Spatial scale is rarely considered in population-level assessments of contaminant impacts on wild animals; as a result misinterpretation of the relationship between contaminant exposure and population status may occur. We assessed the strength of expression of polychlorinated biphenyl (PCB) exposure effects at local vs. regional spatial scales on population status in five species of waterbirds, "bioaccumulators" often promoted as indicators of contaminant effects in aquatic ecosystems. Our focus was the upper Hudson River where PCBs occur at levels reported to have adverse impacts on wild birds. At the local scale, waterbird habitat occupancy was estimated from 220 repeat surveys made between 2001 and 2010 along the same survey route divided into 25 contiguous river segments with markedly different PCB concentrations. At the regional scale, waterbird habitat occupancy in relation to proximity to the upper Hudson River was estimated across 1248 Breeding Bird Atlas survey blocks while controlling for region-wide variation in habitat availability. At the local scale, many associations of habitat and sampling covariates with species detection probabilities were evident but none, including PCB concentration, with habitat occupancy, extinction or colonization of a given river segment. At the regional scale, survey effort and habitat factors not related to PCB exposure were the most important drivers of waterbird occurrence although two species were more likely to occur farther from the contaminated river segment. Spatial scale clearly mediates expression of contaminant impacts on wild bird populations; large-scale, expert-generated databases provide an underused opportunity for better delineating the spatial scales at which population impacts occur and risk assessments should be performed.
Remarkably few studies on the effects of PCBs on wild turtles have been conducted. We contrasted population-level parameters of 2 species of freshwater turtles, the snapping turtle Chelydra serpentina and the painted turtle Chrysemys picta, across a strong PCB concentration gradient in the upper Hudson River, New York State, USA. Our study employed standardized capture methods that yielded 465 turtles during 1968 trap-nights over 2 yr (2006 and 2007) at 246 trapping sites. Individuals of both turtle species were relatively heavier in a contaminated river segment, but PCB concentration did not otherwise influence relative abundance of turtles, probabilities of habitat occupancy or detection, sex or age ratios, body size, incidence of deformities, external parasite occurrence, or several movement parameters in a manner consistent with adverse impacts to wild turtles. This lack of spatial trend in parameters of wild populations of aquatic turtles across a strong gradient of PCB contamination contrasts with predictions of adverse impacts derived from population modeling, dose-response studies, and tissue residue analyses.
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