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Herbivores and Algae: Food Utilization, Growth and Reproduction ...
4.6 Individual Mortality and Population Losses
In natural zooplankton populations, mortality losses are composed of at
least two component parts; mortality due to vertebrate and invertebrate
predation, and nonpredatory mortality due to starvation and senescence.
The impact of predatory mortality on zooplankton populations has been
shown to be strongly dependent on the type of predator involved; carnivorous crustaceans and insects exhibit a selective mortality on small species
and/or juvenile stages, while planktivorous stages of fish and amphibians
have a strong selectivity for large species and/or adults (Zaret 1980; Kerfoot
and Sih 1987; Gliwicz and Pijanowska 1989).
Since the pioneering work of Holling (1966), much progress has been
made in identifying the component processes of predation, where a
successful capture of a prey is described by a Markov chain of probabilities
for encounter, attack, and ingestion (e.g. Williamson and Gilbert 1980).
The encounter probabilities have been accurately modeled as functions of
prey visibility and the swimming speeds of prey and predator by Gerritsen
and Strickler (1977). Compared to the detailed studies carried out by numerous workers on predatory mortality in Daphnia and other zooplankters,
the processes involved in mortality from starvation and senescence have
received less attention and are generally much less well understood. As
noticed by Lynch (1983,1989), reliable size-specific mortality rates in natural Daphnia populations are almost completely lacking, so that the main
source of information on nonpredatory mortality rates is from life-table
experiments under laboratory conditions.
Daphnia Survival Under Food-Sufficient Conditions. While mortality most
certainly is a binary process on the individual level (an individual is either
alive or dead), it is usually observed on the population or cohort level as a
survival curve describing the fraction remaining alive as the initial cohort is
aging. In the typical life-table experiment, a cohort of newborn is raised
under identical conditions while the state of each individual (dead or alive)
is being monitored. In Appendix A4 it is shown that the empirical survival
curve of a finite cohort is an unbiased and consistent estimator for the true
survival function of the population. The survival function relates the accumulated mortality loss on the population level to the instantaneous
mortality rate (or force of mortality) on the individual level.
The problems associated with maintaining animals at a constant food
level throughout their lives, as discussed in Section 4.4, suggest an
approach to mortality modeling similar to the model of growth and reproduction developed earlier in this chapter: to start with a model of the
mortality process in populations growing at nonlimiting food levels and
then try to expand this model to situations where survival is modulated by
food limitation and starvation. The zooplankton literature contains a rich
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
4.6 Individual Mortality and Population Losses
In natural zooplankton populations, mortality losses are composed of at
least two component parts; mortality due to vertebrate and invertebrate
predation, and nonpredatory mortality due to starvation and senescence.
The impact of predatory mortality on zooplankton populations has been
shown to be strongly dependent on the type of predator involved; carnivorous crustaceans and insects exhibit a selective mortality on small species
and/or juvenile stages, while planktivorous stages of fish and amphibians
have a strong selectivity for large species and/or adults (Zaret 1980; Kerfoot
and Sih 1987; Gliwicz and Pijanowska 1989).
Since the pioneering work of Holling (1966), much progress has been
made in identifying the component processes of predation, where a
successful capture of a prey is described by a Markov chain of probabilities
for encounter, attack, and ingestion (e.g. Williamson and Gilbert 1980).
The encounter probabilities have been accurately modeled as functions of
prey visibility and the swimming speeds of prey and predator by Gerritsen
and Strickler (1977). Compared to the detailed studies carried out by numerous workers on predatory mortality in Daphnia and other zooplankters,
the processes involved in mortality from starvation and senescence have
received less attention and are generally much less well understood. As
noticed by Lynch (1983,1989), reliable size-specific mortality rates in natural Daphnia populations are almost completely lacking, so that the main
source of information on nonpredatory mortality rates is from life-table
experiments under laboratory conditions.
Daphnia Survival Under Food-Sufficient Conditions. While mortality most
certainly is a binary process on the individual level (an individual is either
alive or dead), it is usually observed on the population or cohort level as a
survival curve describing the fraction remaining alive as the initial cohort is
aging. In the typical life-table experiment, a cohort of newborn is raised
under identical conditions while the state of each individual (dead or alive)
is being monitored. In Appendix A4 it is shown that the empirical survival
curve of a finite cohort is an unbiased and consistent estimator for the true
survival function of the population. The survival function relates the accumulated mortality loss on the population level to the instantaneous
mortality rate (or force of mortality) on the individual level.
The problems associated with maintaining animals at a constant food
level throughout their lives, as discussed in Section 4.4, suggest an
approach to mortality modeling similar to the model of growth and reproduction developed earlier in this chapter: to start with a model of the
mortality process in populations growing at nonlimiting food levels and
then try to expand this model to situations where survival is modulated by
food limitation and starvation. The zooplankton literature contains a rich