In comparison with other habitats, tidal marshes support few nonaquatic vertebrate species, but their unique characteristics have led to the evolution of species and subspecies that are endemic (found nowhere else). These endemic species and subspecies, which seem to be largely restricted to North America, have adaptations that suit them to life in a harsh environment in which seed abundance is low, salinity is high, and flooding is frequent.

Yet they face a broad variety of threats from human-caused environmental damage, according to an assessment in the August 2006 issue of BioScience written by Russell Greenberg of the Smithsonian Migratory Bird Center and colleagues.

Tidal marshes occur in mid to high latitudes, along coasts that are protected within estuaries or behind barrier islands. They are most common in North America and China. Some tidal marsh species are protected from high salinity by relatively impermeable skin, and others have kidneys that can concentrate salts from large volumes of water or specialized glands that exude salt. Many are gray or black in color, which is believed to be an advantage because it matches the dark color of the soils often found in tidal marshes.

Why endemic tidal marsh species seem to be largely restricted to North America--which has 24 of the worldwide total of 25--is not clear. Although it could reflect differing taxonomic practices in different countries, it may be related to the history of glaciation or of agriculture.

Endemic tidal marsh species are vulnerable to coastal development and to sea level rise, both of which are rapidly reducing the area of tidal marshes. They are also threatened by toxic wastes and invasive species.

Greenberg and his coauthors argue for an expanded research program to try to understand how species will respond to these threats.

BioScience publishes commentary and peer-reviewed articles covering a wide range of biological fields, with a focus on Organisms from Molecules to the Environment. The journal has been published since 1964 by the American Institute of Biological Sciences, an umbrella organization for professional scientific societies and organizations that are involved with biology. It represents some 200 member societies and organizations with a combined membership of about 250,000.

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