Data from: Evolution of resource use along a gradient of stress leads to increased facilitation

The stress-gradient hypothesis (SGH) posits that the relative importance of facilitative interactions versus negative interactions increases as levels of abiotic stress increase. Originally formulated in empirical studies of plant populations, in recent years the SGH has been found to describe how interactions change in response to stress in a wide range of species including algae, mussels and moths. However, there has been little theory attempting to predict patterns from first principles in relation to different types of interactions. Here, we use mathematical models of microbial populations to investigate whether patterns consistent with the SGH arise when species interact through resource use and allelopathy. Evolution alters the degree to which competition for resource use versus facilitation (cross-feeding) occurs. Our results are consistent with the SGH; species interactions evolve to be more facilitative as average stress intensifies. This occurs because at greater stress the species evolve to become specialists on either of the two resources thereby decreasing overlap in resource use and increasing facilitation through cross-feeding. In addition, the production of toxic allelopathic compounds decreases as stress intensifies due to density-dependent effects. Our results suggest that the SGH could arise through fundamental interactions that are common to many organisms and therefore that the SGH could be a more widespread phenomenon than previously recognised.胁迫梯度假说 (stress-gradient hypothesis, SGH) 提出，随着非生物胁迫水平升高，种间促进作用相较于负相互作用的相对重要性会逐步提升。该假说最初由植物种群的实证研究提出，近年来已有研究表明，其可用于描述包括藻类、贻贝与蛾类在内的诸多物种中，种间相互作用随胁迫水平变化的规律。然而，目前鲜有理论能够基于第一性原理，针对不同类型的种间相互作用预测其变化模式。本研究借助微生物种群的数学模型，探究当物种通过资源利用与化感作用 (allelopathy) 产生相互作用时，是否会出现符合胁迫梯度假说的变化模式。进化过程会调控资源利用竞争与促进作用（即交叉摄食 (cross-feeding)）的相对强度。本研究结果与胁迫梯度假说一致：随着平均胁迫水平加剧，物种间的相互作用会朝着更强的促进方向演化。这一现象的成因在于，当胁迫水平升高时，物种会演化成为两种资源中某一类的特化利用者，从而降低资源利用的重叠度，并通过交叉摄食增强种间促进作用。此外，受密度依赖效应影响，有毒化感化合物的合成量会随胁迫水平加剧而降低。本研究结果表明，胁迫梯度假说可源于众多生物共有的基础相互作用，因此该假说的适用范围可能比此前认知的更为广泛。