Estimating population sizes and historical trends for birds in a vulnerable ecosystem
Population estimates for tidal marsh birds of high conservation concern in the northeastern USA from a design-based survey. Wiest, W.A., Correll, M.D., Olsen, B.J., Elphick, C.S., Hodgman, T.P., Curson, D.R. and Shriver, W.G. 2016. The Condor: Ornithological Applications. DOI: 10.1650/CONDOR-15-30.1 VIEW
Predictors of specialist avifaunal decline in coastal marshes. Correll, M.D., Wiest, W.A., Hodgman, T.P., Shriver, W.G., Elphick, C.S., McGill, B.J., O’Brien, K.M. and Olsen, B.J. 2016 (in press). Conservation Biology.
Tidal salt marsh ecosystems are spread across thousands of kilometers of coastline in predominately temperate latitudes. The global areal extent, however, actually only covers ~45,000 square km, and over a third of these marshes are along the Atlantic and Gulf coasts of the United States. North American tidal marshes are home to 56% of endemic salt marsh species worldwide (Greenberg 2006). While few species are truly restricted to these habitats due to the wide-ranging hydrology, salinity, and soil conditions, several avian species have evolved reproductive strategies to contend with the tidal flooding events that maintain these ecosystems.
In the northeastern U.S., five bird species are considered tidal marsh specialists, spending their entire annual life cycle in the salt marsh; clapper rail (Rallus crepitans), willet (Tringa semipalmata), Nelson’s sparrow (Ammodramus nelsoni), saltmarsh sparrow (A. caudacutus), and seaside sparrow (A. maritimus). These species nest near the ground; successful reproduction is therefore necessarily tied to the ability to withstand flooding (Gjerdrum et al. 2005, 2008). Increases in flooding from sea-level rise and severe coastal storms due to global climate change threaten the ability of these birds to successfully rear young, and as a result many of these species are currently designated as high conservation priorities. Unfortunately, fundamental population size and trend information that is essential in developing effective species conservation plans have, until recently, been unavailable for these vulnerable species.
To address this critical need for tidal marsh bird population size and trajectory information, we undertook a large-scale, comprehensive avian survey under the umbrella of the Saltmarsh Habitat and Avian Research Program (SHARP). Over the course of two breeding seasons (2011-2012), we performed point count and call-broadcast surveys at 1,780 sampling locations in tidal marshes from Maine to Virginia (Figure 1). Using the data from these surveys and modern quantitative techniques we were able to produce the first set of robust population estimates for these tidal marsh birds. We estimated that there were 151,000 clapper rails, 117,000 willets, 5,000 Nelson’s sparrows, 53,000 saltmarsh sparrows, and 230,000 seaside sparrows in northeastern tidal marshes. The regional population estimates can be broken down at finer spatial-scales, such as the subregion or state levels, and used by wildlife agencies to help identify focal habitat areas critical for maintaining tidal marsh bird populations within each state.
Another facet of the survey work was to collect point count datasets from various organizations along the Atlantic coast and combine them to form a historical point count database. Datasets were donated from over 20 federal, state, and non-profit organizations spanning 18 years and 10 U.S. states. We then used these data to produce population trends for our five focal species, and explored trends in the tidal marsh bird community as a whole. Additionally, we used remote sensing methods to quantify some of the modifications inflicted on tidal marshes by humans to explore potential drivers for the declines we observed in this historical data. Humans have modified many U.S. marshes locally for centuries; marsh ditching has been systematically employed in coastal marshes for mosquito abatement, and road crossings driving tidal restriction of marshes upriver of the obstruction can permanently alter marsh hydrology. Climate change, specifically through extreme storm events and sea-level rise, can also drive marsh pattern and process over time (Kirwan et al. 2016).
Our results, unfortunately, paint a bleak outlook both for the saltmarsh sparrow and the tidal marsh specialist bird community. We found a 9% annual decline in the global breeding population for saltmarsh sparrows and an overall 2.3% decline in the specialist bird community. We also observed declines in clapper rail and Nelson’s sparrow populations within our study area. Combining these trends with the global population estimate of ~53,000 saltmarsh sparrow individuals in 2012, the saltmarsh sparrow population will likely be reduced to under 500 individuals in 50 years, indicating a very high risk of extinction for this species within our lifetimes.
We found correlative relationships between several types of marsh modifications and the population trends we observed in our data. 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. More surprisingly, we found that tidal marsh birds exhibited no trend at all in marshes with no road crossings, but declined in tidally restricted locations, superseding sea-level rise and extreme storm events as a short-term predictor of bird declines. We believe that this is because tidal restrictions have the ability to limit the amount of sediment supply to marshes, therefore slowing vertical marsh growth known as accretion.
Figure 2 Saltmarsh sparrow nestlings experiencing a tidal flooding event
© Jeanna Mielcarek
While these findings could initially drive managers to remove all tidal restrictions in their local marshes, the answer is not that simple. Tidal restrictions, on shorter timescales, may provide refugia to breeding saltmarsh sparrows because of their ability to dampen tidal extremes that can be fatal to saltmarsh sparrow nestlings (Culp 2012, Ruskin 2015; Figure 2). This creates a paradox for the marsh manager; remove restrictions and endanger short-term success of these birds, or keep restrictions but risk loss of long-term marsh resilience to rising sea levels through slowed marsh accretion. While some strategies exist to restore sediment supplies to marshes (e.g., dam removal, thin layer deposition), the solutions are not without drawbacks, and a clear management solution for the saltmarsh sparrow remains elusive.
Extensive historical losses of marsh habitat and continuing habitat degradation from terrestrial and marine sources place an inordinate pressure on the integral structure of tidal marshes and the species populations they support. Restoration and management of salt marsh ecosystems, particularly in light of accelerated sea-level rise, are critical to the persistence of tidal marsh bird populations. As tidal marshes and the species they host struggle to adapt to the severe changes faced from global climate change, a consistent marsh bird monitoring program, such as the one introduced here, provides opportunities to assess bird responses to habitat changes and develop effective plans for species conservation.
Culp, L. 2012. Roads in salt marshes: flooding, vegetation, and sharp-tailed sparrow habitat quality in tidally restricted marshes. Master’s thesis, The University of Maine, Orono, Maine, USA.
Gjerdrum, C., Elphick, C.S. and Rubega, M. 2005. Nest site selection and nesting success in saltmarsh breeding sparrows: The importance of nest habitat, timing, and study site differences. The Condor 107: 849–862. View
Gjerdrum, C., Sullivan-Wiley, K., King, E., Rubega, M.A. and Elphick, C.S. 2008. Egg and chick fates during tidal flooding of saltmarsh sharp-tailed sparrow nests. The Condor 110: 579–584. View
Greenberg, R. 2006. Tidal marshes: Home for the few and the highly selected. In: Greenberg, R., Maldonado, J.E., Droege, S., and MacDonald, M.V. (Eds) Terrestrial Vertebrates of Tidal Marshes: Evolution, Ecology, and Conservation. Studies in Avian Biology 32: 2–9. View
Kirwan, M.L., Temmerman, S., Skeehan, E.E., Guntenspergen, G.R. and Fagherazzi, S. 2016. Overestimation of marsh vulnerability to sea level rise. Nature Climate Change 6: 253–260. View
Ruskin, K. 2015. Investigating sharp-tailed sparrow reproductive biology in an ecological and adaptive framework. PhD dissertation, The University of Maine, Orono, Maine, USA.
Saltmarsh Habitat and Avian Research Program (SHARP). 2016. View
Featured image: Saltmarsh Sparrow, Ammodramus caudacutus © Dominic Sherry | CC-BY-SA-2.0 via Wikimedia Commons
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