Location and General Description
More than 800 km from South America, St. Peter and St. Paul’s Rocks (0°56’N, 29°21’W) are a small group of islands in the equatorial Atlantic Ocean. The island group is 250 m across, and their highest point is 19.5 m (Smith et al. 1974). Composed of mylonitic peridotite, the submarine mountain of which these rocks are the pinnacles extends 4000 meters into the ocean depths (Smith et al. 1974). St. Peter and St. Paul’s Rocks were one of the few sites visited by Darwin on the February 16, 1832 Beagle expedition.
These small islands are part of the mid-oceanic ridge of the Atlantic Ocean, one of the world’s longest mountain chains. The Earth’s crust separates along this ridge, and new ocean floor is created. Underwater volcanoes and lava flows occur all along the ridge. In some areas, such as St. Peter and St. Paul’s Rocks or Iceland, the ridge is so high that it forms islands. The only source of freshwater on the St. Peter and St. Paul’s Rocks is from rain, and there is almost no vegetation. A lack of vascular vegetation is most likely due to constant sea-spray bombarding the Rocks. The meager terrestrial vegetation that does occur includes very simplistic fungus and algae. In 1971, Smith et al. identified a filamentous blue-green algae as Lyngbya sp. and a small green algae as Stichococcus bacillaris (Smith et al. 1974).
While terrestrial flora is scarce, the isolated islands provide habitat for a rich benthic and littoral marine biota. This food source supports many seabirds, which are the only vertebrate wildlife found on the islands. Breeding seabirds found on the Rocks during the 1971 survey included brown booby (Sula leucogaster), brown noddy (Anous stolidus), and black noddy (Anous minutus) (Smith et al. 1974). All life-cycle stages of the booby were found during this survey, suggesting that their breeding was aseasonal. The birds’ eggs occassionally fall prey to crabs (Grapsus grapsus), a marine invertebrate that is present in large numbers on the Rocks (Smith et al. 1974). The invertebrate element of the Rocks’ food chain primarily consists of microbial feeders. These include protozoa, nematodes (Acrobeloides, Diploscapter and Panagrolaimus genera), bdelliod rotifers, and mites (Scheloribates spp.) (Smith et al. 1974).
Scientists and military personnel are the only human visitors to these islands and pose very few direct threats. St. Peter and St. Paul’s Rocks are in the center of their ranges for the seabirds found there. Old accounts of visits to the island reported very large numbers of birds on the Rocks. For example, Darwin (1860) stated that he observed "a vast multitude of sea-fowl", and Moseley (1879) noted "birds hovering in thousands". These accounts are not consistent with more current numbers observed, and it is therefore suspected that the seabird population of St. Peter and St. Paul’s Rocks are declining. It is suspected that this is due to human disturbance or possibly to the gradual erosion of the Rocks into the sea (Smith et al. 1974). Excluding this possible decline, the ecology of St. Peter and St. Paul’s Rocks is relatively stable.
Types and Severity of Threats
Threats are not numerous for these isolated island rocks; however, climate change and the following increase in ocean levels could eventually cover this rock ecoregion. Also of concern is the decreasing sea bird populations and the lack of knowledge to identify the cause.
Justification of Ecoregion Delineation
St. Peter and St. Paul’s Rocks, far offshore in the equatorial Atlantic Ocean, represent one of the few places on Earth where an underwater oceanic ridge breaks through the surface of the sea. Isolation from the mainland also provides a habitat of significant ecological and biogeographic interest. The rocks function as an oasis in the deep ocean, providing a niche for marine life nearer the ocean’s surface. While the islands are virtually devoid of terrestrial vegetation, the rich marine flora and fauna provides a food source to the unique assemblage of seabirds that reside here.
Darwin, C. 1860. Journal of Researches into the Natural History and Geology of the Countries Visited during the voyage of H. M. S. Beagle round the World. Murray, London.
Moseley, H. N. 1879. Notes by a naturalist on the "Challenger". Macmillan, London.
Smith, H. G., Hardy, P., Leith, I. M., Spaull, V. W., and Twelves, E. L. 1974. A biological survey of St. Paul's Rocks in the equatorial Atlantic Ocean. Biol. J. Linn. Soc. 6:89-96.
Prepared by: Leann Trowbridge
Reviewed by: In process