Seasonal variation in the population growth rate of a dominant zooplankter: what determines its population dynamics?

Abstract

Summary Most demographic studies predict population dynamics from known vital rates. In natural plankton populations, in contrast, the ecologist faces the ‘inverse problem’ to estimate the vital rates from observations of a population's dynamics. Since this problem is complex, many studies only present time series of abundance data from the field and vital rates estimated from laboratory experiments. Here, however, we estimate vital rates of the copepod Arctodiaptomus salinus from a stage‐dependent matrix model developed from time series of abundances. We show how each vital rate contributes to the observed variation in the population growth rate (retrospective analyses) and how potential changes in vital rates could affect this growth rate (prospective analyses). Such perturbation analyses can be expected to become very useful management tools, because they show how sensitive the population is to changes in its different life traits and to detect potential disturbances in its habitat. The models showed that the finite population growth rate (λ) varied seasonally. The population decreased during the summer and was very close to steady state in autumn and winter. The drop in λ in summer was caused mainly by reductions in fertility and in the probability of growth of pre‐adult stages. Fertility and survival of pre‐adult and adult stages were the vital rates contributing most to the variation on λ in autumn. The increment of pre‐adult stage survival determined the increment of λ in winter. During autumn and winter λ was more sensitive to changes to the probabilities of survival than to the probabilities of growth to the following stage, but in summer the opposite was observed. During the whole of the study period, λ was not very sensitive to changes to fertility. Since at high temperatures λ decays and the study pond is warm most of the year, global warming might have dramatic consequences for this A. salinus population even in the short term. Results suggest that pre‐adult stages of A. salinus may over‐summer as quiescence stages, which could be a useful strategy for coping with the warming of the pond and, consequently, global warming.