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Approaching Planktonic Food Webs: Competition. Coexistence. and Chaos
6.S Summary and Conclusions
The models presented in this Chapter represent the first few steps towards
extending the formalism of pelagic nutrient cycling, as developed in Chapter 2, to real food webs. All models focus on aspects of resource competition among the organisms constituting the food source for secondary
producers and thus forming the base of the food web. The main issue has
been to investigate the mechanisms by which grazing can promote or
impede coexistence among prey species, and how grazing effects on
plankton community structure can be influenced by the external nutrient
loading conditions.
The basic assumption behind all studies of interspecific competition is
the existence of evolutionary tradeoffs between different competitive traits,
thus excluding the possibility of a "superspecies" which is competitively
superior under all conditions. Different models in this Chapter have
focused on the effects of competitive tradeoffs with respect to: nutrient
uptake and growth capacity (Section 6.1), predator defence and competitive ability for limiting nutrients (Section 6.2), uptake and utilization of
different limiting nutrients (Section 6.3).
In contrast to the model of Chapter 5, which was sufficiently simple to
allow formal stability analysis of all equilibrium points, all the models
considered in this Chapter have too many state variables (from six to nine)
to make such analysis feasible. The analysis of stationary states has therefore necessarily been more heuristic than in Chapter 5, relying heavily on
numerical techniques, such as exhaustive simulations from random initial
conditions in the search for stationary states and reciprocal invasion
experiments to identify conditions for coexistence and competitive exclusion.
The simulation experiments in Section 6.1 show that when the stationary
state of the system is in a periodic orbit, it becomes possible for up to three
phytoplankton species to coexist on a single limiting nutrient. Since the
coexistence is caused by the species exploiting different phases of the preypredator cycle, it is argued that this situation of predator-mediated coexistence does not represent a strict violation of the competitive exclusion
principle as stated by Hardin (1961). The models presented in Sections 6.2
and 6.3 demonstrate two mechanisms that seem to counteract such predator-mediated coexistence.
In the model of Section 6.2, where grazers are assumed to have different
feeding efficiencies on different prey species, no predator-mediated coexistence is possible, independently of whether the system is attracted to a
stable equilibrium or a periodic orbit. The model is therefore unable to
represent the often observed coexistence among ungrazed "canopy"
species and edible "undergrowth" species, sensu Sommer et al. (1986). If
the large, inedible species are able to migrate vertically (like large dinoflagellates and colonial cyanobacteria), coexistence could result from
Approaching Planktonic Food Webs: Competition. Coexistence. and Chaos
6.S Summary and Conclusions
The models presented in this Chapter represent the first few steps towards
extending the formalism of pelagic nutrient cycling, as developed in Chapter 2, to real food webs. All models focus on aspects of resource competition among the organisms constituting the food source for secondary
producers and thus forming the base of the food web. The main issue has
been to investigate the mechanisms by which grazing can promote or
impede coexistence among prey species, and how grazing effects on
plankton community structure can be influenced by the external nutrient
loading conditions.
The basic assumption behind all studies of interspecific competition is
the existence of evolutionary tradeoffs between different competitive traits,
thus excluding the possibility of a "superspecies" which is competitively
superior under all conditions. Different models in this Chapter have
focused on the effects of competitive tradeoffs with respect to: nutrient
uptake and growth capacity (Section 6.1), predator defence and competitive ability for limiting nutrients (Section 6.2), uptake and utilization of
different limiting nutrients (Section 6.3).
In contrast to the model of Chapter 5, which was sufficiently simple to
allow formal stability analysis of all equilibrium points, all the models
considered in this Chapter have too many state variables (from six to nine)
to make such analysis feasible. The analysis of stationary states has therefore necessarily been more heuristic than in Chapter 5, relying heavily on
numerical techniques, such as exhaustive simulations from random initial
conditions in the search for stationary states and reciprocal invasion
experiments to identify conditions for coexistence and competitive exclusion.
The simulation experiments in Section 6.1 show that when the stationary
state of the system is in a periodic orbit, it becomes possible for up to three
phytoplankton species to coexist on a single limiting nutrient. Since the
coexistence is caused by the species exploiting different phases of the preypredator cycle, it is argued that this situation of predator-mediated coexistence does not represent a strict violation of the competitive exclusion
principle as stated by Hardin (1961). The models presented in Sections 6.2
and 6.3 demonstrate two mechanisms that seem to counteract such predator-mediated coexistence.
In the model of Section 6.2, where grazers are assumed to have different
feeding efficiencies on different prey species, no predator-mediated coexistence is possible, independently of whether the system is attracted to a
stable equilibrium or a periodic orbit. The model is therefore unable to
represent the often observed coexistence among ungrazed "canopy"
species and edible "undergrowth" species, sensu Sommer et al. (1986). If
the large, inedible species are able to migrate vertically (like large dinoflagellates and colonial cyanobacteria), coexistence could result from