by inverse population oscillations of copepods and Oikopleura dioica in the Baltic
Sea.
Mortality Rates and Age Distribution of
Mortality
(^) A key issue is how mortality is apportioned among the life stages of plankton animals:
what does the survivorship curve (lx, proportion surviving, vs. x, age) look like from
birth to the oldest ages attained? The shape of this curve affects every aspect of a
population’s productivity and reproductivity. For example, if only a few individuals
ever enter the larger size classes whose growth can contribute significant amounts of
tissue production per capita, then stock production can only be small. Moreover, if
most potential reproduction is lost to early mortality, then high individual egg output
is required for survival over multiple generations, and secondary production comes to
focus in the adult female. Survivorship curves can be expected to vary among
generations of the same species growing up at different seasons or in different
locations.
(^) There have been relatively few attempts to draw survivorship curves for plankton,
because advection and patchiness of sampled stocks make the estimation process
difficult and the results uncertain. Attempts fall into classic categories defined by
ecologists long ago, the horizontal and vertical life-table approaches. A horizontal life
table is obtained by enumerating a cohort of very young individuals born at about the
same time, and then following their numbers until eventually they are all dead. A
classic exercise of this kind in intertidal ecology is to scrape a square of rock and
count all of the barnacles that arrive there during one pulse of larval settlement from
the plankton. A map is made showing each individual. Then this square is visited at
low tide for several years as those barnacles grow, push each other off, get bored into
by snails, reproduce, and finally are gone entirely. A vertical life table is made by
visiting a stock of animals once at some suitable season and determining the age
structure. The relative numbers at different ages are tabulated and survivorship
estimated from the decline of numbers with age. Abundance in horizontal tables
cannot increase with age, because cohort entry is closed and the only source of change
is death. Vertical tables take statistical rambles, because production of young may
have been unusually high (or low) at the time when animals now middle-aged were
born. A reproductive peak occurred in the American human population after World
War II; giving birth became stylish, generating the baby boom. That massive pulse
remains in the population, and it produced its own, lesser pulse of children – the
“echo” of the baby boom. Thus, the age structure does not exactly represent the
mortality schedule, and vertical life tables aren’t reliable as predictors of future