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      Recruitment constraints in Singapore's fluted giant clam ( Tridacna squamosa) population—A dispersal model approach

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          Abstract

          Recruitment constraints on Singapore's dwindling fluted giant clam, Tridacna squamosa, population were studied by modelling fertilisation, larval transport, and settlement using real-time hydrodynamic forcing combined with knowledge of spawning characteristics, larval development, behaviour, and settlement cues. Larval transport was simulated using a finite-volume advection-diffusion model coupled to a three-dimensional hydrodynamic model. Three recruitment constraint hypotheses were tested: 1) there is limited connectivity between Singapore's reefs and other reefs in the region, 2) there is limited exchange within Singapore's Southern Islands, and 3) there exist low-density constraints to fertilisation efficacy (component Allee effects). Results showed that connectivity among giant clam populations was primarily determined by residual hydrodynamic flows and spawning time, with greatest chances of successful settlement occurring when spawning and subsequent larval dispersal coincided with the period of lowest residual flow. Simulations suggested poor larval transport from reefs located along the Peninsular Malaysia to Singapore, probably due to strong surface currents between the Andaman Sea and South China Sea combined with a major land barrier disrupting larval movement among reefs. The model, however, predicted offshore coral reefs to the southeast of Singapore (Bintan and Batam) may represent a significant source of larvae. Larval exchange within Singapore's Southern Islands varied substantially depending on the locations of source and sink reefs as well as spawning time; but all simulations resulted in low settler densities (2.1–68.6 settled individuals per 10,000 m 2). Poor fertilisation rates predicted by the model indicate that the low density and scattered distribution of the remaining T. squamosa in Singapore are likely to significantly inhibit any natural recovery of local stocks.

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          Connectivity of marine populations: open or closed?

          Most marine populations are thought to be well connected via long-distance dispersal of larval stages. Eulerian and Lagrangian flow models, coupled with linear mortality estimates, were used to examine this assumption. The findings show that when simple advection models are used, larval exchange rates may be overestimated; such simplistic models fail to account for a decrease of up to nine orders of magnitude in larval concentrations resulting from diffusion and mortality. The alternative process of larval retention near local populations is shown to exist and may be of great importance in the maintenance of marine population structure and management of coastal marine resources.
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            Consequences of the Allee effect for behaviour, ecology and conservation.

            Warder C. Allee brought attention to the possibility of a positive relationship between aspects of fitness and population size 50 years ago. Until recently, however, this concept was generally regarded as an intriguing but relatively unimportant aspect of population ecology. Increasing appreciation that Allee effects must be incorporated into models of population dynamics and habitat use, together with recent interest in the implications of sociality for conservation, have shown that for ecology and conservation the consequences of the Allee effect are profound. The Allee effect can be regarded not only as a suite of problems associated with rarity, but also as the basis of animal sociality.
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              Assessing minimum viable population size: Demography meets population genetics.

              The discussion of a population's minimum viable size provides a focus for the study of ecological and genetic factors that influence the persistence of a threatened population. There are many causes of extinction and the fate of a specific population cannot generally be predicted. This uncertainty has been dealt with in two ways: through stochastic demographic models to determine how to minimize extinction probabilities; and through population genetic theory to determine how best to maintain genetic variation, in the belief that the ability to evolve helps buffer a population against the unknown. Recent work suggests that these two very different approaches lead to very similar conclusions, at least under panmictic conditions. However, defining the ideal spatial distribution for an endangered species remains an important challenge. Copyright © 1993. Published by Elsevier Ltd.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2013
                13 March 2013
                27 March 2013
                : 8
                : 3
                : e58819
                Affiliations
                [1 ]Experimental Marine Ecology Laboratory, Department of Biological Sciences, National University of Singapore, Singapore, Singapore
                [2 ]Tropical Marine Science Institute, National University of Singapore, Singapore, Singapore
                [3 ]Sinclair Knight Merz (SKM), Perth, Australia
                [4 ]The UWA Oceans Institute, University of Western Australia, Crawley, Australia
                [5 ]Deltares, (formerly Delft Hydraulics), Delft, The Netherlands
                Institute of Marine Research, Norway
                Author notes
                * E-mail: dbspat@ 123456nus.edu.sg (Peter A. Todd)

                Competing Interests: The authors confirm that they have the following interests: co-author Paul Erftemeijer is employed by Sinclair Knight Merz (SKM). There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

                Conceived and designed the experiments: MLN PE JvB DvM PT. Performed the experiments: MLN PE JvB DvM. Analyzed the data: MLN PE JvB DvM. Contributed reagents/materials/analysis tools: PE JvB DvM. Wrote the paper: MLN PE JvB DvM PT ST.

                Article
                PONE-D-13-02237
                10.1371/journal.pone.0058819
                3596324
                23555597
                94661ad0-c1e1-4b21-be89-92596c6cf602
                Copyright @ 2013

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 12 January 2013
                : 7 February 2013
                Page count
                Pages: 12
                Funding
                The research presented in this work was carried out as part of the Building with Nature Singapore Supportive Modelling project (1201442.002). This study was also supported by the National Parks Board's Coastal & Marine Environment grant number R-154-000-504-490 and Singapore-Delft Water Alliance's Marine & Coastal Research Programme (Theme 2) grant number R-264-001-001-272. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology
                Ecology
                Autecology
                Behavioral Ecology
                Biodiversity
                Coastal Ecology
                Conservation Science
                Ecosystems
                Marine Ecology
                Marine Biology
                Coastal Ecology
                Marine Conservation
                Marine Ecology
                Zoology
                Malacology
                Computer Science
                Computer Modeling
                Computerized Simulations

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                Uncategorized

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