Meadow No More?

The sea’s undulating green carpet is at risk of melting away in murky waters. Nearly one-third of the world’s known seagrass species are in decline and ten species are at risk of extinction, often due to declining water clarity, according to a landmark global analysis. The trend threatens a critical marine habitat that supports fish and shellfish that feed millions of people.

Seagrasses appeared millions of years ago, after land-based flowering plants re-entered the ocean where their ancestors had evolved. Today, the marine grasses can form vast meadows, blooming and bearing seed in relatively clear, shallow waters around the world. In addition to providing key nursery habitat for countless other marine species, seagrass is a major food some sea turtles and other species. One study found that the slender plants provide ecological services worth at least $34,000 per hectare per year.

Coastal pollution and development, however, has taken a toll on seagrasses. Researchers estimate that seagrass habitat eroded by 110-square-kilometers per year between 1980 and 2006. It wasn’t clear, however, which species were facing the greatest threats.

To rate the risks, in 2007 the International Union for the Conservation of Nature (IUCN) asked 21 leading experts to analyze the available information. Over four years, they carefully examined what was known about the world’s 72 recognized seagrass species. In particular, they asked whether a species faced problems due a shrinking geographic range, or population declines. Now, in Biological Conservation, they present their sobering conclusions.

Overall, they rated three species as “endangered,” seven as “vulnerable” and five as “near threatened.” Forty-eight received a “least concern” designation, and the researchers were unable to classify nine species due to a lack of information. Twenty-two species (31%) have declining populations, while five have increasing numbers (including two species that have jumped across the Pacific and Atlantic Oceans to become invasive species).

One of the three endangered species, called Phyllospadix japonicas, is disappearing from China’s coastline due to seaweed aquaculture and land reclamation. Another, Zostera chilensis, is known only from two locations on the coast of Chile, one of which researchers couldn’t find the last time they looked, in the 1980s. The third endangered species, little-known Korean species named Zostera geojeensis, had one of its two known populations destroyed by coastal development within the last decade.

“Globally, the primary impact to seagrasses is loss of water clarity and quality due to” plankton and seaweed blooms and sedimentation, the team reports. Eleven of the 15 most vulnerable species, for instance, face problems due to cloudier waters. “The most common threat to seagrasses is human activity,” they add, which ranges from coastal development to dredging and trawling. “For species with small spatial ranges, coastal development can be devastating,” they note.

“In the big picture,” they conclude, “our findings elevate the seagrass crisis.” And stopping and reversing the decline, they say, will require “a powerful combination of reduced exploitation, habitat protection and monitoring, and improved water clarity… Both policy and action are imperative to protect seagrass habitats and species from degradation and extinction.” David Malakoff | May 9, 2011

Source: Short, F., Polidoro, B., Livingstone, S., Carpenter, K., Bandeira, S., Bujang, J., Calumpong, H., Carruthers, T., Coles, R., Dennison, W., Erftemeijer, P., Fortes, M., Freeman, A., Jagtap, T., Kamal, A., Kendrick, G., Judson Kenworthy, W., La Nafie, Y., Nasution, I., Orth, R., Prathep, A., Sanciangco, J., Tussenbroek, B., Vergara, S., Waycott, M., & Zieman, J. (2011). Extinction risk assessment of the world’s seagrass species. Biological Conservation DOI: 10.1016/j.biocon.2011.04.010

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