Page 677 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

now
geosystems
Invasive Species
Arrive at Tristan
dGa Cunha
eosystems Now in Chapter 6 de- scribes an accident that left an oil- drilling platform adrift in the currents
of the South Atlantic Ocean. Eventually the rig ran aground in Trypot Bay on the remote island of Tristan da Cunha, carry- ing new organisms into an existing marine ecosystem (Figure gN 20.1).
Aquatic Species on the Rig Because the rig had not been cleaned before towing, the platform was carefully checked for unwelcome organisms after making land- fall in Tristan. Although the living quar- ters inside the rig showed no rodents or other terrestrial animals, the underwater portion of the rig carried a virtually intact subtropical reef community of 62 species, all non-native to the island.
A non-native species, also known as an exotic or alien species, is one that origi- nates in a different ecosystem from where it is now found. If a non-native species be- comes invasive in a new environment, it outcompetes native species for resources and can introduce new predators, patho- gens, or parasites into an ecosystem. In- vasive species can devastate biodiversity, especially in isolated island ecosystems. The arrival of a marine community on the drilling platform thus provided an oppor- tunity for scientists to assess a potential biological invasion.
The Scientific Assessment Scientists made numerous dives to survey the newly arrived organisms to Tristan da Cunha. They developed a system of four assessed risk levels for a potential invasion: (1) no threat from species that perished in transit; (2) low threat from species that persist on the rig alone; (3) medium threat from species that could spread from the rig; and (4) high threat from breeding species with strong invasive potential.
At level 1, scientists found corals that died in transit, providing skeleton micro- habitats for various non-native worms, small crabs, amphipods, and other species that posed a level-2 threat. Also present were living and dead barnacles; the living acorn barnacles, the largest on the rig, were assessed at level 3. Among the shells left by the dead barnacles (assessed at level 1) lived small sponges, brown mussels, and urchins, assessed at level 2 (Figure gN 20.2).
A population of free-swimming finfish species was found around the rig, appar- ently having swum along with the rig as it drifted—the first report of such finfish becoming established after this type of unintentional movement. Two of these species, the silver porgy (with more than 60 counted around the rig) and the variable blenny (having a reproductive doubling time of 15 months), pose the greatest threat to native communities— a level-4 risk. Scientists also found por- celain crabs and a balanidae (a type of barnacle), both assessed as presenting a level-4 invasion risk.
▲Figure GN 20.2 Coral skeletons and dead barnacle shells on the submerged portion of the oil-drilling rig. [Sue Scott.]
Effects and Lessons Learned Tristan’s population of 267 people depends on rock lobster (a crawfish) for export, the island’s main source of income (Figure gN 20.1b). In 2007, to prevent in- vasive species from threatening the rock lobster industry, the drilling platform was salvaged and then towed to distant deep water and sunk, at a total cost of USD $20 million. Nine months after the rig’s removal, scientists failed to find any exotic species at the grounding site. How- ever, the risk of invasion will continue for an unknown period. The scientific team concluded that the towing of rigs that have not been properly cleaned presents “unexcelled opportunities for invasion to a wide diversity of marine species.”
The arrival of the oil platform and its associated species into Tristan’s waters demonstrates how globalization af- fects even the most distant archipelago. Aquatic and terrestrial biomes and the ef- fects of invasive species are the subjects of this chapter.
geosystems now online go to Chapter 20 on the MasteringGeography website (www.masteringgeography.com) for more on non-native and invasive species, and see the Tristan website at www .tristandc.com/. Also, see r. M. Wanless, S. Scott, et al., 2009, Semi-submersible rigs: A vector transporting entire marine communities around the world, Biological Invasions 10 (8): 2573–2583.
(a) Natural kelp forest.
(b) Native Tristan rock lobster.
▲Figure GN 20.1 Native marine environment offshore from Tristan da Cunha. [Sue Scott.]
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