108
Herbivores and Algae: Food Utilization, Growth and Reproduction •..
(D. pulex) makes it possible to construct a model with a high level of internal
consistency, but leaves open the question of how well such a model can be
adapted or generalized to other zooplankton taxa or aggregated functional
groups.
Several earlier attempts to construct general zooplankton models relied
heavily on allometric relationships between size and vital rates (e.g., Hall et al.
1976; Lynch 1977). When examined critically in the light of recent experimental results, many size trends observed within a major taxon, like rotifers
or cladocerans, break down when comparisons are made across major taxa
(DeMott 1989). There is more uniqueness to common zooplankton taxa than
is revealed by size alone; thus, most likely other structural or functional characters will be more instrumental in capturing the essential features of a given
zooplankton taxon in a simple model.
From a simple life-history model, Allan (1976) concluded that one would
expect the intrinsic rates of increase among the major zooplankton taxa to be
ranked as rotifers > cladocerans > copepods (shaded areas in Fig. 4.22). Since
then, many common zooplankton species have been successfully cultured
and studied in life-table experiments (Table AI0.7 provides a partial list of ..t'
values estimated from a range of species). Although the frequency distribution in Fig. 4.22 shows some resemblance with the predicted pattern, the
apparent variability within the major taxa is much higher than predicted by
Allan (1976). While the highest values of ..t' are generally found in rotifers,
and the lowest in the few copepod species that have been investigated, there
are also notable exceptions: both the rotifer Keratella cochlearis and the cladoceran Chydorus sphaericus have maximal intrinsic rates of increase comparable to those of copepods. The maximal intrinsic rate of increase predicted
by the present model (0.38 day-') is seen to be quite close to the median of the
frequency distribution in Fig. 4.22 (0.36 day·').
In a situation where two zooplankton species are involved in exploitative
competition for a common food resource, the population with the lowest
threshold food level will eventually depress the food resource to a level where
the other population is unable to maintain positive net population growth.
The threshold food level is therefore thought to be a key parameter in determining the competitive ability of a zooplankton species (e.g., DeMott 1989).
In a collection of published threshold food levels from several zooplankton
species (Table AIO.8), the interspecific variability extends over almost two
orders of magnitude, with a frequency distnbution resembling the lognormal
(Fig. 4.23). The threshold food level predicted by the present model [0.073
(mg C) 1"' ] is seen to be quite close to the median of the frequency distnbution
in Fig. 4.23 [0.075 (mg C) 1"'].
Although available data are limited, copepods appear to have the lowest
threshold food levels, rotifers the highest, while cladocerans are in an intermediate position. The low threshold food level found in the rotifer Keratella
cochlearis and the high value found in the marine copepod Acartia tonsa are
both exceptions to this pattern. Still, there appears to be a tendency for
Herbivores and Algae: Food Utilization, Growth and Reproduction •..
(D. pulex) makes it possible to construct a model with a high level of internal
consistency, but leaves open the question of how well such a model can be
adapted or generalized to other zooplankton taxa or aggregated functional
groups.
Several earlier attempts to construct general zooplankton models relied
heavily on allometric relationships between size and vital rates (e.g., Hall et al.
1976; Lynch 1977). When examined critically in the light of recent experimental results, many size trends observed within a major taxon, like rotifers
or cladocerans, break down when comparisons are made across major taxa
(DeMott 1989). There is more uniqueness to common zooplankton taxa than
is revealed by size alone; thus, most likely other structural or functional characters will be more instrumental in capturing the essential features of a given
zooplankton taxon in a simple model.
From a simple life-history model, Allan (1976) concluded that one would
expect the intrinsic rates of increase among the major zooplankton taxa to be
ranked as rotifers > cladocerans > copepods (shaded areas in Fig. 4.22). Since
then, many common zooplankton species have been successfully cultured
and studied in life-table experiments (Table AI0.7 provides a partial list of ..t'
values estimated from a range of species). Although the frequency distribution in Fig. 4.22 shows some resemblance with the predicted pattern, the
apparent variability within the major taxa is much higher than predicted by
Allan (1976). While the highest values of ..t' are generally found in rotifers,
and the lowest in the few copepod species that have been investigated, there
are also notable exceptions: both the rotifer Keratella cochlearis and the cladoceran Chydorus sphaericus have maximal intrinsic rates of increase comparable to those of copepods. The maximal intrinsic rate of increase predicted
by the present model (0.38 day-') is seen to be quite close to the median of the
frequency distribution in Fig. 4.22 (0.36 day·').
In a situation where two zooplankton species are involved in exploitative
competition for a common food resource, the population with the lowest
threshold food level will eventually depress the food resource to a level where
the other population is unable to maintain positive net population growth.
The threshold food level is therefore thought to be a key parameter in determining the competitive ability of a zooplankton species (e.g., DeMott 1989).
In a collection of published threshold food levels from several zooplankton
species (Table AIO.8), the interspecific variability extends over almost two
orders of magnitude, with a frequency distnbution resembling the lognormal
(Fig. 4.23). The threshold food level predicted by the present model [0.073
(mg C) 1"' ] is seen to be quite close to the median of the frequency distnbution
in Fig. 4.23 [0.075 (mg C) 1"'].
Although available data are limited, copepods appear to have the lowest
threshold food levels, rotifers the highest, while cladocerans are in an intermediate position. The low threshold food level found in the rotifer Keratella
cochlearis and the high value found in the marine copepod Acartia tonsa are
both exceptions to this pattern. Still, there appears to be a tendency for