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Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
formulation of Hardin (1960; two species can coexist if they exploit the
environment differently). This interpretation also explains why it is the
extremes of the r-K continuum (species 1 and 3) that form a stable coexistence in the domain of the limit cycle, while species with intermediate
competitive abilities (like species 2) are competitively excluded from both
temporal niches (except for a limited range ofloading rates). The exclusion
of intermediate species makes it unlikely that the dynamics of the simple
limit cycle can support the coexistence of more than a few phytoplankton
species from a unidimensional r-K continuum.
6.2 Differential Loss Rates and Invadability of Equilibria
Phytoplankton ecologists have traditionally been divided into two different
schools, emphasizing either differential reproductive rates (e.g., Tilman et
aI. 1982) or differential loss rates (e.g., Kalff and Knoechel 1978) as the
dominating forces behind phytoplankton succession. Still, a balanced view
seems to be emerging, where it is recognized that differential loss processes
can play an important role by tilting the balance of resource competition
(Kilham 1987; Sommer 1988b).
Differential losses result when competing phytoplankton species have
different susceptibilities to grazing, or different abilities to remain suspended in the pelagic zone. These two major loss processes differ fundamentally in that the impact of selective grazing can be expected to be
density-dependent, while differential sinking losses will generally be
density-independent. Furthermore, while sinking is always a net loss of
nutrients from the pelagic zone, the nutrient content of grazed phytoplankton cells will either be immediately recycled or remain suspended for
some time as a constituent of grazer biomass.
Many phytoplankton species show morphological or biochemical adaptations that can be interpreted as forms of predator defence. Increasing size
is generally accepted as an advantageous adaptation against consumption
by common filter feeders, but because it also reduces the relative adsorptive surface area as well as increases the sinking rate (Reynolds 1984), one
would expect this strategy of predator defence to have its costs in terms of
reduced competitive ability for essential resources and increased sinking
losses. It thus appears that no single cell shape can be optimal for all combinations of loss and nutrient supply rates, and that phytoplankton species
seem to be facing a tradeoff between minimizing loss rates or maximizing
growth rates in order to maximize their evolutionary fitness (cf. Kilham
and Hecky 1988).
Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
formulation of Hardin (1960; two species can coexist if they exploit the
environment differently). This interpretation also explains why it is the
extremes of the r-K continuum (species 1 and 3) that form a stable coexistence in the domain of the limit cycle, while species with intermediate
competitive abilities (like species 2) are competitively excluded from both
temporal niches (except for a limited range ofloading rates). The exclusion
of intermediate species makes it unlikely that the dynamics of the simple
limit cycle can support the coexistence of more than a few phytoplankton
species from a unidimensional r-K continuum.
6.2 Differential Loss Rates and Invadability of Equilibria
Phytoplankton ecologists have traditionally been divided into two different
schools, emphasizing either differential reproductive rates (e.g., Tilman et
aI. 1982) or differential loss rates (e.g., Kalff and Knoechel 1978) as the
dominating forces behind phytoplankton succession. Still, a balanced view
seems to be emerging, where it is recognized that differential loss processes
can play an important role by tilting the balance of resource competition
(Kilham 1987; Sommer 1988b).
Differential losses result when competing phytoplankton species have
different susceptibilities to grazing, or different abilities to remain suspended in the pelagic zone. These two major loss processes differ fundamentally in that the impact of selective grazing can be expected to be
density-dependent, while differential sinking losses will generally be
density-independent. Furthermore, while sinking is always a net loss of
nutrients from the pelagic zone, the nutrient content of grazed phytoplankton cells will either be immediately recycled or remain suspended for
some time as a constituent of grazer biomass.
Many phytoplankton species show morphological or biochemical adaptations that can be interpreted as forms of predator defence. Increasing size
is generally accepted as an advantageous adaptation against consumption
by common filter feeders, but because it also reduces the relative adsorptive surface area as well as increases the sinking rate (Reynolds 1984), one
would expect this strategy of predator defence to have its costs in terms of
reduced competitive ability for essential resources and increased sinking
losses. It thus appears that no single cell shape can be optimal for all combinations of loss and nutrient supply rates, and that phytoplankton species
seem to be facing a tradeoff between minimizing loss rates or maximizing
growth rates in order to maximize their evolutionary fitness (cf. Kilham
and Hecky 1988).