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      Spurious and functional correlates of the isotopic composition of a generalist across a tropical rainforest landscape

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          Abstract

          Background

          The isotopic composition of generalist consumers may be expected to vary in space as a consequence of spatial heterogeneity in isotope ratios, the abundance of resources, and competition. We aim to account for the spatial variation in the carbon and nitrogen isotopic composition of a generalized predatory species across a 500 ha. tropical rain forest landscape. We test competing models to account for relative influence of resources and competitors to the carbon and nitrogen isotopic enrichment of gypsy ants ( Aphaenogaster araneoides), taking into account site-specific differences in baseline isotope ratios.

          Results

          We found that 75% of the variance in the fraction of 15N in the tissue of A. araneoides was accounted by one environmental parameter, the concentration of soil phosphorus. After taking into account landscape-scale variation in baseline resources, the most parsimonious model indicated that colony growth and leaf litter biomass accounted for nearly all of the variance in the δ 15N discrimination factor, whereas the δ 13C discrimination factor was most parsimoniously associated with colony size and the rate of leaf litter decomposition. There was no indication that competitor density or diversity accounted for spatial differences in the isotopic composition of gypsy ants.

          Conclusion

          Across a 500 ha. landscape, soil phosphorus accounted for spatial variation in baseline nitrogen isotope ratios. The δ 15N discrimination factor of a higher order consumer in this food web was structured by bottom-up influences - the quantity and decomposition rate of leaf litter. Stable isotope studies on the trophic biology of consumers may benefit from explicit spatial design to account for edaphic properties that alter the baseline at fine spatial grains.

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          Most cited references24

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          Isotopic ecology ten years after a call for more laboratory experiments.

          About 10 years ago, reviews of the use of stable isotopes in animal ecology predicted explosive growth in this field and called for laboratory experiments to provide a mechanistic foundation to this growth. They identified four major areas of inquiry: (1) the dynamics of isotopic incorporation, (2) mixing models, (3) the problem of routing, and (4) trophic discrimination factors. Because these areas remain central to isotopic ecology, we use them as organising foci to review the experimental results that isotopic ecologists have collected in the intervening 10 years since the call for laboratory experiments. We also review the models that have been built to explain and organise experimental results in these areas.
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            Explaining the abundance of ants in lowland tropical rainforest canopies.

            The extraordinary abundance of ants in tropical rainforest canopies has led to speculation that numerous arboreal ant taxa feed principally as "herbivores" of plant and insect exudates. Based on nitrogen (N) isotope ratios of plants, known herbivores, arthropod predators, and ants from Amazonia and Borneo, we find that many arboreal ant species obtain little N through predation and scavenging. Microsymbionts of ants and their hemipteran trophobionts might play key roles in the nutrition of taxa specializing on N-poor exudates. For plants, the combined costs of biotic defenses and herbivory by ants and tended Hemiptera are substantial, and forest losses to insect herbivores vastly exceed current estimates.
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              Landscape-scale variation in forest structure and biomass in a tropical rain forest

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                Author and article information

                Journal
                BMC Ecol
                BMC Ecology
                BioMed Central
                1472-6785
                2009
                24 November 2009
                : 9
                : 23
                Affiliations
                [1 ]Department of Biology, California State University Dominguez Hills,1000 E Victoria St Carson, CA 90747, USA
                [2 ]Department of Biology, Occidental College, Los Angeles, CA, USA
                [3 ]East Valley High School, Los Angeles Unified School District, North Hollywood, CA, USA
                Article
                1472-6785-9-23
                10.1186/1472-6785-9-23
                2788522
                19930701
                28c5675f-bca2-47f6-98fb-91b9eaa867df
                Copyright ©2009 McGlynn et al; licensee BioMed Central Ltd.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 29 July 2009
                : 24 November 2009
                Categories
                Research article

                Ecology
                Ecology

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