Pathways and consequences of contaminant flux to Acadian Flycatchers (Empidonax virescens) in urbanizing landscapes of Ohio, USA
Linnea M. Rowse, MS
Advisor: Amanda Rodewald
Widespread increases in urbanization with a growing global population can result in a diverse assortment of threats to wildlife, including exposure to harmful chemical contaminants. Among environmental contaminants hazardous to humans and wildlife, mercury is of special concern due to its prevalence, mobility in aquatic systems, and persistence in sediments. Aquatic systems, in particular, can have high amounts of mercury in its bioavailable form (methylmercury), which can then bioaccumulate in insects and be transported to terrestrial food webs (i.e., to aerial insectivores). Such contaminant flux from aquatic to terrestrial systems is expected to disproportionately affect species reliant upon aquatic emergent insects, such as with aerial insectivorous birds. To understand the pathways of contaminant flux and the role that contaminants play in reproductive success, I addressed two broad questions: (1) Is avian exposure to metal contaminants influenced by territory placement at local and landscape scales? and (2) Do metal contaminant loads negatively impact individual body condition and reproductive output? From April-August 2011-2012 I tracked reproductive success of Acadian flycatchers (Empidonax virescens) in 19 mature riparian forest fragments located across an urban to rural land-use gradient in central Ohio, USA. I collected blood samples from adult (n = 76) and nestling (n = 17) Acadian flycatchers and collected samples from riparian systems (sediment, aquatic emergent insect, and water) to test for contaminants and examine relationships between landscape factors and contaminant concentrations. I used reproductive data and flycatcher contaminant loads to evaluate the impact of mercury on reproductive success and body condition of adult flycatchers.
Factors that were most responsible for contaminant transfer to flycatchers remain unclear. Landscape factors, including proximity of flycatcher territories to rivers (i.e., access to aquatic emergent insects) and amount of urbanization surrounding forest sites, were not related to mercury levels in flycatchers, sediment, water, or insects. However, when separately analyzed, I detected a positive relationship between mercury in flycatchers and sediments in urban landscapes, and the opposite, a negative relationship, in rural landscapes. Unlike previous research, mercury concentrations in aquatic insects were not predicted by concentrations in sediments. The overall lack of concordance among mercury levels in Acadian flycatchers, aquatic insects, and sediments raises the possibility that flycatchers consumed less aquatic prey than expected or perhaps that the common aquatic insects were not the most important vectors of aquatic-to-terrestrial contaminant flux.
Although levels of mercury in the blood of Acadian flycatchers in this study were relatively low and appeared not to affect body condition, reproductive success declined with increasing mercury loads. My results are consistent with other studies that have documented reduced reproductive success in contaminated areas, though mercury loads were at trace levels in the flycatchers that I studied. Reduced reproductive success in my study system could possibly be explained by changes in adult behavior or by egg hatching success, driven by mercury loads in flycatchers; these factors need to be further examined as sublethal effects of mercury contamination in birds.
As a whole, my research contributes to a growing body of knowledge regarding pathways for contaminant flux from aquatic to terrestrial systems, and also to our understanding of mercury contamination effects on free-living songbirds. My research suggests that pathways for contaminant flux from aquatic to terrestrial systems are complex and do not always follow predictable routes. Moreover, that reduced reproductive success was associated with trace levels of mercury in a free-living songbird is a cautionary note that wildlife managers should not dismiss the impact of trace levels of contaminants that may biomagnify.