Mapping the correlations and gaps in studies of complex life histories

For species with complex life histories, phenotypic correlations between
life-history stages constrain both ecological and evolutionary
trajectories. Studies that seek to understand correlations across life
history differ greatly in their experimental approach: some follow
individuals (“individual longitudinal”), while others follow cohorts
(“cohort longitudinal”). Cohort longitudinal studies risk confounding
results through Simpson’s Paradox, where correlations observed at the
cohort-level do not match that of the individual-level. Individual
longitudinal studies are laborious in comparison, but provide a more
reliable test of correlations across life-history stages. Our
understanding of the prevalence, strength and direction of phenotypic
correlations depends on the approaches that we use, but the relative
representation of different approaches is unknown. Using marine
invertebrates as a model group, we used a formal, systematic literature
map to screen 17,000+ papers studying complex life histories, and
characterized the study type (i.e. cohort longitudinal, individual
longitudinal or single stage), as well as other factors. For 3,315
experiments from 1,716 articles, most (67.4%) focused on a single stage,
and just 1.7% used an individual longitudinal approach. While life-history
stages have been studied extensively, we suggest that the field prioritize
individual longitudinal studies to understand the phenotypic correlations
among stages.