Department of Biological Sciences, The George Washington University, Washington, D.C., 20052, USA.
Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, 2751, Australia.
Ecology. 2019 Sep;100(9):e02790. doi: 10.1002/ecy.2790. Epub 2019 Jul 24.
Environmental forces and biotic interactions, both positive and negative, structure ecological communities, but their relative roles remain obscure despite strong theory. For instance, ecologically similar species, based on the principle of limiting similarity, are expected to be most competitive and show negative interactions. Specious communities that assemble along broad environmental gradients afford the most power to test theory, but the communities often are difficult to quantify. Microbes, specifically fungal endophytes of wood, are especially suited for testing community assembly theory because they are relatively easy to sample across a comprehensive range of environmental space with clear axes of variation. Moreover, endophytes mediate key forest carbon cycle processes, and although saprophytic fungi from dead wood typically compete, endophytic fungi in living wood may enhance success through cooperative symbioses. To classify interactions within endophyte communities, we analyzed fungal DNA barcode variation across 22 woody plant species growing in woodlands near Richmond, New South Wales, Australia. We estimated the response of endophytes to the measured wood environment (i.e., 11 anatomical and chemical wood traits) and each other using latent-variable models and identified recurrent communities across wood environments using model-based classification. We used this information to evaluate whether (1) co-occurrence patterns are consistent with strong competitive exclusion, and (2) a priori classifications by trophic mode and phylum distinguish taxa that are more likely to have positive vs. negative associations under the principle of limiting similarity. Fungal endophytes were diverse (mean = 140 taxa/sample), with differences in community composition structured by wood traits. Variation in wood water content and carbon concentration were associated with especially large community shifts. Surprisingly, after accounting for wood traits, fungal species were still more than three times more likely to have positive than negative co-occurrence patterns. That is, patterns consistent with strong competitive exclusion were rare, and positive interactions among fungal endophytes were more common than expected. Confirming the frequency of positive vs. negative interactions among fungal taxa requires experimental tests, and our findings establish clear paths for further study. Evidence to date intriguingly suggests that, across a wide range of wood traits, cooperation may outweigh combat for these fungi.
环境因素和生物相互作用（包括正相互作用和负相互作用）共同构建了生态群落，但尽管理论基础很强，它们的相对作用仍然不清楚。例如，根据限制相似性原则，生态上相似的物种预计会具有最强的竞争力，并表现出负相互作用。沿着广泛的环境梯度组装的似是而非的群落为检验理论提供了最大的能力，但这些群落往往难以量化。微生物，特别是木材中的真菌内生菌，特别适合用于检验群落组装理论，因为它们相对容易在具有明确变化轴的综合环境范围内进行采样。此外，内生菌介导着关键的森林碳循环过程，虽然来自枯木的腐生真菌通常具有竞争性，但活木中的内生真菌可能通过共生合作来提高成功率。为了在内生菌群落中分类相互作用，我们分析了澳大利亚新南威尔士州里士满附近林地的 22 种木本植物中真菌 DNA 条码的变异。我们使用潜在变量模型估计内生菌对所测木材环境（即 11 种木材解剖和化学特性）的反应，以及彼此之间的反应，并使用基于模型的分类法识别木材环境中的重复群落。我们利用这些信息来评估以下两种情况：(1) 共生模式是否符合强烈的竞争排斥；(2) 基于营养模式和门的先验分类是否能区分在限制相似性原则下更有可能产生正相互作用或负相互作用的分类群。真菌内生菌种类繁多（平均每个样本 140 个分类群），群落组成的差异受木材特性的影响。木材水分含量和碳浓度的变化与群落的巨大变化有关。令人惊讶的是，在考虑木材特性后，真菌物种的正共现模式出现的可能性仍然是负共现模式的三倍多。也就是说，与强烈竞争排斥一致的模式很少见，真菌内生菌之间的正相互作用比预期的更为常见。要证实真菌分类群之间的正相互作用与负相互作用的频率，需要进行实验测试，而我们的研究结果为进一步研究提供了明确的途径。迄今为止的证据表明，在广泛的木材特性范围内，合作可能比这些真菌之间的竞争更为普遍。
