Field manipulations of resources mediate the transition from intraspecific competition to facilitation

1. Population density affects individual performance, though its effects are often mixed. For sessile species, increases in population density typically reduce performance. Still, cases of positive density dependence do occur in these systems and demand explanation. The stress gradient hypothesis (SGH) predicts that under stressful conditions, positive effects of facilitation may outweigh the negative effects of competition. 2. While some elements of the SGH are well studied, its potential to explain intraspecific facilitation has received little attention. Further, there have been questions regarding whether the SGH holds if the stressor is a resource. Most studies of interactions between the environment and intraspecific facilitation have relied on natural environmental gradients; manipulative studies are much rarer. 3. To test the effects of intraspecific density and resources, we manipulated resource availability over natural population densities for the marine bryozoan Watersipora subtorquata. 4. We found negative effects of density on colony performance in low resource environments, but mainly positive density-dependence in high resource environments. By adding resources, competition effects were reduced and the positive effects of facilitation were revealed. 5. Our results suggest that resource availability mediates the relative strength of competition and facilitation in our system. We also suggest that intraspecific facilitation is more common than may be appreciated and that environmental variation may act as mediators to the balance between negative and positive density-dependence.

1. 种群密度会对个体表现产生影响，但其效应往往兼具复杂性与两面性。对于固着生物（sessile species）而言，种群密度升高通常会降低个体表现，但该类群中仍存在密度正效应的案例，亟待针对性解释。胁迫梯度假说（Stress Gradient Hypothesis, SGH）预测，在胁迫环境下，生物间促进作用的正效应可能超过竞争带来的负效应。
2. 尽管胁迫梯度假说的部分核心内容已得到充分研究，但其用于解释种内促进作用（intraspecific facilitation）的潜力却鲜少受到学界关注。此外，学界仍存在争议：当胁迫因子为资源类物质时，胁迫梯度假说是否依然成立？目前，多数探究环境与种内促进作用交互效应的研究均依托自然环境梯度开展，操控性实验则相对稀缺。
3. 为验证种群密度与资源可获得性的效应，我们针对海洋苔藓虫（marine bryozoan）Watersipora subtorquata，在其自然种群密度梯度下开展了资源可获得性的操控实验。
4. 研究结果显示，在低资源环境中，种群密度对苔藓虫群体表现存在显著负效应；而在高资源环境中，则主要呈现密度正相关效应。通过人为补充资源，竞争效应得到抑制，促进作用的正效应得以显现。
5. 本研究结果表明，资源可获得性可调控本研究体系中竞争与促进作用的相对强度。我们同时提出，种内促进作用的普遍性可能高于此前认知，且环境变异可作为介导正负密度依赖效应平衡的关键因子。