The Economy of Essential Elements
87
maternity in terms of carapace length, they also avoided a rather embarrassing property of other models, including the present one, where maturation is described in terms of body weight; that adults might starve themselves back to immaturity. The weight-for-Iength index makes the model
specially suitable to describe the regular within-instar starvation ep~sodes
experienced by animals in transfer cultures, as is well illustrated by the impressive performance of their model against a validation suite based on
Daphnia pulex culture data. On the other hand, the level of biological detail
in the model of Gurney et al. (1990) and McCauley et al. (1990) also has its
costs, in both a large number of parameters (18), and numerical complications arising from representing clutch production and molting as discrete
events. The latter problem seems to have hindered McCauley and coworkers from extending their model from single individuals to whole populations; in their only attempt on Daphnia population modeling so far (Nisbet
et al. 1989), they resorted to a delay-differential equation formulation sacrificing much of the biological insight gained in their previous models.
The present model represents an intermediate level of complexity
between extremes described above, and as such, it contains tradeoffs which
affect both analytical tractability and predictive power compared to the
models of Kooijman (1986), and Gurneyet al. (1990) and McCauley et al.
(1990). Such compromises, based on sound biological judgement, seem
unavoidable in constructing a practical model of natural zooplankton
populations within the constraints given by realism, experimental evidence,
and structural and computational complexity.
4.5 The Economy of Essential Elements
Although some workers have demonstrated direct uptake of inorganic ions
like phosphate in Daphnia (e.g., Parker and Olson 1966), this flux will
probably only be of any importance at concentrations several orders of
magnitude above what is common in natural waters (Peters 1987). It is
therefore likely that aquatic herbivores are dependent solely on their food
to obtain the amounts of biomolecules and essential elements that are
necessary to support life.
The Importance of Blemental Composition in Determining Food Quality.
Most of the work on nutrition and growth limitation in Daphnia and other
zooplankton has focused on the effects of food quantity, while food quality
usually has been associated with differences among functional or taxonomic groups (e.g., Arnold 1971; Geller 1975; Lampert 1977; Pace et al. 1983;
Infante and Litt 1985). More recently, a growing body of evidence shows
that other aspects, like chemical and biochemical composition, can also be
of vital importance to the nutritional quality of food.
87
maternity in terms of carapace length, they also avoided a rather embarrassing property of other models, including the present one, where maturation is described in terms of body weight; that adults might starve themselves back to immaturity. The weight-for-Iength index makes the model
specially suitable to describe the regular within-instar starvation ep~sodes
experienced by animals in transfer cultures, as is well illustrated by the impressive performance of their model against a validation suite based on
Daphnia pulex culture data. On the other hand, the level of biological detail
in the model of Gurney et al. (1990) and McCauley et al. (1990) also has its
costs, in both a large number of parameters (18), and numerical complications arising from representing clutch production and molting as discrete
events. The latter problem seems to have hindered McCauley and coworkers from extending their model from single individuals to whole populations; in their only attempt on Daphnia population modeling so far (Nisbet
et al. 1989), they resorted to a delay-differential equation formulation sacrificing much of the biological insight gained in their previous models.
The present model represents an intermediate level of complexity
between extremes described above, and as such, it contains tradeoffs which
affect both analytical tractability and predictive power compared to the
models of Kooijman (1986), and Gurneyet al. (1990) and McCauley et al.
(1990). Such compromises, based on sound biological judgement, seem
unavoidable in constructing a practical model of natural zooplankton
populations within the constraints given by realism, experimental evidence,
and structural and computational complexity.
4.5 The Economy of Essential Elements
Although some workers have demonstrated direct uptake of inorganic ions
like phosphate in Daphnia (e.g., Parker and Olson 1966), this flux will
probably only be of any importance at concentrations several orders of
magnitude above what is common in natural waters (Peters 1987). It is
therefore likely that aquatic herbivores are dependent solely on their food
to obtain the amounts of biomolecules and essential elements that are
necessary to support life.
The Importance of Blemental Composition in Determining Food Quality.
Most of the work on nutrition and growth limitation in Daphnia and other
zooplankton has focused on the effects of food quantity, while food quality
usually has been associated with differences among functional or taxonomic groups (e.g., Arnold 1971; Geller 1975; Lampert 1977; Pace et al. 1983;
Infante and Litt 1985). More recently, a growing body of evidence shows
that other aspects, like chemical and biochemical composition, can also be
of vital importance to the nutritional quality of food.