Mysid population responses to resource limitation differ from those predicted by cohort studies

Effects of anthropogenic stressors on animal populations are often evaluated by assem- bling vital rate responses from isolated cohort studies into a single demographic model. However, these models are difficult to translate into ecological predictions because stressor effects observed in isolated cohorts may differ from those occurring in populations with overlapping generations. This problem is evident in many areas of stressor-response research, including the burgeoning work on ocean acidification. To address this problem, we compared vital rates in experimental cohorts and populations of the mysid Americamysis bahia at 4 levels of resource limitation. This required devel- opment of a novel observational scheme that allows inverse estimation of stage-specific vital rates in mixed-age populations without the use of cohort isolation or marking. Using digitally imaged time series observations of stage abundances, the most strongly supported inverse models indicated opposing (i.e. compensatory) effects of resource limitation on adult survival and juvenile maturation. The model with adult survival response only (i.e. no compensation) was also strongly supported. This contrasts with cohort results, where feeding effects on fecundity were strongest. These results sug- gest that emphasis in stressor-response studies on early life stages and cohorts of uniformly aged individuals may miss important demographic responses and should be augmented by observations of intact populations, especially as methods such as ours become more available.