100
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
normal survival properties decreases with decreasing food level, no
attempts have been made to develop any more detailed model of the relationship between reallocation of material in the starvation part of the
transfer cycle and the quality of eggs laid in the next clutch.
In the course of this section, we have presented evidence that Daphnia
survival in food-limited transfer culture might be dominated by maternal
effects to an extent where the death of an individual bears little relationship
to the food level the individual itself has experienced throughout its lifetime. The maternal effects seem to be caused by the decoupling of oocyte
and yolk precursor accumulation in the dramatic "feast and famine" cycles
experienced by adults at low food treatments in transfer culture, and, as
such, mostly artifacts caused by the time scales enforced by the experimental design. Such decoupling of oocyte and yolk precursor accumulation
might not be common when food levels change on the time scales experienced by natural populations, as indicated by the results of Tessier and
Goulden (1982), who found lipid levels and clutch sizes in natural populations of Daphnia and Holopedium to be closely correlated.
This interpretation of survival curves from transfer culture experiments
calls for extreme caution when extrapolating to natural populations, and
implies that we have virtually no reliable information on how survival
might be affected by long-term food limitation in natural populations. A
parsimonious model might then be that Daphnia mortality in natural
populations is a food level-independent process that is described by a
convex age-dependent function like Eq. (4.27), at least as long as food is
sufficient to support positive net growth.
4.7 Population Survival and Proliferation: Demography
The fate of animal populations is determined by the survival and fecundity
of its individual members; a female contributes nothing to population
growth if she dies before reaching maternity. The egg production rate of a
female in the Daphnia growth model [Eqs. (4.14)-(4.17)] is a function of
animal size and food level. If we assume that an experimental arrangement
can be made such that food level can be held at a constant level while a
population is expanding (even though this might be complicated in practice), there will be a fixed relationship between age and body size, implying
that fecundity can be regarded as a function of female age alone. Ifboth the
survival and fecundity schedules at a given food level are age-dependent
functions, standard demographic theory (e.g., Pollard 1973; Nisbet and
Gurney 1982; or Frauenthal 1986) proves that the population will eventually converge to a stable age distribution with all age classes increasing or
decreasing exponentially at the same rate A. (day"\ called the intrinsic rate
of increase (see Appendix AS).
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
normal survival properties decreases with decreasing food level, no
attempts have been made to develop any more detailed model of the relationship between reallocation of material in the starvation part of the
transfer cycle and the quality of eggs laid in the next clutch.
In the course of this section, we have presented evidence that Daphnia
survival in food-limited transfer culture might be dominated by maternal
effects to an extent where the death of an individual bears little relationship
to the food level the individual itself has experienced throughout its lifetime. The maternal effects seem to be caused by the decoupling of oocyte
and yolk precursor accumulation in the dramatic "feast and famine" cycles
experienced by adults at low food treatments in transfer culture, and, as
such, mostly artifacts caused by the time scales enforced by the experimental design. Such decoupling of oocyte and yolk precursor accumulation
might not be common when food levels change on the time scales experienced by natural populations, as indicated by the results of Tessier and
Goulden (1982), who found lipid levels and clutch sizes in natural populations of Daphnia and Holopedium to be closely correlated.
This interpretation of survival curves from transfer culture experiments
calls for extreme caution when extrapolating to natural populations, and
implies that we have virtually no reliable information on how survival
might be affected by long-term food limitation in natural populations. A
parsimonious model might then be that Daphnia mortality in natural
populations is a food level-independent process that is described by a
convex age-dependent function like Eq. (4.27), at least as long as food is
sufficient to support positive net growth.
4.7 Population Survival and Proliferation: Demography
The fate of animal populations is determined by the survival and fecundity
of its individual members; a female contributes nothing to population
growth if she dies before reaching maternity. The egg production rate of a
female in the Daphnia growth model [Eqs. (4.14)-(4.17)] is a function of
animal size and food level. If we assume that an experimental arrangement
can be made such that food level can be held at a constant level while a
population is expanding (even though this might be complicated in practice), there will be a fixed relationship between age and body size, implying
that fecundity can be regarded as a function of female age alone. Ifboth the
survival and fecundity schedules at a given food level are age-dependent
functions, standard demographic theory (e.g., Pollard 1973; Nisbet and
Gurney 1982; or Frauenthal 1986) proves that the population will eventually converge to a stable age distribution with all age classes increasing or
decreasing exponentially at the same rate A. (day"\ called the intrinsic rate
of increase (see Appendix AS).