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Biodiversity of remote islands

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  • Bob Conrich
    http://www.eurekalert.org/pub_releases/2008-08/uoc--soi082608.php Public release date: 26-Aug-2008 Contact: George Foulsham george.foulsham@ia.ucsb.edu
    Message 1 of 1 , Aug 26, 2008
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      Public release date: 26-Aug-2008
      Contact: George Foulsham
      University of California - Santa Barbara
      Study of islands reveals surprising extinction results
      Steve Gaines.

      (Santa Barbara, Calif.) – It's no secret that humans are having a huge
      impact on the life cycles of plants and animals. UC Santa Barbara's
      Steven D. Gaines and fellow researcher Dov Sax decided to test that
      theory by studying the world's far-flung islands.

      Their research, published this month in the Proceedings of the
      National Academy of Sciences, sheds surprising light on the subject of
      extinction rates of species on islands. The paper, "Species Invasions
      and Extinction: The Future of Native Biodiversity on Islands," is one
      in a series of reports by this team studying how humans have altered
      the ecosystems of the planet.

      Gaines and Sax started the project with a question: What effect are
      humans really having on biological diversity? "The presumption at the
      time was that we are driving biodiversity to lower levels," said
      Gaines, who directs UCSB's Marine Science Institute. "Certainly, if
      you think about it at the global level, this is true because humans
      have done a lot of things that have driven species extinct."

      However, when studied on the smaller scale of islands, the findings
      showed something completely different. Diversity is on the rise –
      markedly so in some instances. Diversity has gone up so dramatically
      that it might cause some to wonder if the health of the ecosystems
      might not be better because the number of species is twice as high as
      it used to be. But it's not that simple, Gaines said.

      "What Dov and I worked on a few years ago is the fact that the vast
      majority of introductions (of species) don't have large negative
      effects," Gaines said. "Indeed, most species that get introduced don't
      have much effect at all. It doesn't mean that they're not altering the
      ecosystem, but they're not driving things extinct like some of the big
      poster-child stories we've been hearing about."

      Still, the study showed that human colonization has had a massive
      impact on ecosystems of islands, with the introduction of new, exotic
      plants and animals. In New Zealand, for example, there were about
      2,000 native species of plants. Since colonization, about 2,000 new
      plant species have become naturalized. Over the same period, there
      have been few plant extinctions, so the net effect is that humans have
      transformed New Zealand's landscape by bringing in so many new species.

      Sax, a former postdoctoral researcher at UCSB who is now assistant
      professor of ecology and evolutionary biology at Brown University, did
      much of the fact-finding for this report by painstakingly digging
      through data that had been collected over hundreds of years on islands
      around the world. "This is Dov's specialty," Gaines said. "Finding
      really old data sets that are very interesting."

      "The dramatic increase in the number of species has changed how the
      system functions," Sax said. "Changing the abundance of natives versus
      exotics affects all of the other species that used to depend on the
      natives for food or shelter. So, it's not in any way to say that
      increasing biodiversity is a good thing."

      With birds, it's a different story. The number of bird species on
      islands today is almost exactly the same as it was prior to human
      colonization, but the species of birds on the islands are very
      different. About 40 percent of the species of birds that you find on
      islands today are introduced species, Sax said, which means that a
      comparable number of birds has gone extinct. "In the case of birds,"
      he said, "lots of extinctions, no change in total biodiversity."

      All of this caused Gaines and Sax to ask new questions:

      * Are the islands undersaturated? Can you still keep throwing
      species in there, with the result that nothing is going to happen?

      * Are they now oversaturated? Are there limits in how many species
      an ecosystem can hold?

      * Are we building an extinction debt? "Which means," Gaines said,
      "that by going in and mucking up the system, we may have already
      created the setting where too many species have been packed in, and we
      just haven't waited long enough to see these extinctions start to happen.

      "The whole point of this study was to start looking down the path to
      see which of these wildly different scenarios might be right," Gaines
      added. "We haven't nailed the answer yet, but we've set the stage for
      answering whether islands are now saturated or not."

      What made the research possible was that many of the explorers who
      colonized the islands included naturalists on their boats. From the
      time they landed on the islands, the naturalists were busy cataloging
      and documenting the plants and animals of each colony.

      "It was very surprising to find such a strong correlation between the
      number of native and exotic plant species on islands around the
      world," Sax said. "In ecological research, a 'strong' correlation
      often explains 50 percent of the variation. Here, the correlation
      between native and exotics explains almost 100 percent of the
      variation. In other words, if you know how many native plants are on
      an oceanic island then you can predict almost perfectly how many
      exotic plants are there."

      The study, which took a year and a half, included islands such as Lord
      Howe Island east of Australia and Tristan da Cunha, a group of remote
      volcanic islands in the south Atlantic Ocean, among others.

      "These were all oceanic islands," Gaines said, "which means islands
      that are far enough away from a continent that they're not getting
      regular exchanges with the mainland."

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