Tidal marsh birds are declining– SHARP population trends are published

A major goal of SHARP’s initial work plan was to produce population trends for the saltmarsh sparrow and other specialist species. Maureen “Mo” Correll, a recent graduate from the University of Maine’s PhD program in Ecology and Environmental Science, led an effort to collect point count datasets from various organizations along the Atlantic coast and combine them into a single database as part of her dissertation work. Over 20 federal, state, and non-profit organizations donated historical data spanning 18 years and 10 US states. Mo then used these data to produce population trends for SHARP’s five focal species and explore trends in the tidal marsh bird community as a whole. She also used remote sensing (data about the earth’s surface derived from satellites overhead) to quantify direct modifications to tidal marshes by humans and quantified recent change in sea levels, all which have the potential to drive marsh pattern and process over time.

Mo’s work was recently published in the journal Conservation Biology. Her findings do not paint a hopeful future for the saltmarsh sparrow nor the tidal marsh specialist bird community; she and her coauthors found a 9% annual decline in the global breeding population for saltmarsh sparrows and an overall 2.3% decline in the specialist bird community. Simple extrapolations of these numbers estimate the saltmarsh sparrow population will likely be reduced to fewer than 500 individuals in 50 years, indicating a very high risk of extinction for this species within our lifetimes.

 

Tidal marsh birds in decline

Tidal marsh specialists, or bird species that rely on tidal marsh for habitat, are declining across the northeastern U.S. Figure is from Correll et al. 2016 Conservation Biology.

Further, they found the presence of a tidal restriction downriver of a marsh explains population declines both in the endemic saltmarsh sparrow and in the entire specialist avifaunal community. Tidal marsh birds exhibited no trend at all in marshes without road crossings, but declined in tidally restricted locations. This may be because tidal restrictions limit the amount of sediment supply to marshes, therefore slowing vertical marsh growth, known as accretion, which impedes the marsh’s capability to keep pace with sea level rise.

 

Tidal marsh bird decline and tidal restrictions

Tidal marsh specialist decline is associated with tidal restrictions. Areas of the marsh that are behind more restrictions that modify the tide (often road crossings) are experiencing higher rates of population decline. Marshes that are not modified by tidal restrictions host stable tidal marsh bird populations. Figure is from Correll et al. 2016 Conservation Biology.

While these findings seem to suggest removal of all tidal restrictions to save this declining suite of species, the answer is not that simple. Tidal restrictions may provide refugia to breeding saltmarsh sparrows through their ability to dampen tidal extremes that can be fatal to nestlings. Removal of restrictions could therefore endanger short-term success of these birds, but restrictions left in place present a risk of loss of marsh resilience to rising sea levels through slowed marsh accretion. While some strategies exist to restore sediment supply from upriver (dam removal, thin layer deposition) a clear management solution for the saltmarsh sparrow remains elusive.

 

For more questions or comments, please contract Mo Correll at maurren.correll@maine.edu. This published work can be found at: http://onlinelibrary.wiley.com/doi/10.1111/cobi.12797/full.

 

 

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