Projected population‐level effects of thiobencarb exposure on the mysid, Americamysis bahia, and extinction probability in a concentration‐decay exposure system

Abstract

Population-level effects of the mysid, Americamysis bahia, exposed to varying thiobencarb concentrations were estimated using stage-structured matrix models. A deterministic density-independent matrix model estimated the decrease in population growth rate (lambda) with increasing thiobencarb concentration. An elasticity analysis determined that survival of middle stages provided the largest contribution to lambda. Decomposing the effects of lambda in terms of changes in the matrix components determined that reduced reproduction had a large influence on population dynamics at lower thiobencarb concentrations, whereas reduced survivorship had the largest impact on populations at higher concentrations. A simulation model of a concentration-decay system was developed to demonstrate the importance of integrating chemical half-life and management practices in determining population viability. In this model, mysids were originally exposed to a high thiobencarb concentration (300 microg/L) that decayed an order of magnitude in the number of mysid generations corresponding to thiobencarb half-life values under three different exposure regimes. Environmental stochasticity was added to the model to estimate the cumulative extinction probability of mysids exposed to fluctuating concentrations of thiobencarb in random environments. The cumulative extinction probability increased with thiobencarb half-life, stochasticity, and concentration present at the time of a new exposure. The model demonstrated the expansion of population projection models in determining the ecological impact of a population exposed to pesticides.

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

“…Moreover, our population study suggests that stage-based models, which allow effects on maturation in addition to survival and fecundity, are superior to age-based models for laboratory populations of Americamysis bahia. Raimondo & McKenney (2005) used stage-based formulations because of evidence for stressor effects on brood release timing in their cohort observations. Further comparison of their model projections with our observations of intact populations would be difficult because of the need to discriminate between multiple adult stages in their model, but their use of stage-based models is supported by our results.…”

“…In addition, Raimondo & McKenney (2005) used LTRE decomposition analysis of their cohort results and, as in our cohort study, reported that fecundity was the driver of population response to sublethal stressor concentrations. Table 4.…”

“…In addition to the many studies of chemical lethality, cohort-based observations of A. bahia stress responses have been combined with matrix element substitution to predict effects on abundance or population growth rate (Kuhn et al 2000, 2001, Raimondo & McKenney 2005. Similarly, Gentile et al (1982) used Euler-Lotka formulations to estimate population growth rate from A. bahia cohort data.…”

Mysid crustaceans as standard models for the screening and testing of endocrine-disrupting chemicals

“…Unfortunately, long-term population field studies are typically not feasible, and it is generally difficult to isolate the effects of a toxicant on a field population that is also exposed to natural environmental variation. Population models are useful tools for linking organism-level effects with population-level responses and afford the opportunity to integrate probabilistic approaches into ecological risk assessments (Raimondo and McKenney 2005b). …”

“…reduced reproduction) for the observed effect. Risk assessment of thiobencarb exposure included probabilistic assessment of mysid population viability under various environmental conditions and management scenarios (Raimondo and McKenney, 2005b). Elasticity analyses performed with these models determined that survival of intermediate life stages (approximately 10-16 days old) was the most critical vital rate to mysid population growth rate and impairments to it would have the most dramatic effect on growth rate McKenney, 2005a, 2005b).…”