Data from: Compounded perturbations in coastal areas: contrasting responses to nutrient enrichment and the regime of storm-related disturbance depend on life-history traits

1.Natural systems are exposed to compounded perturbations, whose changes in temporal variance can be as important as those in mean intensity for shaping the structure of assemblages. Specifically, climate-related physical disturbances and nutrient inputs due to natural and/or anthropogenic activities occur concomitantly, but experimental tests of the simultaneous effects of changes in the regime of more than one perturbation are generally lacking. Filling this gap is key to understand ecological responses of natural assemblages to climate-related change in the intensity and temporal patterning of physical disturbance combined with other global stressors.

2.Responses to factorial manipulations of nutrient enrichment, mean intensity and temporal variability of storm-like mechanical disturbance were examined, using benthic assemblages of tide-pools as model system.

3.Response variables were mean abundance values and temporal variances of taxa with different life-traits. Consistent negative effects of disturbance intensity were observed for the mean cover of long-living taxa (algal canopies and the polychaete Sabellaria alveolata), whose temporal fluctuations were also reduced by more severe mechanical stress. More resilient taxa (ephemeral algae, mostly green of the genus Ulva) increased under enriched conditions, particularly when low intensity events were irregularly applied over time. Opposite effects of disturbance intensity depending on nutrient availability occurred on filamentous algae (e.g. red of the genus Ceramium). This was probably due to the fact that, although nutrient enrichment stimulated the abundance of both algal groups, when this condition was combined with relatively mild physical disturbance the competitively superior ephemeral green algae tended to become dominant over filamentous red algae. The same did not occur under high intensity of disturbance since it likely damaged large, foliose fronds of Ulva-like forms more than small, filamentous fronds of Ceramium-like forms. Grazers were positively affected by nutrients, likely responding indirectly to more food available.

4.A direct relationship between the mean abundance of most organisms and their temporal fluctuations was documented. However, all organisms persisted throughout the study, even under experimental conditions associated to the largest temporal variation of their abundance, likely due to their ability to resist to/quickly recover from, the applied perturbations. Therefore, in systems with great recovery abilities of dominant organisms (e.g. rocky intertidal, grasslands), effects of traits of the regime of disturbance and nutrient enrichment may modulate the fluctuations of populations not through the elimination and substitution of species, but through changes in relative abundances of the same species. This contrasts with the theory that temporal variation in abundance would be directly related to the risk of local extinction. Present findings enable more accurate predictions of the consequences of climatic and non-climatic scenarios on the biodiversity of marine and terrestrial systems sharing analogous functional traits of organisms. Future more intense physical disturbances are expected to exert negative effects on slow growing/recovering species (e.g. habitat-formers) irrespectively of the temporal patterning of the same disturbances and nutrient inputs. On the contrary, more resistant species (e.g. encrusting algae on rocky shores or below-ground vegetation in grasslands) are expected to benefit from intense physical disturbance. Species whose abundance is more directly related to the availability of nutrients (e.g. filamentous and ephemeral algae or herbs) are expected to generally increase under enriched conditions, but their ability to eventually become dominant would depend on their ability to grow fast and attain cover large enough to overwhelm any possible control of concomitant disturbance intensity on their abundance. If, such as in the present examined system, virtually all organisms can persist, over the temporal scale of the experiment, under any combination of physical disturbance and nutrient availability, the resulting overall diversity is not predicted to change drastically. Nevertheless, low intensity events evenly distributed and high intensity events irregularly distributed appear as the conditions supporting the highest richness of taxa, independently of the availability of nutrients.

1. 自然生态系统常遭受复合扰动，其时间方差的变化对于群落（assemblages）结构塑造的重要性，可与平均强度的变化相媲美。具体而言，气候相关的物理扰动，以及自然或人为活动带来的营养盐输入往往同时发生，但目前针对多种扰动格局变化的联合效应的实验检验普遍缺失。填补这一研究空白，对于理解自然群落在物理扰动的强度、时间模式结合其他全球胁迫因子的情况下，对气候相关变化的生态响应至关重要。
2. 本研究以潮池（tide-pools）底栖群落（benthic assemblages）为模式系统，探究了营养盐富集、风暴式机械扰动的平均强度与时间变异性的多因素操控实验对生物响应的影响。
3. 响应变量为不同生活史特征（life-traits）类群的平均丰度值与时间方差。研究发现，扰动强度对长寿类群（藻类冠层（algal canopies）以及多毛类（polychaete）*Sabellaria alveolata*）的平均盖度具有一致的负向效应，且更强的机械胁迫会降低这类群的时间波动幅度。恢复力较强的类群（短命藻类，主要为石莼属（Ulva）绿藻）在营养盐富集条件下丰度上升，尤其当低强度扰动事件呈不规则时间分布时更为显著。对于丝状藻类（例如杉藻属（Ceramium）红藻），扰动强度的效应会因营养盐可获得性的不同而呈现相反趋势。这一现象的潜在原因在于：尽管营养盐富集会促进两类藻类的丰度，但当该条件与相对温和的物理扰动结合时，竞争优势更强的短命绿藻往往会取代丝状红藻成为优势类群；而在高强度扰动条件下则不会出现这一情况，因为高强度机械胁迫对石莼类群大型叶状藻体的损伤程度，远大于杉藻类群小型丝状藻体。植食者（grazers）的丰度会因营养盐供给而正向增加，这可能是其间接响应于可获得食物增多的结果。
4. 本研究证实，多数生物的平均丰度与其时间波动幅度之间存在直接关联。然而，所有受试生物在整个实验期间均得以存续，即便在其丰度波动幅度最大的实验处理组中，这大概率得益于它们对施加的扰动的抵抗能力或快速恢复能力。因此，在优势类群具备强恢复能力的生态系统（例如潮间带岩滩、草原）中，扰动格局与营养盐富集的效应并非通过物种的消除与替代来调控种群波动，而是通过同一物种的相对丰度变化实现。这与“丰度的时间变化与局域灭绝风险直接相关”的理论相悖。本研究结果可更为精准地预测气候与非气候情景下，具备相似功能性状（functional traits）的海洋与陆地生态系统的生物多样性变化。未来更强的物理扰动，无论其时间分布模式与营养盐输入情况如何，都将对生长/恢复缓慢的物种（例如生境构建者（habitat-formers））产生负向影响。与之相反，抗性更强的物种（例如岩岸的壳状藻类（encrusting algae）或草原的地下植被）则会从高强度物理扰动中获益。丰度与营养盐可获得性关联更为直接的类群（例如丝状藻类、短命藻类或草本植物），在营养盐富集条件下丰度通常会上升，但其最终能否成为优势类群，取决于它们能否快速生长并达到足够的盖度，以抵消伴随扰动强度对其丰度的潜在调控作用。若如本研究的实验系统一般，几乎所有生物在实验的时间尺度内，均可在任意物理扰动与营养盐供给的组合条件下存续，那么最终的群落整体多样性预计不会发生剧烈变化。不过，无论营养盐供给情况如何，均匀分布的低强度扰动与不规则分布的高强度扰动，均是维持最高类群丰富度的实验条件。