and state of change at all scales
of variability, both spatial and temporal. Although for most practical purposes we regard
the circulation of ocean and atmosphere as having a “normal” behavior (we call it the
“average climate”), we really know that this is a moving average and capable of important
excursions. At least some of these must be accommodated in any model of how biota
are distributed and interact in the oceans. And so they must be accommodated in any
account of how marine ecosystems are distributed and function in today’s oceans.
115