Interaction Assessment: A modeling tool for predicting population dynamics from field data

Life-history phenology strongly influences population vulnerability to toxicants: a case study with the mudsnail Potamopyrgus antipodarum

One of the main objectives of ecological risk assessment is to evaluate the effects of toxicants on ecologically relevant biological systems such as populations or communities. However, the effects of toxicants are commonly measured on selected subindividual or individual endpoints due to their specificity against chemical stressors. Introducing these effects into population models is a promising way to predict impacts on populations. The models currently employed are very simplistic, and their environmental relevance needs to be improved to establish the ecological relevance of hazard assessment. The present study with the gastropod Potamopyrgus antipodarum combines a field experimental approach with a modeling framework. It clarifies the role played by seasonal variability of life-history traits in the population's vulnerability to the alteration of individual performance, potentially due to toxic stress. The present study comprised 3 steps: 1) characterization of the seasonal variability in life-history traits of a local population over 1 yr by using in situ experiments on caged snails, coupled with a demographic follow-up; 2) development of a periodic matrix population model that visualizes the monthly variability of population dynamics; and 3) simulation of the demographic consequences of an alteration in life-history traits (i.e., fertility, juvenile, and adult survival). The results revealed that demographic impacts strongly depend on the season when alterations of individual performance occur. Model analysis showed that this seasonal variability in population vulnerability is strongly related to the phenology of the population. The authors emphasize that improving the realism of population models is a major objective for ecological risk assessment, and that taking into account species phenology in modeling approaches should be a priority.

Developing methods to assess and predict the population level effects of environmental contaminants

The field of ecological toxicity seems largely to have drifted away from what its title implies--assessing and predicting the ecological consequences of environmental contaminants--moving instead toward an emphasis on individual effects and physiologic case studies. This paper elucidates how a relatively new ecological methodology, interaction assessment (INTASS), could be useful in addressing the field's initial goals. Specifically, INTASS is a model platform and methodology, applicable across a broad array of taxa and habitat types, that can be used to construct population dynamics models from field data. Information on environmental contaminants and multiple stressors can be incorporated into these models in a form that bypasses the problems inherent in assessing uptake, chemical interactions in the environment, and synergistic effects in the organism. INTASS can, therefore, be used to evaluate the effects of contaminants and other stressors at the population level and to predict how changes in stressor levels or composition of contaminant mixtures, as well as various mitigation measures, might affect population dynamics.

Do animals select habitat at small or large scales? An experiment with meadow voles (Microtus pennsylvanicus)

Habitat and patch use are crucial to the dynamics of populations and the structure of ecological communities. But ecologists have not rigorously tested whether animals choose habitat at small or large scales. If individuals base their patch and habitat choices on fine-scale differences in habitat, then their use of different sites should correspond with measures of microhabitat at those sites. But if individuals use density to assess and respond to habitat at larger spatial scales, then site use should correspond with the scale of density-dependent habitat selection. We tested these predictions with experiments that measured microhabitat and monitored the use of capture sites by meadow voles ( Microtus pennsylvanicus Ord, 1815) in 0.25 ha old-field enclosures. We varied the density of voles in pairs of adjacent enclosures and tested for density-dependent habitat selection. Then we assessed whether their frequency of captures at trapping stations was best predicted at the small scale of microhabitat or at the much larger scale of enclosures where density varied. The voles selected habitat at different scales. When the use of enclosures was predicted by density, the scale of density-dependent choice trumped the use of small-scale patches. And when voles selected amongst different small-scale patches, their use of enclosures was independent of density. These results suggest that assessments of spatial scale in habitat use must include tests for both scale- and density-dependent habitat choice.