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The USGS evaluated the habitat preferences of seabeach amaranth to better understand factors contributing to the species’ significant decline at Assateague Island National Seashore. The National Park Service can use this information to make management decisions that can help increase seabeach amaranth population size.

Seabeach amaranth (Amaranthus pumilus) is a threatened beach dependent plant species that serves as an indicator of a healthy beach ecosystem and contributes to coastal resiliency.

The USGS released a Scientific Investigations Report by Ben Gutierrez and Erika Lentz about a study evaluating the habitat preferences of seabeach amaranth (Amaranthus pumilus) at Assateague Island National Seashore (ASIS) in Maryland and Virginia. The study was undertaken in collaboration with ASIS Natural Resources biologists, Bill Hulslander and Jonathan Chase, who have been tracking a significant decline in the species at ASIS since 2008.

Prior to the early 20th century, seabeach amaranth was widespread on beaches and barrier islands along the U.S. Atlantic coast from southern New England to South Carolina. However, few seabeach amaranth plants were observed from the early 1920s through the late 1990s, with declines presumed due to increasing development on the coast, including construction of seaside resorts, and efforts to control shoreline changes.

chart showing the number of plants each year from 2001 to 2020 accompanied by photo of seabeach amaranth
Figure 1. Time-series of number of seabeach amaranth observed in end-of-season surveys by National Park Service (NPS) biologists. Shaded regions specify years where drought conditions persisted through the amaranth growing season. Vertical lines denote strong storms that occurred during the observation period. Inset photo shows a seabeach amaranth plant photographed at Assateague Island (photo source: NPS staff). 

Following the discovery of seabeach amaranth at ASIS in 1997, the National Park Service conducted a three-year effort to re-establish the species on the barrier island. Between 2000 and 2002, National Park Service biologists planted approximately 5,000 cultivated plants on ASIS and have continued monitoring plant presence through today. Although the population reached a high of 2,200 plants in 2007, the population has since declined, with the largest drop occurring between 2008 and 2010 to approximately 1,000 plants, and then to only a few hundred in subsequent years (Figure 1).

This USGS study helps to better understand the species’ habitat preferences and determine potential causes of this significant population decline, such as changes to the physical characteristics of the island, as well as climatological and biological factors.

Habitat Preferences

The study found that seabeach amaranth tended to occur more frequently in lower-elevation locations, presumably where periodic washover by high-water conditions was more likely. This is consistent with the long-held belief that seabeach amaranth requires a barrier island setting where periodic disturbances such as washovers naturally occur, which serves to remove vegetation that may outcompete seabeach amaranth. 


Potential Causes of Population Decline

The authors determined the largest changes in population coincided with a multi-year period where drought conditions persisted during the growing season and a combination of in-season storms and large storms out-of-season caused significant impacts to the island’s landscape (Nor’Ida, H. Irene, Superstorm Sandy). This led authors to speculate that seabeach amaranth seed stock (seed bank) may have been effectively removed from the system during large storms or buried too deeply to germinate the following season. 


A Peek Behind the Curtain—Data and Methods Used to Obtain Results

To better understand factors that may have contributed to seabeach amaranth decline, the USGS researchers evaluated seabeach amaranth population trends and compared these to climatological indices. In addition, the study relied on data obtained from USGS Coastal Change Hazards work to sample physical measurements from observed seabeach amaranth locations and randomly selected locations where the plant did not occur.

The study used measurements from 2008, 2010, and 2014 to evaluate the physical characteristics of Assateague Island locations where amaranth was and was not found during these seasons. The authors then developed probabilistic models, using Bayesian networks to evaluate the spatial and temporal distribution of the plant during the three years examined in detail.

In addition to the 20 years of field observations of the species provided by the National Park Service, the work of current and former USGS scientists, Rachel Henderson, Julia Heslin, Travis Sterne, and Emily Sturdivant, was instrumental in developing the datasets and subsequent data releases of barrier island metrics that informed the analysis. 

Image: Assateague Inland Scrub
Inland scrub plants on Assateague Island. Inland scrub plays an important role in stabilizing the sand dunes on a barrier island.

This study broadens the portfolio of climate change and habitat studies that Sara Zeigler, Ben Gutierrez, and others have been undertaking over the last 10 years to develop barrier island and habitat models for another threatened species, the piping plover. Being able to quantify favorable habitat characteristics for threatened species that National Park Service staff are responsible for managing, helps to optimize management strategies and access to our coastal national parks, while providing species the best chance of success at not just persisting, but thriving.

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