Urban habitats - attractive, but bad for nature? On fruits, metals, and birds
Urban habitats provide valuable refuge for wildlife; however, these sites are often polluted. Therefore, it is vital to determine if urban habitats pose a greater risk than benefit to the species they attract. In this study, we examined attraction versus risk by investigating the trophic transfer of heavy metals and avian frugivory at a brownfield in Liberty State Park (LSP) in Jersey City, New Jersey. Previous research has shown that plants and birds are accumulating metals at LSP. To map the trophic transfer of metals at LSP and determine if these metals are biomagnified, we collected avian, invertebrate, and fruit samples from LSP and a control site. The samples were tested for metal load using atomic absorption spectroscopy. It is well known that fruits attract birds, but little is know about the potential for metal transfer from fruits to birds. We aimed to determine which species of birds feed on which species of fruit at LSP and if frugivores are at risk for metal consumption. Field observations were conducted at ten individual plants from five fleshy-fruiting species totaling fifty sites. We recorded the species and number of individual birds that visited and fed from the fruits of each plant.
Results of the LSP analysis for Cd, Cr, Cu, Pb and Zn indicate 1) bioaccumulation in fruits, birds and invertebrates, and 2) biomagnification among invertebrates. Levels of Pb and Cd were significantly higher at LSP when compared to a control site. We found significant variation in metal levels among fruit species at LSP. Cd and Pb were higher in fruit collected from LSP than from a control site. The results of our field observations of frugivory showed significant differences in fruit selection among the 25 observed visiting bird species. American Robins (Turdus migratorius), Gray Catbirds (Dumetella carolinensis), and Red-winged Blackbirds (Agelaius phoeniceus) made up over 80% of the visitations to fruiting plants. Visitation rates were strongly influenced by plant species, bird species, and the distance from the edge of the brownfield. These results suggest the potential for metal transfer from fruits to birds depends on a suite of factors and, dependent upon life history traits, certain bird species may be at higher risk than others.