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Biology

Sea Turtles, Sea Grasses, and Sharks

Heithaus MR, Alcoverro T, Arthur R, Burkholder DA, Coates KA, Christianen MJA, Kelkar N, Manuel SA, Wirsing AJ, Kenworthy WJ and Fourqurean JW (2014) Seagrasses in the age of sea turtle conservation and shark overfishing. Front. Mar. Sci. 1:28. doi: 10.3389/fmars.2014.00028

Background

Successful green sea turtle population recovery programs may be linked to sea grass populations in some ecosystems. For example, if sea turtle populations increase and are naturally maintained at historic levels, it could improve sea grass meadow health. Herbivorous sea turtles feeding on sea grasses helps ecosystem dynamics by reducing sea grass biomass and reducing sediment hypoxia.   Sea turtle populations should be controlled by predation by sharks, but their populations are decreasing due to overfishing. As a result, some recovery populations of sea turtles are exceeding historic levels and their excess grazing could threaten the survival of sea grass meadows. Dying sea grass meadows could seriously damage an ecosystem and reverse sea turtle conservation efforts.

Methods

Studies focused on successful sea turtle populations in Bermuda (Northwest Atlantic Ocean), Shark Bay (Western Australia, Eastern Indian Ocean), Derawan (Indonesia), and Lakshadweep (India, Central Indian Ocean).   In each study researchers compared sea grass grazing by sea turtles in a low risk predation area with sea grass grazing in a high risk predation area.

Results

Results varied spatially but there was an overall trend between over grazing and destroyed sea grass meadows. Over grazing was observed to impact density, heights, productivity, community composition, and persistence of sea grasses in the study areas.   Shark populations between the sites vary so it is difficult for researchers to determine their exact role in sea turtle density, however it was observed that in areas with low shark populations, like Bermuda and Derawan, sea turtles had a much greater role in declining sea grass meadows compared to areas where shark predation was high risk (Figure 1).

Illustration of potential ecosystem responses to loss of turtles (moving from the top to the left) or loss of large sharks with turtle conservation (moving from the top to the right).

Illustration of potential ecosystem responses to loss of turtles (moving from the top to the left) or loss of large sharks with turtle conservation (moving from the top to the right).

Importance

Sea grass health is important because sea grasses are key players in ecosystems, influencing water quality and erosion, and because they are a carbon sink. Sea grass populations are already threatened by poor coastal management, and as discussed in this paper, may also by threatened by trophic dynamics. Ecosystem management and top down forcing in populations are important to understand so that the fate of sea grasses may be monitored as the climate and oceans change

Discussion

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