Mangroves are invading Texas salt marshes: what are the consequences?

2014-2016 - $261,939

Dr. Steven Pennings
Department of Biology and Biochemistry
University of Houston

Dr. Anna Armitage
Department of Marine Biology
Texas A&M University


On the Texas coast, mangroves have historically expanded into salt marshes during periods with warm winters, and contracted during periods with hard freezes (Sherrod and McMillan 1981, McMillan 1986). In the future, mangrove distributions are expected to expand further due to rising temperatures (Montagna et al. 2009). Models predict that an increase in winter minimum temperatures of 2-4o C may lead to black mangroves replacing salt marsh on 100% of the TX coast and 95% of the LA coast (Osland et al. 2013). Because salt marsh and mangrove plants are both foundation species that create habitats and dominate ecological processes, this transition will likely have important consequences for wildlife, fisheries, and human economies (Ellison et al. 2005). Coastal wetlands in Texas support commercial and recreational fisheries (e.g., for red drum, Sciaenops ocellatus, and brown shrimp, Farfantepenaeus aztecus), improve water quality, reduce storm damage and erosion, and provide habitat for endangered species and for species such as birds that support a vigorous tourism industry (Moulton et al. 1997, Kennish 2001, Butzler and Davis 2006). Mangrove expansion will likely affect all these services, and require changes in the way that coastal resources are managed. Scientists have begun to compare ecosystem services in mangroves and salt marshes (Bloomfield and Gillanders 2005, Moseman et al. 2009, Perry and Mendelssohn 2009), but how mangrove expansion will affect these services remains poorly understood. Recognizing the importance of this issue, Texas Sea Grant previously funded us to begin an experimental study of the consequences of mangrove expansion. The objective of our study is to rigorously identify the consequences of mangrove expansion into salt marsh habitats, and to identify non-linearities and thresholds in ecosystem responses to mangrove density. We have set up a large field experiment near Port Aransas, begun to sample adjacent marsh and mangrove sites for comparison, and begun an effort to engage citizen scientists in helping us monitor birds. We found immediate effects of mangrove density on microclimate (temperature, wind) and will document initial biotic responses in 2013. We request funding to continue the work for two more years. We envision this as a 20-year experiment, and will use results from Sea Grant funding to write a compelling NSF proposal to support the next phase of the work.