112
Herbivores and Algae: Food Utilization, Growth and Reproduction •..
compatible with a non-negative nutrient excretion rate. For this strategy
there will be no threshold food elemental composition for positive grazer
growth; thus positive net assimilation will be possible for all food sources
with nonzero nutrient content. From analysis of a data set on Daphnia P
release (Olsen et al. 1986b), it appears that the phosphorus utilization strategy of Daphnia is intermediate between these two extremes. For this intermediate strategy, the net assimilation rate will be directly proportional to
food P content, as long as the food has a lower P content than the grazers.
This leads to a particularly simple formulation of C- and P-limited growth
in Daphnia.
Thus far, only processes at the individual level have been considered;
when proceeding to the population level, population mortality losses must
also be taken into account. In Appendix A4 it is shown that the small
cohort sizes used in most life-table studies will introduce a strong stochastic element into the observed survival curves, which would tend to mask
out differences between mortality schedules. In Section 6 it is shown that
different sets of Daphnia pulex survival data obtained under food-sufficient
conditions generally agree to within the expected level of experimental
error variance. The survival curves generally conform to a convex mortality
schedule with high mortality in young and old individuals, and a minimum
at an intermediate age.
Practically all experiments on survival in food-limited zooplankton
populations have been performed as transfer-culture studies. In Section 6
of this chapter it is argued that the interaction between acute starvation
and yolk allocation in the transfer culture regime might create strong
maternal effects on offspring survival. A set of Daphnia pulex survival
curves obtained under varying degrees of food limitation (Frank et al.
1957) could be represented as two distinct subcohorts of eggs; one with the
normal mortality schedule of a food-sufficient population, the other with a
fixed, high mortality rate, as would be expected for individuals developing
from yolk-deficient eggs. This suggests that the immediate effects of food
limitation on mortality are limited, at least as long as the animals are able
to maintain positive net growth, and that increased juvenile mortality in
food-limited transfer cultures is mostly an artifact caused by acute maternal starvation at the end of the brood cycle.
In Section 7 of this chapter, the key processes that have been described in
the preceding sections: growth, reproduction and mortality, are assembled
into a model of Daphnia demography. The model properties are investigated in terms of the asymptotic population structure resulting from
unconstrained growth at constant food concentration. The stable age
distribution is shown to be dominated by eggs and early juveniles in all
populations with positive net growth rate. When the net population growth
rate is decomposed into population-specific, asymptotic birth and death
rates, it is shown that the death rate is close to 5% of the maximal birth rate
for all food levels supporting positive population growth.
Herbivores and Algae: Food Utilization, Growth and Reproduction •..
compatible with a non-negative nutrient excretion rate. For this strategy
there will be no threshold food elemental composition for positive grazer
growth; thus positive net assimilation will be possible for all food sources
with nonzero nutrient content. From analysis of a data set on Daphnia P
release (Olsen et al. 1986b), it appears that the phosphorus utilization strategy of Daphnia is intermediate between these two extremes. For this intermediate strategy, the net assimilation rate will be directly proportional to
food P content, as long as the food has a lower P content than the grazers.
This leads to a particularly simple formulation of C- and P-limited growth
in Daphnia.
Thus far, only processes at the individual level have been considered;
when proceeding to the population level, population mortality losses must
also be taken into account. In Appendix A4 it is shown that the small
cohort sizes used in most life-table studies will introduce a strong stochastic element into the observed survival curves, which would tend to mask
out differences between mortality schedules. In Section 6 it is shown that
different sets of Daphnia pulex survival data obtained under food-sufficient
conditions generally agree to within the expected level of experimental
error variance. The survival curves generally conform to a convex mortality
schedule with high mortality in young and old individuals, and a minimum
at an intermediate age.
Practically all experiments on survival in food-limited zooplankton
populations have been performed as transfer-culture studies. In Section 6
of this chapter it is argued that the interaction between acute starvation
and yolk allocation in the transfer culture regime might create strong
maternal effects on offspring survival. A set of Daphnia pulex survival
curves obtained under varying degrees of food limitation (Frank et al.
1957) could be represented as two distinct subcohorts of eggs; one with the
normal mortality schedule of a food-sufficient population, the other with a
fixed, high mortality rate, as would be expected for individuals developing
from yolk-deficient eggs. This suggests that the immediate effects of food
limitation on mortality are limited, at least as long as the animals are able
to maintain positive net growth, and that increased juvenile mortality in
food-limited transfer cultures is mostly an artifact caused by acute maternal starvation at the end of the brood cycle.
In Section 7 of this chapter, the key processes that have been described in
the preceding sections: growth, reproduction and mortality, are assembled
into a model of Daphnia demography. The model properties are investigated in terms of the asymptotic population structure resulting from
unconstrained growth at constant food concentration. The stable age
distribution is shown to be dominated by eggs and early juveniles in all
populations with positive net growth rate. When the net population growth
rate is decomposed into population-specific, asymptotic birth and death
rates, it is shown that the death rate is close to 5% of the maximal birth rate
for all food levels supporting positive population growth.