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2nd
ASEAN Symposium and Workshop on
Sea Turtle Biology and Conservation
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SESSION
2: Nesting and Foraging Populations
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ABSTRACTS
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ROLE OF
THE GREEN TURTLE IN THE SEAGRASS ECOSYSTEM
Lemnuel V. Aragones
Department of Tropical Environment
Studies and Geography, James Cook University, Queensland
In order to examine the role of the
green turtle particularly the effects of herbivory, on the community structure,
productivity, and nutritional composition of seagrass, experiments simulating
intensive turtle cropping were carried out in intertidal seagrass beds
at Cardwell and Ellie Point on the northeast Queensland coast. Some aspects
of the interactions between green turtles (and dugongs) and tropical seagrasses
were examined. Cropping experiments at Cardwell and Ellie Point were monitored
monthly for a year before the seagrass samples were harvested. An additional
short-term experiment was also carried out at Cardwell only, wherein samples
from the cropping plots were harvested after four months. Video recording
was used for monitoring temporal changes in the species composition and
abundance in tropical seagrass communities. Near infra-red reflectance
spectroscopy (NIRS) was used to measure the concentrations of the following:
nitrogen, organic matter, neutral detergent fibre, acid detergent fibre,
lignin, water soluble carbohydrate, and starch and in vitro digestibility
of dry matter.
The nature and extent of the effects
of cropping was related to the nature of the seagrass community, including
its species composition and location. The species composition of a monospecific
bed of Halodule uninervis was not affected by cropping. Cropping increased
the net above-ground biomass productivity of H. ovalis and Halodule uninervis.
Recovery times varied for H. ovalis, Zostera/Cymodocea and H. uninervis.
In short-term experiments, cropping
increased the leaf nitrogen concentration of H. uninervis. The digestibility
(in vitro) of dry matter of H. uninervis moderately increased after cropping.
Cropping has variable effects on the fibre and lignin contents of H. uninervis
depending on the plant part, nature and intensity of herbivory and duration
of the recovery.
Green turtles (and dugongs) appear
to optimise their diet by selecting food species that maximise digestible
nutrients. This is achieved by selecting seagrass species that are more
digestible and have higher nutrients (e.g. nitrogen and carbohydrates/starch)
and/or species which can compensate for grazing. Grazing and cropping improved
the seagrass bed as grazing habitat for green turtles (and dugongs). |
