Rosencranz JA, Thorne KM, Buffington KJ, Overton KJ, Takekawa JY, Casazza ML, McBroom J, Wood JK, Nur N, Zembal R, MacDonald GM & Ambrose RF (2018) Rising tides: Assessing habitat vulnerability for an endangered salt marsh-dependent species with sea-level rise. Wetlands. https://doi.org/10.1007/s13157-018-1112-8
Beyond the beaches of California lie coastal marshes. These marshes are dense with short vegetation, sprouting from saturated, salt-ridden soils. Activity is constantly shifting along with the tides. At high tides, the tidal creeks overflow with seawater. As the tides recede, the soils become exposed to air and the leftover materials brought in by the tide. It is a nutrient rich ecosystem, full of wonderful wildlife.
The Ridgway’s rail, Rallus obsoletus, lives in this shifting environment. It nests in the tall dense cordgrass found along the tidal creek. The rails exist in only two subpopulations, one is found in southern California and the other in the San Francisco Bay area. Seldom do these birds leave their territory. However, as sea levels continue to rise, there is an increasing concern for the future of this endangered species.
Dr. Rosencranz led a team from the University of California Los Angeles and the U.S. Geological Survey to describe how the rail’s habitat may change over the next 100 years with sea level rise.
Salt marshes persist at land’s edge; the vegetation grows just above mean sea level. As sea level has increased in the past, marsh subsist by trapping in sediment delivered from the sea and growing vertically, or by migrating landward. But over the next 100 years, coastal California and much of the world is expected to experience much higher rates of sea-level rise than what has been recorded in human history. The rising sea will have its consequences.
Coastal California & Flooding
Rosencranz’s team assessed the conditions of the species’ current habitat in 14 different marshes in California. They collected information related to the elevation of the marshes within a single centimeter of accuracy, they examined the soils of the marshes, they assessed the tidal creeks which many of the rails rely on for food as well as examined the vegetation throughout the marsh complexes.
In order to assess the rail’s future environment, the team utilized tools from WARMER. WARMER is a data source which make future sea-level rise projections based on historical records. The team examined three different flooding scenarios: a low rate (44cm), a moderate rate (93cm) and a high rate (160cm/100years) and assessed the habitat from these 14 different marshes every ten years from 2010 to 2110.
Under the low rate of sea-level rise (44cm/100years) the researchers found that by the end of the 21st century the amount of suitable habitat for the Ridgway’s rail will actually increase by 37%. With a low increase in sea-level, more exposed low elevation marsh will be created.
This pattern is also observed in the moderate rate of sea-level rise (93cm/100years). By 2050, they expect a 32% increase in habitat available for the rails in these 14 observed salt marshes. However, as the century ends the marshes will begin to flood excessively and 57% of suitable habitat is expected to be lost.
Under the highest rate of sea-level rise (166cm/100years), nearly 85% of suitable habitat in these marshes will no longer exist for the rail.
The future of the Ridgway’s rail in California in unclear. With limited to no habitat available in the future for a bird that is already endangered does not look promising. But these are predictions. The researchers acknowledge they only examined 14 marsh complexes and that with increasing sea level, there may be new habitat available closer to land that was not included in these predictions.
However, this research provides motivation to act. As sea levels rise, marshes will need to accrete more sediment or migrate landward. To support the future of the Ridgway rail and other species it’s imperative that land managers and governmental agencies address certain strategies that may help marsh migration; such as the removal of barriers or alter levels of sediment reaching the marshes.