MEPS 486:59-78 (2013)  -  DOI: https://doi.org/10.3354/meps10418

ABSTRACT: We use an individual-based cohort model of the planktonic calanoid copepod Acartia tonsa to examine how variability among individuals in bioenergetic traits affects cohort dynamics. The model follows a cohort of individual zooplankton through the hourly processes of feeding and energy distribution among growth, development, and reproduction, under high and low rates of size-dependent mortality. The degree of variability in traits among individuals was quantified by the coefficient of variation (CV). In our first simulation experiment (Expt 1), population growth rate (λ) increased with increasing variability among individuals (increasing CV) when overall mortality was high; egg production increased with CV under high and low mortality rates, and survival was unaffected by the degree of trait variability. Expt 2 tested the effect of genetic makeup on cohort dynamics; when mortality rate was high, increasing CV resulted in individuals with correlated traits having higher egg production and λ than individuals with randomly assigned traits. Expt 3 tested how environmental conditions altered the effect of CV on survival, egg production, and λ. Population growth rate increased with increasing CV under high mortality rate and suboptimal conditions. Our analyses demonstrate how small differences in the values of individual-level traits can influence growth and reproduction which can in turn translate into ecologically important effects on cohort dynamics. The effect of increased variability among individuals on cohort dynamics was particularly important when environmental conditions were poor, when the mortality rate was high, and under certain types of size-dependent mortality. These results may have important implications for understanding the dynamics of populations experiencing stress.

KEY WORDS: Variability among individuals · Fitness · Cohort dynamics · Individual-based model · Acartia tonsa · Environmental conditions · Size-dependent mortality