Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesis

1.  The stress gradient hypothesis (SGH) predicts a shift from net negative interactions in benign environments towards net positive in harsh environments in ecological communities. While several studies found support for the SGH, others found evidence against it, leading to a debate on how nature and strength of species interactions change along stress gradients, and to calls for new empirical and theoretical work.
2.  In the latest attempt in this journal, it is successfully argued how the SGH should be expanded by considering different life strategies of species (stress tolerance versus competitive ability) and characteristics of abiotic stress (resource versus non-resource based) over wider stress gradients (opposed to low–high contrasts), but the crucial role of biotic stress by consumers is largely ignored in this refinement.
3.  We point out that consumers strongly alter the outcome of species interactions in benign and harsh environments, and show how inclusion of consumer-incurred biotic stress alters the predicted outcome of interactions along resource- and non-resource-based stress gradients for stress-tolerant and competitive benefactors and beneficiaries.
4.   Synthesis. New studies should include stress gradients consisting of both abiotic and biotic components to disentangle their impacts, and to improve our understanding of how species interactions change along environmental gradients.     

Positive interactions by cushion plants in high mountains: fact or artifact?

Aims Positive interactions are defined as non-trophic interactions where at least one of the interacting species is benefited in terms of fitness and the other remains unaffected. Nevertheless, the bidirectional feedbacks between species may be positive, neutral or negative. Thus, if facilitated species induce negative effects on their 'nurses', the assumed definition of positive interactions could be reconsidered.Methods We assessed if ecological interactions between cushions of Azorella madreporica and their facilitated species are positive. Specifically, we tested if cover of facilitated species has any costs for cushion plants from an ecophysiological perspective, and if these costs increase with the amount of cover of facilitated species. In addition, through pathway analysis and correlations, we assessed if cover and richness of facilitated species have a direct and/or indirect effect on the fitness of cushion plants.Important findings We found that facilitated plant species induced a significant cost for their nurses (cushion plants), and this cost increases with cover of the facilitated species. Additionally, the facilitated species exert a strong direct negative effect on the cushion's fitness and a moderate indirect negative cost evident through the nutrient status and physiological performance of cushion plants. We thus contribute evidence that positive interactions between high mountain cushion plants of central Chile and their 'facilitated' species may be an artifact more than a fact, especially when bidirectional effects are considered; contrasting with the majority of studies that document only one side of the interaction.     

遮阴对两种泥炭藓植物生长及相互作用的影响

以大泥炭藓和喙叶泥炭藓为材料,研究遮阴对其生长及植物相互作用的影响.结果表明:在单种群中,遮阴处理明显促进了大泥炭藓的高生长,但对喙叶泥炭藓的生长以及大泥炭藓生物量和分枝数未产生影响;在混合群中,喙叶泥炭藓抑制了大泥炭藓生物量和分枝数的增长,而大泥炭藓对喙叶泥炭藓的生长无影响.随遮阴胁迫的增加,邻体对喙叶泥炭藓竞争加剧,当胁迫进一步增强,邻体效应有转变为正相互作用的趋势,但邻体对大泥炭藓的效应始终为竞争,未随胁迫增加而变化.     

水淹对克隆植物空心莲子草种内关系的影响

为检验“水淹程度可以改变植物种内关系的类型和强度”的假说, 将克隆植物空心莲子草(Alternanthera philoxeroides)的3种不同密度的植株(每盆种植1、4或16株)置于4种不同的水淹处理下(水位分别为-20 (不水淹)、0、20或40 cm), 研究不同水淹程度对空心莲子草种内关系的影响。随着植株密度和水淹程度的增加, 空心莲子草的生长显著减慢, 但密度效应在不同的水淹处理下显著不同。在不发生水淹的情况下, 植株密度对生长的负面(竞争)效应最强; 在水位为0和20 cm的情况下, 植株密度对生长的效应仍为负面的, 但影响强度相对减小; 而在水位为40 cm的情况下, 空心莲子草植株的生物量随着植株密度的增大而倾向于增加。进一步分析相对邻体效应时发现, 随着水淹程度的增加, 相对邻体效应显著增加, 并且数值从负值(不水淹)逐渐变为正值(40 cm水位下)。这些结果支持胁迫梯度假说, 表明水淹可以影响植物的种内关系, 即随着水淹程度的增加, 植物种内竞争作用减弱, 而易化作用增强。     

气候变化和人类活动对中国北方旱区植被变绿的定量贡献

气候变化和大规模的生态恢复使中国北方旱区植被发生了显著变化,量化气候变化和人类活动对植被动态的相对贡献,对于旱区生态系统管理和应对未来气候变化具有重要意义。目前,中国北方旱区植被变化影响因素的时间动态（2000年大规模生态恢复工程实施前后）和空间异质性（沿干旱梯度）仍需进一步的定量研究。基于多源数据,采用趋势分析、偏相关分析和随机森林模型等方法,分析了1981-2018年中国北方旱区气候和植被的时空变化规律,量化了2000年前后气候变化和人类活动对植被动态的相对贡献并分析其在干旱梯度上的空间差异性。结果表明：（1）1981-2018年期间,中国北方旱区的叶面积指数（LAI）平均增加速率为（0.0037±0.0443） a^-1,且增加速率沿干旱梯度增大。2000年前仅10.46%（P<0.05）的地区显著变绿,而2000年后达到36.84%,且植被变绿主要归因于非树木植被。（2）2000年后降水对植被变绿的正效应在不同干旱梯度均增加,而在半干旱区和亚湿润干旱区,温度对植被变绿由正向促进转为负向抑制,而辐射在干旱区由负效应转向正效应。（3）2000年前后,气候变化均主导着植被的动态,贡献率分别为96.07%和73.72%,人类活动的贡献在2000年后进一步增强（从3.93%增加到26.28%）,且沿着干旱梯度而增加,其中人类活动对植被变绿的贡献在半干旱地区增加最显著（+0.0289 m² m^-2 a^-1,P<0.05）。研究结果可为未来气候变化下中国北方旱区的植被恢复和可持续发展提供科学依据。     

Testing the facilitation-competition paradigm under the stress-gradient hypothesis: decoupling multiple stress factors

While the facilitation-competition paradigm under the stress-gradient hypothesis has received recent attention, its rigorous testing is yet to be explored. Most of the studies have considered a switch in the net interactions from competition to facilitation with increasing environmental stress as primary evidence supporting the hypothesis, though few studies examined changes in interaction along a full range of a stress gradient. Here, we have conceptualized possible variations in the patterns of change in interaction strength along such gradient. Based on this, we empirically evaluated the temporal shift in the interaction between two marine sessile animals, goose barnacles (Capitulum mitella) and mussels (Septifer virgatus), under multiple stress factors. The net effect of goose barnacles on mussel survivorship was positively related to the total stress gradient encompassing two stress factors, physical disturbance and thermal stress, while no negative value occurred even under mild conditions. When the two stress factors were treated separately, however, the net effect demonstrated apparently different patterns: monotonic increase with physical disturbance versus a quasi-asymptotic pattern (no change over a wide range) with thermal stress. These variable situations have not previously been recognized in this discipline, and the present study emphasizes the importance of an integrative and mechanistic approach to testing and deciphering the facilitation-competition paradigm.