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. 自然生态系统会暴露于复合扰动（compounded perturbations）之中，扰动的时间变异特征与平均强度特征，二者在塑造群落结构的过程中具有同等重要的作用。具体而言，与气候相关的物理扰动，以及由自然和/或人为活动引发的养分输入，往往同时发生；但目前鲜有实验同时检验多种扰动的动态变化所产生的联合效应。填补这一研究空白，是理解自然群落在物理扰动的强度、时间格局与其他全球胁迫因子共同作用下，对气候相关变化产生的生态响应的关键所在。
2. 本研究以潮池（tide-pools）底栖群落（benthic assemblages）为模式系统，开展了养分富集、风暴类机械扰动的平均强度与时间变异性的因子设计操控实验，以探究群落对各处理的响应。
3. 本研究的响应变量为不同生活史特征类群（taxa）的平均丰度值与时间方差。结果显示，扰动强度对长寿命类群的平均盖度产生了一致的负向影响，这类类群包括藻类冠层（algal canopies）以及多毛类（polychaete）物种蜂巢沙蚕（Sabellaria alveolata），且更强的机械胁迫会进一步降低其种群的时间波动幅度。对养分富集条件响应更具弹性的类群，主要为短命藻类（ephemeral algae，以石莼属Ulva绿藻为主），其丰度在养分富集环境中会有所提升，尤其当低强度扰动事件随机发生时优势更为显著。对于丝状藻类（如仙菜属Ceramium红藻）而言，扰动强度的效应会随养分可利用性发生反转。这一现象可能源于：尽管养分富集会同时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，竞争力更强的短命绿藻会逐渐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的大型叶状藻体的损伤程度，远高于仙菜类群的小型丝状藻体。植食性牧食者（grazers）的丰度会因养分输入而正向增加，这可能是其间接响应于更多可利用食物资源的结果。
4. 本研究发现，多数生物的平均丰度与其种群时间波动幅度之间存在直接正相关关系。不过，所有受试生物在整个实验周期中均得以存续，即便处于其丰度时间变异最大的实验处理条件下，这可能源于它们对施加的扰动具备抗性或可快速恢复的能力。因此，在优势物种具备较强恢复能力的生态系统（如潮间带岩礁生境、草原生境）中，扰动动态与养分富集的效应并不会通过物种的灭绝与替代来调控种群波动，而是通过同一物种种群的相对丰度变化实现。这一结论与“种群丰度的时间变异直接与局地灭绝风险相关”的经典理论相悖。本研究结果可帮助更精准地预测气候与非气候情景下，具有相似生物功能性状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，无论其时间分布格局与养分输入状况如何，都会对生长/恢复速度缓慢的物种（如造境物种）产生负向影响。与之相反，抗性更强的物种（如岩礁生境的壳状藻类或草原的地下植被）则会从高强度物理扰动中获益。丰度与养分可利用性直接相关的类群（如丝状藻类、短命藻类或草本植物），在养分富集条件下通常会出现丰度提升，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高的盖度，以抵消伴随扰动强度对其丰度产生的调控作用。若如本研究所考察的系统一般，几乎所有生物都能在实验周期内的任意物理扰动与养分可利用性组合条件下存续，那么最终的群落总多样性不会发生剧烈变化。不过，无论养分可利用性如何，均匀分布的低强度扰动与随机分布的高强度扰动，均为维持最高类群丰富度的最优实验条件。