Population Threats to Black Rails
The reason for the dramatic decline of black rails is not completely known but may be a result of one or more factors. Negative impacts can include habitat loss and degradation, predation, and environmental contaminants.
The Atlantic and Gulf coasts have lost nearly 9 % of their estuarine marsh cover since the 1950s. Most of these losses are attributable to the conversion of marsh to agriculture or coastal development, such as urbanization, suburbanization, and road/bridge construction. Black rails rely on the highest portions of marshes that are rarely inundated by tidal waters. Because of the drier hydrology, the high marsh has typically been the most favored wetland zone for filling and development.
Coastal development can also lead to a host of negative impacts on black rails beyond direct habitat loss. Many salt marshes positioned near development have been ditched for mosquito control, submerged by water control structures, and used as grazing areas for livestock. Development of upland areas are known to reduce the bird diversity and abundance of marshes they interface. Black rail territories are typically located directly adjacent to upland habitats so are particularly vulnerable to conversion of adjoining forest land to human uses.
Coastal counties now support some of the largest urban concentrations in the United States. The demand for new development by an ever burgeoning human population within coastal zones will bring another era of impacts on black rail habitats unless appropriate management is undertaken.
The earth’s seas are projected to rise 0.2 to 2 meters before the end of the 21st century. Because of their low position within the landscape, salt marshes will be one of the first habitats consumed by rising seas. One way salt marshes can continue to exist with sea-level rise is to migrate upslope where topography is favorable. However, barriers to salt marsh transgression such as sea-walls and the impermeable surfaces of developed land will eventually squeeze marshes out of existence. The high marsh zone that black rails rely on for breeding are particularly susceptible to be converted to low marsh where barriers to landward migration are present.
The Chesapeake Bay region has long been known as a population stronghold for breeding black rails. Simulations of the impact of sea-level in this region indicate that 51 % of black rail habitats will become submerged and vanish by the year 2100. Moreover, simulations made under an expected acceleration in the rate of sea-level rise would eliminate 99 % of black rail habitats in the Chesapeake Bay region.
The coastal zone of the Atlantic and Gulf coasts are suffering from an ever growing human pressure of development. Improved regulation and spatial planning of development and barriers to marsh transgression are needed to proactively reduce the impacts of sea-level rise on black rails and their habitats.
Although the impacts of nest predators on black rails is relatively understudied, evidence from other marsh nesting birds indicate that the probability of nest predation is related to water level and the surrounding habitats in the landscape. Marsh birds experience higher levels of nest predation when water levels are low and where marshes are surrounded by development. Black rails may be particularly vulnerable to nest predators because they rely exclusively on high marsh zones for breeding. The high marsh zone is infrequently flooded by tidal waters providing mammalian predators greater access. Additionally, high marsh zones are positioned directly adjacent to upland habitats that are the source of mammalian nest predators.
The impact of contaminants on black rails is not known. Black rails are exposed to a number of contaminants found in marsh sediments and the prey they ingest such as polychlorinated biphenyls (PCBs), DDT and its metabolites, and heavy metals. Many of these contaminants have been shown to reduce reproduction or cause mortality in other bird species including those that use marshes.
Marshes provide resources for a broad array of organisms and ecological services. In many situations, prescribed marsh management creates a class of winners and losers according to the species affinity for the management action being applied. Controlling water levels and prescribed fire are the two most widely used methods for marsh management but in many cases the timing or extent of these tools are being used to benefit one species are in direct conflict with requirements of the black rail. Conducting prescribed burns or raising water levels of impounded marshes during inappropriate times or inappropriate lengths of time can negatively impact black rails during the breeding and non-breeding season. There is very little information on the compatibility of these widely used management tools for black rails. This notion suggests the need for research and summary of how management directly influences black rail populations.
Exotic plants and animals can alter the structure of function of salt marshes and disrupt food webs. One specific example is Phragmites australis, a noxious, exotic reed that rapidly invades marshes along the Atlantic and Gulf coasts. Phragmites aggressively colonizes the narrow ecotone between the upland and salt marsh creating a monoculture stand. The tall-reed like structure of phragmites differs markedly from the grass-like savannah of the high marsh where black rails breed. One other example that may be specific only to a certain area, but devastating in its impact is the destruction of marsh by foraging nutria (Myocaster coypus). Nutria are exotic rodents originally introduced in the 1950s to bolster the fur trade. Nutria forage on the roots of marsh vegetation and can quickly turn healthy marsh into mudflats in short time. Dedicated programs to control nutria populations have been successful but are expensive and difficult to implement over large scales.