Where inter-specific facilitation favors the establishment of high densities of a beneficiary species, strong intra-specific competition may subsequently impede beneficiary performance. Consequently, the negative influence of intra-specific competition between beneficiary individuals could potentially outweigh the positive influence of inter-specific facilitation when, for example, higher densities of a beneficiary are negated by the negative effect of crowding on beneficiary reproduction. The aim of this study was, therefore, to examine the impact of an inter-specific interaction on the outcome of intra-specific interactions within the context of plant-plant facilitation. We used the cushion-forming Azorella selago and a commonly co-occurring dominant perennial grass species, Agrostis magellanica, on sub-Antarctic Marion Island as a model system. We assessed the impact of an inter-specific interaction (between A. selago and A. magellanica) on the outcome of intra-specific interactions (between A. magellanica individuals), by testing if the impact of A. magellanica density on A. magellanica performance is mediated by its interaction with A. selago. We observed evidence for competition among A. magellanica conspecifics, with a decreasing proportion of A. magellanica individuals being reproductive under higher conspecific density. This negative intra-specific effect was greater on A. selago than on the adjacent substrate, suggesting that the facilitative effect of A. selago changes the intensity of intra-specific interactions between A. magellanica individuals. However, experimentally reducing A. magellanica density did not affect the species' performance. We also observed that the effect of A. selago on A. magellanica was positive, and despite the negative effect of intra-specific density on the proportion of reproductive A. magellanica individuals, the net reproductive effort of A. magellanica (i.e. the density of reproductive individuals) was significantly greater on A. selago than on the adjacent substrate. These results highlight that, in abiotically-severe environments, the positive effects of inter-specific facilitation by a benefactor species may outweigh the negative effects of intra-specific competition among beneficiaries. More broadly, these results suggest that both positive inter- and intra-specific biotic interactions may be key to consider when examining spatial and temporal variation in species' performance.

中文翻译：

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种间促进介导跨海拔梯度的种内相互作用的结果

在种间促进有利于建立高密度受益物种的情况下，强烈的种内竞争可能随后会阻碍受益物种的表现。因此，受益人之间种内竞争的负面影响可能超过种间促进的积极影响，例如，当受益人的较高密度被拥挤对受益人繁殖的负面影响所抵消时。因此，本研究的目的是检查种间相互作用对植物 - 植物促进背景下种内相互作用结果的影响。我们使用形成垫层的 Azorella selago 和亚南极马里恩岛上常见的优势多年生草种 Agrostis magellanica 作为模型系统。我们通过测试 A. magellanica 密度对 A. magellanica 的影响，评估了种间相互作用（A. selago 和 A. magellanica 之间）对种内相互作用（A. magellanica 个体之间）结果的影响性能由其与 A. selago 的相互作用介导。我们观察到 A. magellanica 同种之间竞争的证据，在更高的同种密度下繁殖的 A. magellanica 个体比例下降。这种负面的种内效应对 A. selago 比对相邻基质更大，表明 A. selago 的促进作用改变了 A. magellanica 个体之间种内相互作用的强度。然而，通过实验降低 A. magellanica 密度并没有影响该物种的性能。我们还观察到 A. selago 对 A. magellanica 的影响是积极的，尽管种内密度对繁殖 A. magellanica 个体的比例有负面影响，但 A. magellanica 的净繁殖努力（即密度繁殖个体）在 A. selago 上显着大于在相邻基质上。这些结果强调，在非生物严酷的环境中，受益物种种间促进的积极影响可能超过受益人种内竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。尽管种内密度对繁殖 A. magellanica 个体的比例有负面影响，但 A. magellanica 的净繁殖努力（即繁殖个体的密度）在 A. selago 上显着大于在相邻基质上。这些结果强调，在非生物严酷的环境中，受益物种种间促进的积极影响可能超过受益人种内竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。尽管种内密度对 A. magellanica 繁殖个体的比例有负面影响，但 A. magellanica 的净繁殖努力（即繁殖个体的密度）在 A. selago 上显着大于在相邻基质上。这些结果强调，在非生物严酷的环境中，受益物种种间促进的积极影响可能超过受益人种内竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。繁殖个体的密度）在 A. selago 上显着大于在相邻基质上。这些结果强调，在非生物严酷的环境中，受益物种种间促进的积极影响可能超过受益人种内竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。繁殖个体的密度）在 A. selago 上显着大于在相邻基质上。这些结果强调，在非生物严酷的环境中，受益物种种间促进的积极影响可能超过受益人种内竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。受益人种间种间促进的积极影响可能超过受益人种间竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。受益人种间种间促进的积极影响可能超过受益人种间竞争的负面影响。更广泛地说，这些结果表明，在检查物种表现的空间和时间变化时，积极的种间和种内生物相互作用可能是考虑的关键。