Aug. 6, 2009 For the first time, researchers have definitively shown that shipwrecks and other man-made structures increase the potential for large invasions of unwanted species into coral reefs, even comparatively pristine ones. These unwanted species can completely overtake a reef and eliminate native corals, dramatically decreasing the diversity of marine organisms on the reef. Coral reefs can undergo fast changes in their dominant life forms, a phenomenon referred to as phase shift.
Scientists have speculated on many possible causes of phase shift, but this study is the first one to clearly show that a rapid change in the dominant life forms on a coral reef is associated with man-made structures.
In September 2007, USGS researcher Dr. Thierry Work, Dr. Greta Aeby from the Hawaii Institute of Marine Biology, and Dr. James Maragos from U.S. Fish and Wildlife Service studied a 100-foot vessel that wrecked in 1991 on isolated Palmyra Atoll in the central Pacific Ocean. They found extremely high numbers of an invasive species related to anemones and corals, Rhodactis howesii, on and around the shipwreck site. The density of this species progressively decreased with distance from the ship, and it was rare or absent in other parts of the atoll. Likewise, the researchers confirmed high densities of R. howesii around several buoys installed on the atoll in 2001.
Even though phase shifts can have long-term negative effects for coral reefs, eliminating organisms responsible for phase shifts can be difficult, particularly if they cover a large area. The extensive R. howesii invasion and subsequent loss of coral reef habitat at Palmyra highlights the importance of rapid removal of shipwrecks on corals reefs to help prevent reefs from being overgrown by invasive species.
"Why this phenomenon is occurring remains a mystery," said Work, a scientist at the USGS National Wildlife Health Center's Honolulu Field Station. One possibility, he said, is that iron leaching from the ship and mooring buoy chains, accompanied with other environmental factors particular to Palmyra Atoll, are somehow promoting the growth of Rhodactis.
This research was presented at the 58th annual meeting of the Wildlife Disease Association (WDA) held on August 2-7, 2009, in Blaine, Wash.
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