Data from: Facilitation within species: a possible origin of group-selected superorganisms

Facilitation—positive interactions—has emerged as a dominant ecological mechanism in many ecosystems. Its importance has recently been expanded to include intraspecific interactions, creating the potential for higher level natural selection within species. Using multiple lines of evidence we show that conspecific facilitation within the southern beech tree, Nothofagus pumilio, appears to overcome competition in two life phases. In a seedling experiment addressing stress and planting density effects, we found that mortality was lowest (~0%) where there was no stress and was indistinguishable across densities. Further, in mature forests (45 years), genetically variable, merged individuals had lower mortality (-50%) compared to unmerged individuals in locations without identifiable stress. Thus, a full understanding of the occurrence of facilitation may require a more general model of resource improvements than the commonly cited Stress Gradient Hypothesis. Additionally, the merged trees showed a density dependent mortality pattern at the level of the groups. These data demonstrate a potential mechanism (facilitation) driving natural selection at this higher level, via stem merging. These merged "superorganisms" would confirm theoretical predictions whereby facilitation acts as an ecological mechanism driving group selection.

促进作用（facilitation）——即正相互作用——现已成为诸多生态系统中居于主导地位的生态学机制。近年来其重要性范畴已拓展至种内相互作用，为物种内更高层级的自然选择提供了可能性。本研究通过多维度证据证实，智利南山毛榉（Nothofagus pumilio）种内的促进作用，似乎可在两个生活史阶段克服竞争压力。在一项针对胁迫与种植密度效应的幼苗实验中，我们发现无胁迫环境下幼苗死亡率最低（约0%），且不同密度组间死亡率无显著差异。此外，在林龄45年的成熟林样地中，无明显胁迫环境下，遗传多样的融合个体死亡率较未融合个体低50%。因此，若要全面理解促进作用的发生机制，或许需要一套比当前广泛引用的胁迫梯度假说（Stress Gradient Hypothesis）更为普适的资源改良模型。此外，融合植株在群体水平上呈现出密度依赖的死亡率格局。上述数据表明，通过茎干融合，促进作用这一潜在机制可推动该层级的自然选择。这类融合形成的“超个体（superorganisms）”将验证相关理论预测：促进作用作为一种生态学机制，可驱动群体选择。