Spatial analysis and ecological risk assessment for lead exposure in Gulf Coast waterfowl: Does environmental lead create an ecological trap?

Background/Question/Methods

Lead contamination is a widespread conservation issue in National Wildlife Refuge (NWR) complexes along the Texas Gulf Coast, affecting both habitat quality and wildlife populations. Sources of anthropogenic lead are numerous, but most prominent is spent lead shot used for hunting waterfowl before being banned on all federal land in 1991.  Waterfowl species are also at disproportionately high risk for lead shot ingestion because of their foraging strategies.  This study assesses the risk of exposure to lead for various biota in coastal wetland habitat and directs effective management and mitigation practices.  We conducted stratified soil samples taken on a grid at three different depths to determine the distribution of lead particles across NWR complexes in Northeastern Texas.  Point data were then spatially interpolated to create a raster risk prediction surface, indicating areas of highest lead concentration. These lead levels were then compared with land cover, wetland habitat classifications, and digital elevation models to define habitats with high lead exposure risk.  Lastly, habitat use by mottled ducks (Anas fulvigula), a waterfowl species native to the Gulf Coast, was assessed for geospatial effects of environmental lead and potential direct effects of contamination on biota, which in turn suggest directed management practices.

Results/Conclusions

Mottled ducks exhibited extremely variable corporeal lead levels, with birds sampled ranging between 0 and 12000 ppb; the upper end of this range is well beyond the toxicity threshold. Black-necked stilts (Himantopus mexicanus), which have been shown to utilize similar habitats and bioaccumulate heavy metals in a similar way to  mottled ducks, have additionally demonstrated both nominal (74.6% of birds sampled) and toxic (4.8% of birds sampled) blood lead levels.  We have additionally estimated 55,290 lead shot pellets per hectare based on soil surveys on NWR complexes, and average soil lead concentrations on the order of 19.90 ppm.  Given these environmental lead levels and extremely high variability in corporeal lead values across species, we conclude that natural and landscape level processes will also cause high variability in spatial lead availability.  Our spatial analyses and creation of a “risk surface” can help direct management efforts and raise awareness of “ecological traps” – areas where forage and shelter are adequate for persistence, but exposure to anthropogenic lead will reduce fecundity and survival.