Box 2: Did the elimination of large vertebrates such as manatees, turtles, and groupers from tropical ecosystems significantly alter biodiversity?

ELIMINATION OF LARGE VERTEBRATES FROM TROPICAL ECOSYSTEMS

Large marine vertebrates (such as whales, manatees, turtles, groupers, and the extinct Steller's sea cow, Caribbean monk seal, and great auk) have been systematically removed from the oceans by humans over the past 500 years. The ecological effects of the reduction or complete elimination of most large vertebrates remain unknown (sea otter impacts on kelp and sea urchins in the Eastern Pacific are an exception).

Eighteenth-century Caribbean explorers found extraordinarily abundant populations of large vertebrate grazers (manatees, turtles, and parrot fish), large invertebrate grazers (conch snails), and large carnivorous fish (groupers). These animals consumed seagrasses, algae, sea urchins, other fish, and many other animals. The removal of these consumers must have substantially affected communities both directly (e.g., altering food pyramids and trophic structure) and indirectly (e.g., the resulting increases in seagrass populations altering coastal sedimentation processes). Unfortunately, scientists arrived in the Caribbean two centuries after the large animal expulsion commenced.

The elimination of large consumers from a broad region is an example of how early historic alterations at one trophic level can markedly impact modern assumptions and interpretations of both natural biodiversity patterns at other trophic levels and overall ecosystem function. The need for an appropriate retrospective context is clear and further argues for the use of human exclusion experiments (see Box 13) to assess the effects of historical hunting on those species that still survive elsewhere.

   

Key References: Estes and Palmisano (1974); May et al., (1979); Hay (1984); Thayer et al., (1984); Duggins et al., (1989); Vermeij (1993); Jackson (1994).

  • Invasive species are increasing dramatically, with more than 3,000 species a day in motion inside the giant aquaria that serve as ballast tanks in oceangoing vessels (Carlton and Geller, 1993), sometimes completely altering the trophic structure of bays and estuaries into which the ballast water is discharged (e.g., Nichols et al., 1990; Horoshilov, 1993).
  • Filling and development of coastal habitat has resulted in total wetland losses of 50 percent in Washington, 74 percent in Maryland and Connecticut, and 91 percent in California (Dahl et al., 1991), with a concomitant loss of critical seagrass and marsh habitat that host a diversity of invertebrates and fish (including many economically important species), protect coastlines from erosion, enhance nutrient cycling, and improve water clarity (Fenchel, 1977).

There are thus significant reasons for concern. The timing is critical for



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