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Dugong grazing and turtle cropping: grazing optimization in tropical seagrass systems?

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Abstract

Grazing by dugongs and cropping by green turtles have the capacity to alter the subsequent nutritional quality of seagrass regrowth. We examined the effects of simulated light and intensive grazing by dugongs and cropping by turtles on eight nutritionally relevant measures of seagrass chemical composition over two regrowth periods (short-term, 1–4 months; long-term, 11–13 months) at two seagrass communities (a mixed species community with Zostera capricorni, Halophila ovalis, Halodule uninervis, Cymodocea rotundata and C. serrulate; and a monospecific bed of Halodule uninervis) in tropical Queensland, Australia. The concentrations of organic matter, total nitrogen, total water-soluble carbohydrates, total starch, neutral detergent fiber, acid detergent fiber, acid lignin, as well as the in vitro dry matter digestibility (IVDMD) were measured in the leaves and below-ground parts of each species using near-infrared reflectance spectroscopy (NIRS). Regrowth of preferred species such as H. ovalis and H. uninervis from simulated intensive dugong grazing after a year exhibited increased (by 35 and 25%, respectively, relative to controls) whole-plant N concentrations. Similarly, regrowth of H. ovalis from simulated turtle cropping showed an increase in the leaf N concentration of 30% after a year. However, these gains are tempered by reductions in starch concentrations and increases in fiber. In the short-term, the N concentrations increased while the fiber concentrations decreased. These data provide experimental support for a grazing optimization view of herbivory in the tropical seagrass system, but with feedback in a different manner. Furthermore, we suggest that in areas where grazing is the only major source of natural disturbance, it is likely that there are potential ecosystem level effects if and when numbers of dugongs and turtles are reduced.

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Acknowledgments

Professor Ross Cunningham of the Statistical Consulting Unit, Australian National University, conducted the statistical analyses. Michelle Larsen and Deanne Haffner ran some of the chemical analyses. Nils Berding of BSES (Meringa Station) allowed access to their NIRS instrument, AUSAID provided Aragones’ PhD scholarship and the Great Barrier Reef Marine Park Authority and James Cook University provided research funds.

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Author notes

  1. Lemnuel V. Aragones

    Present address: Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy, Miami, FL, 33149, USA

  2. William J. Foley

    Present address: School of Botany and Zoology, Australian National University, Canberra, 0200, Australia

Authors and Affiliations

  1. Department of Tropical Environment Studies and Geography, James Cook University, Townsville, QLD, 4811, Australia

    Lemnuel V. Aragones, Ivan R. Lawler & Helene Marsh

  2. Zoology Department, James Cook University, Townsville, QLD, 4811, Australia

    William J. Foley

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  1. Lemnuel V. Aragones
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Correspondence to Ivan R. Lawler.

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Communicated by Marilyn Ball

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Aragones, L.V., Lawler, I.R., Foley, W.J. et al. Dugong grazing and turtle cropping: grazing optimization in tropical seagrass systems?. Oecologia 149, 635–647 (2006). https://doi.org/10.1007/s00442-006-0477-1

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