Understanding the Complex Dynamics of Endangered Species and Pesticide Exposure through Population Modeling
Some species listed under the Endangered Species Act (ESA) occur in agricultural landscapes where their habitats have the potential to be exposed to pesticides. Aquatic species, in particular, may be at risk to pesticides applied to nearby agricultural fields, even though applications are not made directly to the aquatic habitat. Population modeling has emerged as a useful tool in predicting exposure risk and the impacts pesticides have on their populations.
A recent article we published in the Journal of Environmental Toxicology and Chemistry, takes an in-depth look at how a hybrid model—combining an aquatic ecosystem model (CASM) and a species-specific population model for the Topeka shiner (TS-IBM)—can be used to estimate the potential impacts on populations. Through these models, we are able to consider realistic assumptions about the species ecology, ecosystem factors, and potential exposures within the habitat. Additionally, Toxicokinetic/Toxicodynamic (TKTD) models were incorporated into the TS-IBM to capture direct lethal and sublethal effects on individual fish from the time-variable exposures.
In this endangered Topeka shiner (Notropis topeka) study, a hybrid model was applied to compare the effects from potential exposure to a fungicide in oxbow habitats on the shiner population. Hybrid modeling approaches that combine species-specific models with an ecosystem-level model can be advantageous as this type of hybrid model sets population dynamics in the context of species interaction which can further address indirect effects mediated by the food web. We chose the example pesticide due to its known toxicity to fish in standard laboratory studies. The variable exposure scenarios were based on conservative assumptions and made it possible to test the outcomes for the populations of applying a 15-foot vegetative filter strip (VFS) between the treatment area and the waterbody compared to no exposure mitigation measure.
Furthermore, exposure multiplication factors (EMFs) were applied to the exposure scenarios with and without VFS to assess the simulated population-level effects. We found that direct effects on the simulated shiners governed the observed population-level effects, while effects mediated by the food web did not play an important role in the case of the fungicide.
Our conclusions demonstrated that such modeling approaches can help to measure the effectiveness of various mitigation strategies for endangered species protection. We found that the VFS between the treated area and the oxbow habitat resulted in a two to three times reduction in simulated population-level effects compared with the exposure scenario without a VFS, suggesting the effectiveness of the mitigation strategy for Topeka shiner populations in oxbow habitats.