Heavy metal bioaccumulation and avian frugivory in an urban forest

Background/Question/Methods

Urban forests provide valuable habitat for wildlife; however, it is vital to determine if these habitats pose a greater risk than benefit to the species they attract. In this study, we examined the trophic transfer of heavy metals and avian selection of native and non-native fruiting plants at a forested brownfield in Liberty State Park (LSP) in Jersey City, New Jersey. Previous research has shown that insectivorous house wren nestlings are accumulating heavy metals at LSP. In order to identify the source of metals in the house wrens’ diet and to test for biomagnification in the food chain, we determined the metal concentrations of invertebrate samples from LSP and an unpolluted control site. To investigate avian frugivory, we studied avian use of native versus non-native fruiting plants. Non-native fruiting plants can benefit urban birds as a food source; however, the resulting dissemination of their seeds can facilitate invasion. We conducted point counts at fifty sites and recorded the number of birds that fed from native and non-native fruits. Fruit samples from five species of native and non-native plants were collected and tested for heavy metals. These results were compared to fruit metal load at our control site.

Results/Conclusions

Results of the invertebrate analysis show that metal concentrations are higher in predatory invertebrates than in herbivorous invertebrates, indicating bioaccumulation in the food chain. Metal concentrations were higher at LSP than at our control site. Results for Cr, Cu, Zn and Pb were above the minimum detection level for most invertebrate samples from LSP. Results of the fruit metal analysis show that fruits from LSP are accumulating small amounts of metal when compared to an unpolluted site.  Cd was detected in white mulberry, autumn olive, wild grape and porcelain berry.  Pb, Cu and Zn were detected in all of the plant species from LSP. The presence of metal in fruits at LSP illuminates a potential risk to frugivores; however the extent of this risk requires further investigation. Results of our frugivory study show that non-native birds prefer non-native fruits, whereas native birds show no significant selection preference. We discovered a significant difference in overall fruit selection between bird species when fruit and bird origin were not taken into account. These results suggest that the potential for the spread of invasive fruits by birds is, at least in part, dependent upon the bird community structure of specific sites.