514
D. J. TRANTER:
VII. Discussion
In this study, interest is centred on the dynamics and stability of the pelagic ecosystem -the forces which induce fluctuations within the system, and the stabilizing forces
which keep those fluctuations within certain bounds. At the species level, fluctuations
arise as a result of environmental variations in available energy, time-lag phenomena, and
variations in age at death (MACARTHUR, 1955). Similar influences determine stability at
the community (food web) level.
Biological communites are usually much more stable in low latitudes than in high
latitudes (DUNBAR, 1960; MACARTHUR, 1965; PIANKA, 1966). This general principle is
held to apply also to pelagic ecosystems (STEEMANN NIELSEN, 1957; CUSHING, 1958),
the two best known instances being the Sargasso Sea (MENZEL and RYTHER, 1960; 1961)
and the eastern tropical Pacific (BLACKBURN, 1966; BLACKBURN et aI., 1970). Pelagic
ecosystems in the tropical and subtropical Indian Ocean might therefore be expected to
be equally stable. On the other hand, the monsoons of the Indian Ocean area represent
a unique source of instability whose impact on the pelagic ecosystem has never yet been
gauged.
The pelagic ecosystem has many co-ordinates: 3 in space and 2 in time. The time coordinates are succession and translation. Depth, translation, and the rapidity of
succession (especially in warm waters) make the system one of unparalleled complexity.
The form it takes at any point in space (e.g. an oceanographic station) varies from time to
time, like cloud formations in the sky, representing the instantaneous balance between
gain and loss (MARGALEF, 1968).
In view of such complexity, the ecosystem is better considered as a process rather than
as a structure, in analogy with laboratory chemos tats (MALEK and FENCL, 1966). It is one
consequence of such continuous flow systems that too fast a rate of flow or too slow a
rate of reproduction leads to "washout" of the system relative to a fixed point of observation. Time-series changes in estuarine plankton populations have been considered in such
a way (KETCHUM, 1954; BARLOW, 1955; TRANTER and ABRAHAM, 1971).
As a corollary of this, populations disappear from such an ecosystem in reverse order
to their reproductive rate at the point of observation. As a result, trophic sequences
separate in space like pigments on a chromatographic column. VINOGRADOV, VORONINA
and SUKHANOVA (1961) for example, found the spatial maxima of algae, herbivores,
primary carnivores, and secondary carnivores in the Indian Ocean to be displaced at
successive distances from the source of nutrient enrichment whence the food chain had
its orgin, each trophic level in turn developing to a peak at the site of maximal net
recruitment. Similar conclusions have been reached more recently by TIMONIN (1969).
It follows, therefore, that from anyone oceanographic station, the trophic sequence
can be traced a limited distance through the food web; where observations span a wider
area, as in the present study, the sequence can be followed farther. In the discussion which
follows, I have assumed that succession can be traced as far as the second trophic level
(herbivores); beyond this point in the food web, the observed changes are more likely to be
the consequence of translation, a product of food webs generated farther upstream. If
such an assumption is true, even approximately, then nutrient enrichment, primary
production, grazing, excretion (by grazers), and remineralization may profitably be
examined from the window represented by the study section, for cause and effect
relationships.
D. J. TRANTER:
VII. Discussion
In this study, interest is centred on the dynamics and stability of the pelagic ecosystem -the forces which induce fluctuations within the system, and the stabilizing forces
which keep those fluctuations within certain bounds. At the species level, fluctuations
arise as a result of environmental variations in available energy, time-lag phenomena, and
variations in age at death (MACARTHUR, 1955). Similar influences determine stability at
the community (food web) level.
Biological communites are usually much more stable in low latitudes than in high
latitudes (DUNBAR, 1960; MACARTHUR, 1965; PIANKA, 1966). This general principle is
held to apply also to pelagic ecosystems (STEEMANN NIELSEN, 1957; CUSHING, 1958),
the two best known instances being the Sargasso Sea (MENZEL and RYTHER, 1960; 1961)
and the eastern tropical Pacific (BLACKBURN, 1966; BLACKBURN et aI., 1970). Pelagic
ecosystems in the tropical and subtropical Indian Ocean might therefore be expected to
be equally stable. On the other hand, the monsoons of the Indian Ocean area represent
a unique source of instability whose impact on the pelagic ecosystem has never yet been
gauged.
The pelagic ecosystem has many co-ordinates: 3 in space and 2 in time. The time coordinates are succession and translation. Depth, translation, and the rapidity of
succession (especially in warm waters) make the system one of unparalleled complexity.
The form it takes at any point in space (e.g. an oceanographic station) varies from time to
time, like cloud formations in the sky, representing the instantaneous balance between
gain and loss (MARGALEF, 1968).
In view of such complexity, the ecosystem is better considered as a process rather than
as a structure, in analogy with laboratory chemos tats (MALEK and FENCL, 1966). It is one
consequence of such continuous flow systems that too fast a rate of flow or too slow a
rate of reproduction leads to "washout" of the system relative to a fixed point of observation. Time-series changes in estuarine plankton populations have been considered in such
a way (KETCHUM, 1954; BARLOW, 1955; TRANTER and ABRAHAM, 1971).
As a corollary of this, populations disappear from such an ecosystem in reverse order
to their reproductive rate at the point of observation. As a result, trophic sequences
separate in space like pigments on a chromatographic column. VINOGRADOV, VORONINA
and SUKHANOVA (1961) for example, found the spatial maxima of algae, herbivores,
primary carnivores, and secondary carnivores in the Indian Ocean to be displaced at
successive distances from the source of nutrient enrichment whence the food chain had
its orgin, each trophic level in turn developing to a peak at the site of maximal net
recruitment. Similar conclusions have been reached more recently by TIMONIN (1969).
It follows, therefore, that from anyone oceanographic station, the trophic sequence
can be traced a limited distance through the food web; where observations span a wider
area, as in the present study, the sequence can be followed farther. In the discussion which
follows, I have assumed that succession can be traced as far as the second trophic level
(herbivores); beyond this point in the food web, the observed changes are more likely to be
the consequence of translation, a product of food webs generated farther upstream. If
such an assumption is true, even approximately, then nutrient enrichment, primary
production, grazing, excretion (by grazers), and remineralization may profitably be
examined from the window represented by the study section, for cause and effect
relationships.