Comita Liza S, Queenborough Simon A, Murphy Stephen J, Eck Jenalle L, Xu Kaiyang, Krishnadas Meghna, Beckman Noelle, Zhu Yan, Gómez-Aparicio Lorena
Department of Evolution, Ecology and Organismal Biology, The Ohio State University Columbus, OH, 43210, USA ; Smithsonian Tropical Research Institute Box 0843-03092, Balboa, Ancón, Republic of Panama.
Department of Evolution, Ecology and Organismal Biology, The Ohio State University Columbus, OH, 43210, USA.
J Ecol. 2014 Jul;102(4):845-856. doi: 10.1111/1365-2745.12232. Epub 2014 Jun 23.
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.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.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.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.. 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.
简森-康奈尔假说提出，诸如食草动物和病原体等专一性天敌，通过降低靠近繁殖成年植株或同种植物高密度区域的同种种子和幼苗的存活率，来维持植物群落的多样性。据推测，距离依赖性和密度依赖性效应强度的变化可以解释植物物种丰富度沿气候梯度的变化，预计热带地区的效应比温带地区更强，湿润生境中的效应比干燥生境更强。我们进行了全面的文献检索，以确定自该假说首次提出以来40多年间发表的同行评审实验研究。利用这些研究的数据，我们进行了一项荟萃分析，以评估目前支持简森-康奈尔假说距离和密度预测的证据权重。
总体而言，我们发现距离依赖性和密度依赖性预测都得到了有力支持。综合所有研究来看，与远离同种植物相比，靠近同种植物时存活率显著降低，与同种植物低密度区域相比，在高密度区域存活率也显著降低。没有迹象表明这些结果是由发表偏倚导致的。
距离和密度效应的强度在不同研究中差异很大。与预期相反，这种差异与纬度无关，研究区域也没有显著影响。然而，我们确实发现，与年降水量较低的地点相比，在较湿润的地点距离和密度依赖性更强。此外，与种子阶段相比，在幼苗阶段效应显著更强。我们的研究支持了这样一种观点，即距离依赖性和密度依赖性死亡率在全球植物群落中都存在。现有证据表明，天敌常常是造成这种模式的原因，这与简森-康奈尔假说一致，但还需要更多研究来排除其他机制（例如种内竞争）。随着密度和距离依赖性的广泛存在得到明确证实，未来的研究应侧重于评估这些效应在多大程度上允许物种共存并有助于维持植物群落的多样性。
