Chapter 8
Population biology of zooplankton
Population dynamics of protists, including phytoplankton, are relatively simple. A cell
divides into two daughter cells; each runs risks of being eaten or fatally infected, then
divides again. If the risks are small, then the population grows exponentially at a rate
dependent upon the intervals between divisions and upon the risks. If the risks are
large, then the population declines. There can be complications introduced by
occasional mating, auxospore formation in diatoms, resting spore formation in
dinoflagellates, and other life-cycle variants. In comparison, the life cycles of
metazoans are consistently complex. So, their population dynamics involve longer
delays between reproductive periods, chancy mate-finding and mate-selection, in
many cases repetitive and multiple reproduction by single females, changing risks of
death as development proceeds and elaborate strategies for surviving periods of bad
conditions. This applies to zooplankton, benthos (annelids, isopods, clams, ...),
nekton (tuna, squid, porpoises, ...) and seabirds. We cannot cover the details for all of
these; each is a specialized study. To provide examples of the sorts of facts,
measurements, and calculations involved, we present some zooplankton studies that
are familiar to us. In every animal group, reproductive intervals, fecundity, longevity,
and age-specific mortality all vary at the levels of species, regional populations, and
generations, because it is those aspects of adaptation that are most strongly and
immediately tuned by natural selection, and most strongly affected by short-term
conditions.
(^) A good deal of mathematics has been developed describing population dynamics.
Unfortunately, for most organisms, including zooplankton, the assumptions do not fit
well enough to make that mathematics useful. For example, plankton animals in
middle and high latitudes do not tend toward unchanging (“stable”) age- or stage-
frequency distributions. Thus, the classical calculations of population-increase rates,
which depend upon that stability (e.g. calculation of “little r”, the intrinsic rate of
natural increase, from 1 = Σr−xlxmx, the sum over all ages, x, of survivorship to x times
reproduction at x) will not work. In tropical areas, where stable age distributions are
more likely, if not adequately proved, r is usually close to zero and oscillates around
it. So, all of the great math for evaluation of population-dynamical parameters is
useless. Sorry. Our information about abundance cycles is better for many species
than our evaluation of the component birth and death rates that produce the cycles.