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Mysid Population Responses to Resource Limitation Differ from those Predicted by Cohort Studies
Citation:
GREAR, J. S., D. B. HOROWITZ, AND R. E. GUTJAHR-GOBELL. Mysid Population Responses to Resource Limitation Differ from those Predicted by Cohort Studies. MARINE ECOLOGY PROGRESS SERIES. Inter-Research, Luhe, Germany, 432:115-123, (2011).
Impact/Purpose:
EPA program offices – mainly the Office of Water and the Office of Chemical Safety and Pollution Prevention – have requested that ORD develop methods for realistic population level ecological risk assessment. To date, most population models rely on birth and death rates estimated from isolated cohorts (e.g., LC50s, etc) rather than from populations, leading to occasional doubts about the accuracy of risk projections. This manuscript describes an inverse method using multivariate time series analysis for estimating stressor effects on vital rates of marine mysids in whole populations. The method is applied in a laboratory study of feeding rates on population demography, yielding substantially different results from those in cohort studies. Thus, a major conclusion of the manuscript is that cohort- and population-based methods should be viewed as complimentary rather than substitutable. With the population methodology in hand, it will now be feasible to examine effects of stressors on vital rates in intact populations where stage abundance time series can be easily obtained. This will help to address persistent questions in ecological risk assessment regarding the applicability of cohort-based results to field settings. Further, since this method retains emphasis on impairments to birth and death rates rather than total population size, results can be viewed in context with standard protocols that currently rely on dose-response studies of mortality and reproduction.
Description:
Effects of anthropogenic stressors on animal populations are often evaluated by assembling vital rate responses from isolated cohort studies into a single demographic model. However, models constructed from cohort studies are difficult to translate into ecological predictions because stressor effects on cohorts within intact populations may differ from those observed in isolated cohorts. To address this problem, we developed an observational scheme using mixed age Americamysis bahia populations that allows inverse estimation of stage-specific vital rates. We used this system to examine demographic responses to resource limitation, which is a common complication in applied population ecology. We randomly assigned one of four feeding levels to each of 24 laboratory populations. Weekly length measurements from digital images were then compiled into a 10-wk time series of stage abundances for each population. For comparison, this study was preceded by a cohort-based study of resource limitation. Using inverse demographic analysis of these stage-structured time series, the most strongly supported models included opposing (i.e., compensatory) linear effects of resource limitation on two parameters (e.g., adult survival and juvenile maturation). The model with adult survival response only (i.e., no compensation) was also strongly supported. Moreover, the feeding effect on fecundity was negative in the compensatory models, partially offsetting positive logit-linear effects on adult survival. This contrasts with cohort results, where feeding effects on fecundity were strongly positive. Further analysis of population results showed that effects on population fitness were dominated by effects on adult survival. These results suggest that emphasis in stressor-response studies on early life stages and even-aged cohorts may miss important demographic responses and should be augmented by observations of intact populations, especially as methods such as ours become more available.