Does invasive common reed in coastal salt marshes affect dabbling duck food availability?

Abstract

Common reed, Phragmites australis, a nonnative perennial grass, is considered a nuisance species to land managers and wildlife biologists. Common reed thrives in areas with reduced soil salinities, increased nitrogen availability, and anthropogenic shoreline development. The expansion of nonnative common reed into tidal wetlands of North America detrimentally affects native wildlife by altering resource utilization, modifying trophic structures, and changing disturbance regimes. Thus, it also has the potential to drastically affect dabbling duck (Family Anatidae, SubFamily Anatinae, Tribe Anatini) energetic carrying capacity in salt marsh ecosystems. We assessed whether invaded monocultures of common reed in dabbling duck habitat could alter the availability of invertebrate and seed foods for the mallard Anas platyrhynchos, American black duck Anas rubripes, green-winged teal Anas crecca, northern shoveler Spatula clypeata, and northern pintail Anas acuta as compared with wetland type (mudflat, low marsh, high marsh, and impoundments). We compared food and energy availability in . 90% common reed monocultures with noncommon reed-invaded salt marshes in five study areas in Edwin B. Forsythe National Wildlife Refuge, New Jersey, 2015–2016. To estimate wetland-specific food energy supply, we collected sediment core samples, fixed them with formalin, and washed, dried, sorted, and weighed them for seeds and invertebrates. We multiplied biomass (g) by true metabolizable energy values to estimate species-specific dabbling duck food energy availability. We further estimated wetland-specific energetic carrying capacity (duck energy days) on the basis of known species-specific energetic demands. We determined that duck energy days/ha were greater for dabbling ducks in wetlands invaded with common reed because they contained more consumable seed energy and less consumable invertebrate energy. However, future research should explore how accessible these foods are when common reed grass is dense. To aid in restoration efforts once common reed is removed by control efforts, our results indicate that a robust seed bank exists in the soil strata, thus increasing salt-marsh seed biodiversity.