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      Testing predictions of the Janzen–Connell hypothesis: a meta-analysis of experimental evidence for distance- and density-dependent seed and seedling survival


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          1. The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.

          2. We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.

          3. Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.

          4. The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.

          5. Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.

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          Most cited references54

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          Spatial patterns of seed dispersal, their determinants and consequences for recruitment.

          Growing interest in spatial ecology is promoting new approaches to the study of seed dispersal, one of the key processes determining the spatial structure of plant populations. Seed-dispersion patterns vary among plant species, populations and individuals, at different distances from parents, different microsites and different times. Recent field studies have made progress in elucidating the mechanisms behind these patterns and the implications of these patterns for recruitment success. Together with the development and refinement of mathematical models, this promises a deeper, more mechanistic understanding of dispersal processes and their consequences.
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            Plant diversity in tropical forests: a review of mechanisms of species coexistence

            Evidence concerning mechanisms hypothesized to explain species coexistence in hyper-diverse communities is reviewed for tropical forest plants. Three hypotheses receive strong support. Niche differences are evident from non-random spatial distributions along micro-topographic gradients and from a survivorship-growth tradeoff during regeneration. Host-specific pests reduce recruitment near reproductive adults (the Janzen-Connell effect), and, negative density dependence occurs over larger spatial scales among the more abundant species and may regulate their populations. A fourth hypothesis, that suppressed understory plants rarely come into competition with one another, has not been considered before and has profound implications for species coexistence. These hypotheses are mutually compatible. Infrequent competition among suppressed understory plants, niche differences, and Janzen-Connell effects may facilitate the coexistence of the many rare plant species found in tropical forests while negative density dependence regulates the few most successful and abundant species.
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              Is There a Latitudinal Gradient in the Importance of Biotic Interactions?


                Author and article information

                J Ecol
                J. Ecol
                The Journal of Ecology
                BlackWell Publishing Ltd (Oxford, UK )
                July 2014
                23 June 2014
                : 102
                : 4
                : 845-856
                [1 ]Department of Evolution, Ecology and Organismal Biology, The Ohio State University Columbus, OH, 43210, USA
                [2 ]Smithsonian Tropical Research Institute Box 0843-03092, Balboa, Ancón, Republic of Panama
                [3 ]Mathematical Biosciences Institute, The Ohio State University Columbus, OH, 43210, USA
                [4 ]State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences 20 Nanxincun, Xiangshan, Beijing, 100093, China
                Author notes
                *Correspondence author. School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA. E-mail: liza.comita@ 123456yale.edu
                © 2014 The Authors. Journal of Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.

                This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

                : 15 October 2013
                : 18 February 2014
                Special Feature: Standard Paper Meta-Analysis in Plant Ecology

                determinants of plant community diversity and structure,herbivory,maintenance of diversity,natural enemies,pathogens,plant population and community dynamics,review,seed predation,species coexistence,tropical forest


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