Summary and Conclusions
111
the large amount of available data on a single species (D. pulex) makes it
possible to construct a model with a level of internal consistency that probably would have been unattainable for most other common zooplankton
species.
Based on a review of the major features of Daphnia biology, a model
describing the growth and reproduction of Daphnia pulex under foodsufficient conditions has been formulated in Section 2 of this chapter. A
parametrization based on D. pulex data from Lynch (1989) suggests a net
assimilation rate linearly decreasing with body size, and that both the transition to maturity and the fraction of net assimilate allocated to reproduction are size-dependent. Simulation experiments point to the importance
of molting losses in determining the form of the Daphnia body growth
curve; as molting losses are taken from the fraction allocated to body
growth, body growth is halted at a size where egg production is still high.
This effect appears to be responsible for the nearly constant production
rate in adult Daphnia, as observed by Lynch et al. (1986). The model
initially describes the basic events of brood production and molting as
explicit instar transitions; a simplified continuous approximation without
instar transitions is shown to give a good representation of both body
growth and egg production rate in Daphnia pulex.
From a model of the allocation of net assimilate into growth, reproduction, and molting under food-sufficient conditions, the corresponding total
food intake will be given by accounting for the material lost as egesta and
respiration. In Section 3 of this chapter evidence is presented that both
respiration and assimilation efficiency can be considered size-independent
in Daphnia pulex. As net assimilation rate is assumed to be size-dependent;
this implies that the saturated ingestion rate must be linearly decreasing
with body size. Comparing simulation results with body growth curves for
D. pulex under food limitation (Taylor 1985) indicates that the incipient
limiting food level can also be considered size-independent. With the saturated ingestion rate being size-dependent, this means that the threshold
food level for positive body growth should increase with body size, and that
there should be a critical food level where body growth is halted before
reaching the size of maturity.
In Section 5 the implications of maintaining constant elemental ratios in
grazers are investigated. It is shown that while one can construct infinitely
many allocation strategies compatible with the constraints from maintaining constant elemental ratios under variable food composition, all these
strategies are bounded within the two contrasting extremes with respect to
nutrient conservation. The most wasteful strategy would be for the animals
to have a constant nutrient excretion rate, independent of food nutrient
content. For this strategy there must be a threshold food elemental composition such that positive grazer growth is impossible below this level. At the
other extreme, the animals could be able to reutilize metabolic wastes to
such an extent that the maximal growth rate is maintained whenever this is
111
the large amount of available data on a single species (D. pulex) makes it
possible to construct a model with a level of internal consistency that probably would have been unattainable for most other common zooplankton
species.
Based on a review of the major features of Daphnia biology, a model
describing the growth and reproduction of Daphnia pulex under foodsufficient conditions has been formulated in Section 2 of this chapter. A
parametrization based on D. pulex data from Lynch (1989) suggests a net
assimilation rate linearly decreasing with body size, and that both the transition to maturity and the fraction of net assimilate allocated to reproduction are size-dependent. Simulation experiments point to the importance
of molting losses in determining the form of the Daphnia body growth
curve; as molting losses are taken from the fraction allocated to body
growth, body growth is halted at a size where egg production is still high.
This effect appears to be responsible for the nearly constant production
rate in adult Daphnia, as observed by Lynch et al. (1986). The model
initially describes the basic events of brood production and molting as
explicit instar transitions; a simplified continuous approximation without
instar transitions is shown to give a good representation of both body
growth and egg production rate in Daphnia pulex.
From a model of the allocation of net assimilate into growth, reproduction, and molting under food-sufficient conditions, the corresponding total
food intake will be given by accounting for the material lost as egesta and
respiration. In Section 3 of this chapter evidence is presented that both
respiration and assimilation efficiency can be considered size-independent
in Daphnia pulex. As net assimilation rate is assumed to be size-dependent;
this implies that the saturated ingestion rate must be linearly decreasing
with body size. Comparing simulation results with body growth curves for
D. pulex under food limitation (Taylor 1985) indicates that the incipient
limiting food level can also be considered size-independent. With the saturated ingestion rate being size-dependent, this means that the threshold
food level for positive body growth should increase with body size, and that
there should be a critical food level where body growth is halted before
reaching the size of maturity.
In Section 5 the implications of maintaining constant elemental ratios in
grazers are investigated. It is shown that while one can construct infinitely
many allocation strategies compatible with the constraints from maintaining constant elemental ratios under variable food composition, all these
strategies are bounded within the two contrasting extremes with respect to
nutrient conservation. The most wasteful strategy would be for the animals
to have a constant nutrient excretion rate, independent of food nutrient
content. For this strategy there must be a threshold food elemental composition such that positive grazer growth is impossible below this level. At the
other extreme, the animals could be able to reutilize metabolic wastes to
such an extent that the maximal growth rate is maintained whenever this is