Ecologically Scaled Responses of Marsh Birds to Invasive Phragmites Expansion and Water-Level Fluctuations

Abstract

Abstract. We examined effects of Phragmites australis on four marsh-dependent birds [Least Bittern (Ixobrychus exilis), Marsh Wren (Cistothorus palustris), Sora (Porzana carolina), Virginia Rail (Rallus limicola)] during water-level fluctuations within Saginaw Bay, Michigan. During 2002–2004 (pre-Phragmites expansion), 2008–2010 (Phragmites expansion), and 2014–2015 (increasing water levels-decreasing Phragmites coverage), we measured area of native vegetation, area of Phragmites, and distance between native vegetation patches at 21 coastal wetlands. We calculated ecologically scaled landscape indices (ESLIs) to determine changes in carrying capacity and connectivity for each species in the wetland landscape through time. Carrying capacity and connectivity values were greatest for all species during 2002–2004, likely due to the limited influence of Phragmites on the landscape during that period. By 2008-2010, expansion of Phragmites severely reduced marsh bird habitat carrying capacity and connectivity of wetland landscapes. Rising water levels, associated with reduced Phragmites cover, resulted in further slight reductions in connectivity and slight increases in amount of wetland habitat. Data from a subset of focal sites in Saginaw Bay suggested that marsh birds responded positively to increasing water levels. Our study demonstrates utility of ESLIs as a conservation tool for identifying key factors that impact landscape structure and avian community composition over time.