Ecotoxicological simulation modeling: effects of agricultural chemical exposure on wintering burrowing owls

The western burrowing owl, Athene cunicularia hypugaea, is a Federal Species of Concern, whose numbers and range have been drastically reduced from historic levels in Texas. Burrowing owls roost and forage in agricultural areas, and it has been hypothesized that exposure to insecticides may be a factor in the decline of their population. Burrowing owls wintering in southern Texas use agricultural culverts in cotton fields as roost sites, which may increase their risk of exposure to agricultural chemicals, either through ingestion of contaminated prey or through dermal exposure to agricultural runoff. Simulation modeling was used to characterize the risks to individual burrowing owls wintering in agricultural landscapes in southern Texas due to effects of exposure to insecticides or other agricultural chemicals. The simulation model was created using Stella® VII software (High Performance Systems, Inc., New Hampshire, USA). The model is broken into four submodels simulating (1) foraging behavior of burrowing owls, (2) chemical applications to crops, (3) chemical transfer and fate in the crop soil and prey items, and (4) chemical exposure in the burrowing owl. This model was used to evaluate (1) which components of the model most affect the endpoints, (2) the relationship between increased concentrations of agricultural chemicals in culverts and subsequent lethal and sublethal effects from dermal exposure to agricultural runoff, and (3) which agricultural chemicals have the greatest potential to cause adverse effects in burrowing owls. Model results suggested (1) the half-lives of agricultural chemicals in birds caused the most variation in the results, and data gaps exist for several important model components (2), exposure to increased concentrations of agricultural chemicals in culverts is unlikely to result in lethal effects, but is likely to lead to sublethal effects in burrowing owls, and (3) the chemicals with the greatest potential to negatively affect burrowing owls wintering in southern Texas are the OP insecticides chlorpyrifos, dicrotophos, and disulfoton, the oxadiazine insecticide indoxacarb, the herbicide trifluralin, and the defoliants tribufos and paraquat. The results of this model demonstrate the usefulness of simulation modeling to guide future research related to the conservation of burrowing owls