Phytoplankton community assembly in a large boreal lake – deterministic pathways or chaotic fluctuations?

1. The concepts of community assembly and succession are closely related, yet their foci differ slightly. Succession describes the trajectory of species replacements during the temporal development of the community, while assembly also allows that a locality can harbour different communities depending on the events in the near past of community development. 2. The aims of this study were (i) to examine the year‐to‐year variation in phytoplankton community assembly among basins of different trophy and disturbance in the large boreal Lake Hiidenvesi and (ii) to assess community persistence and diversity among basins in relation to prevailing environmental factors. 3. The results showed that the assembly did not follow similar trajectories each year. According to mean similarity analyses, there was a large degree of variability especially among the groups of samples collected in the same months of different years. Similarity between pairs of consecutive samples was highest in a cold year (1998) in all basins. Community assembly was most unpredictable in the basin of highest productivity, perhaps implying that the number of alternative stable states increased towards higher productivity. Our data also showed a strong unimodal relationship between phytoplankton species richness and grazing by cladoceran zooplankton in the basin of highest trophy. 4. This study showed that phytoplankton community assembly exhibited large variability among the years. This implies that different environmental conditions might be the strongest mechanism behind this pattern, given that the degree of community similarity paralleled the year‐to‐year variation in mean temperature. Unravelling the patterns in community assembly has a number of important implications, especially for the monitoring of ecological impacts based only on snapshots of biological assemblages.     

人类活动-气候变化影响下植被生态系统稳态转换研究进展

人类活动和气候变化对植被内部组分的相互作用和反馈机制产生显著而持续的影响,导致植被生态系统状态发生变化。目前围绕植被生态系统多稳态及稳态转换已经形成一系列理论和实证层面的成果。以人类活动和气候变化为特定干扰域,对植被生态系统稳态转换的理论基础演变进行综述,并从机理层面解析植被结构-功能-状态响应过程;系统梳理植被稳态转换的阈值效应,识别出可能指示生态阈值的驱动力-响应关系;最后,从临界域视角整合植被生态系统稳态转换的信号特征,包括临界慢化、空间关系异常和斑块规律性构型等。在对目前研究不足和未来研究重点进行归纳分析的基础上,主张基于阈值的植被生态系统管理路径,但需要建立在认知植被临界响应机制和非线性自组织规律的前提之上。     

Coupling microscale vegetation–soil water and macroscale vegetation–precipitation feedbacks in semiarid ecosystems

At macroscale, land–atmosphere exchange of energy and water in semiarid zones such as the Sahel constitutes a strong positive feedback between vegetation density and precipitation. At microscale, however, additional positive feedbacks between hydrology and vegetation such as increase of infiltration due to increase of vegetation, have been reported and have a large impact on vegetation distribution and spatial pattern formation. If both macroscale and microscale positive feedbacks are present in the same region, it is reasonable to assume that these feedback mechanisms are connected. In this study, we develop and analyse a soil‐vegetation‐atmosphere model coupling large‐scale evapotranspiration–precipitation feedback with a model of microscale vegetation–hydrology feedback to study the integration of these nonlinearities at disparate scales. From our results, two important conclusions can be drawn: (1) it is important to account for spatially explicit vegetation dynamics at the microscale in climate models (the strength of the precipitation feedback increased up to 35% by accounting for these microscale dynamics); (2) studies on resilience of ecosystems to climate change should always be cast within a framework of possible large‐scale atmospheric feedback mechanism (substantial changes in vegetation resilience resulted from incorporating macroscale precipitation feedback). Analysis of full‐coupled modelling shows that both type of feedbacks markedly influence each other and that they should both be accounted for in climate change models.     

Resilience indicators: prospects and limitations for early warnings of regime shifts

In the vicinity of tipping points—or more precisely bifurcation points—ecosystems recover slowly from small perturbations. Such slowness may be interpreted as a sign of low resilience in the sense that the ecosystem could easily be tipped through a critical transition into a contrasting state. Indicators of this phenomenon of ‘critical slowing down (CSD)’ include a rise in temporal correlation and variance. Such indicators of CSD can provide an early warning signal of a nearby tipping point. Or, they may offer a possibility to rank reefs, lakes or other ecosystems according to their resilience. The fact that CSD may happen across a wide range of complex ecosystems close to tipping points implies a powerful generality. However, indicators of CSD are not manifested in all cases where regime shifts occur. This is because not all regime shifts are associated with tipping points. Here, we review the exploding literature about this issue to provide guidance on what to expect and what not to expect when it comes to the CSD-based early warning signals for critical transitions.     

Superorganisms or loose collections of species? A unifying theory of community patterns along environmental gradients

The question whether communities should be viewed as superorganisms or loose collections of individual species has been the subject of a long‐standing debate in ecology. Each view implies different spatiotemporal community patterns. Along spatial environmental gradients, the organismic view predicts that species turnover is discontinuous, with sharp boundaries between communities, while the individualistic view predicts gradual changes in species composition. Using a spatially explicit multispecies competition model, we show that organismic and individualistic forms of community organisation are two limiting cases along a continuum of outcomes. A high variance of competition strength leads to the emergence of organism‐like communities due to the presence of alternative stable states, while weak and uniform interactions induce gradual changes in species composition. Dispersal can play a confounding role in these patterns. Our work highlights the critical importance of considering species interactions to understand and predict the responses of species and communities to environmental changes.     

Positive feedback and alternative stable states in inbreeding, cooperation, sex roles and other evolutionary processes

A large proportion of studies in systems science focus on processes involving a mixture of positive and negative feedbacks, which are also common themes in evolutionary ecology. Examples of negative feedback are density dependence (population regulation) and frequency-dependent selection (polymorphisms). Positive feedback, in turn, plays a role in Fisherian 'runaway' sexual selection, the evolution of cooperation, selfing and inbreeding tolerance under purging of deleterious alleles, and the evolution of sex differences in parental care. All these examples feature self-reinforcing processes where the increase in the value of a trait selects for further increases, sometimes via a coevolutionary feedback loop with another trait. Positive feedback often leads to alternative stable states (evolutionary endpoints), making the interpretation of evolutionary predictions challenging. Here, we discuss conceptual issues such as the relationship between self-reinforcing selection and disruptive selection. We also present an extension of a previous model on parental care, focusing on the relationship between the operational sex ratio and sexual selection, and the influence of this relationship on the evolution of biparental or uniparental care.     

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

Effects of waterfowl, large fish and periphyton on the spring growth of Potamogeton pectinatus L. in Lake Mogan, Turkey

It has been argued that waterfowl and fish may threaten growth of submerged macrophytes, especially in spring during the early growth phase when plant biomass is low. A small reduction of biomass at that time might delay growth or decrease subsequent productivity. We investigated the impact of waterfowl and large fish on the spring growth of fennel pondweed (Potamogeton pectinatusL.) by employing an exclosure experiment in the macrophyte-dominated clear-water Lake Mogan, Turkey. Birds and large fish were excluded from eight plots and both in situvegetation and macrophytes kept in pots were compared to eight open plots. Also, to investigate the effect of periphyton on plant growth it was removed from half of the pot plants. Exclusion of waterfowl and fish may decrease predation on macroinvertebrates, which in turn may affect periphyton, and macrophyte growth, why macroinvertebrates also were sampled. Waterfowl density was high (15–70 ind. of coot, Fulica atraL. ha^–1), abundance of submerged plants was also high with a surface coverage of 70–80%, and benthivorous fish were present, mainly tench, (Tinca tincaL.) and carp, (Cyprinus carpioL.). Exclusion of waterfowl and large fish did not significantly affect the spring growth of pondweed; neither plants growing in situnor kept in pots. Removal of periphyton from the plants in the pots did not favour growth. The density of macroinvertebrates was not affected by the exclusion of waterfowl and large fish, but it was positively related to aboveground biomass of fennel pondweed. We suggest that even if waterfowl and large fish are in high densities, their effect on fennel pondweed spring growth in lakes with abundant submerged vegetation, such as Lake Mogan, is low.     

Synergy between small- and large-scale feedbacks of vegetation on the water cycle

Predictions of the effects of climate change on the extent of forests, savannas and deserts are usually based on simple response models derived from actual vegetation distributions. In this review, we show two major problems with the implicitly assumed straightforward cause–effect relationship. Firstly, several studies suggest that vegetation itself may have considerable effects on regional climate implying a positive feedback, which can potentially lead to large‐scale hysteresis. Secondly, vegetation ecologists have found that effects of plants on microclimate and soils can cause a microscale positive feedback, implying that critical precipitation conditions for colonization of a site may differ from those for disappearance from that site. We argue that it is important to integrate these nonlinearities at disparate scales in models to produce more realistic predictions of potential effects of climate change and deforestation.     

生态系统多稳态研究进展

在多稳态的生态系统中,外力可能导致生态系统状态突然之间发生不可逆转的转变,从而达到一个新的平衡状态。但目前对多稳态理论的系统研究很少,如何使用预警信号来预测生态系统的状态转变依旧是个难题。通过多稳态理论的梳理提出了一个更加综合的多稳态定义,并以放牧模型为例,系统总结了多稳态理论的相关概念,将多稳态理论应用在生态系统演替和扰沌理论的解释中;通过对生态系统稳态转换预警信号的原理、优缺点和应用条件的分析,对不同尺度下多稳态的研究方法进行了归纳;最后提出了目前多稳态领域的研究问题和未来的研究重点。结果表明:(1)将时间和空间预警信号结合在一起,并量化正确预警信号的概率,对错误预警信号的比例进行加权,可能会提供更准确的稳态转换的预报。(2)定量观测试验适用于小尺度的研究,而较大尺度的研究则采用简化的模型来模拟研究,选择正确的尺度极有可能改变预警信号的可靠性。(3)结合多稳态理论研究生态系统临界转换和反馈控制机制,并将基于性状的特征指标和进化动力学纳入其中,是生态系统修复实践的重要研究方向。(4)将多稳态相关理论和生态保护管理政策的实践相结合,是多稳态理论未来应用的前景。本研究为多稳态理论和实践的...     

Long Transients Near the Ghost of a Stable State in Eutrophic Shallow Lakes with Fluctuating Water Levels

Alternative stable states in shallow lakes have received much attention over the past decades, but less is known about transient dynamics of such lakes in the face of stochastic perturbations such as incidental extremes in water levels driven by climatic variability. Here, we report on the ecosystem dynamics of 70 lakes in the floodplains of the Lower Rhine in The Netherlands from 1999 to 2004. In any particular year, most lakes were either in a macrophyte-dominated clear state or in a contrasting state with turbid water and sparse submerged macrophyte cover. Macrophyte dominance was positively related to the occurrence of drawdown, and negatively to lake surface area and mean depth. We did not find a relation with nutrient levels. Remarkably, shifts between the two contrasting states were common, and episodes of low water levels appear to be an important external driver. A dry period before our study and the exceptionally dry summer of 2003 caused widespread drawdown of floodplain lakes, resulting in establishment of submerged macrophytes in the next year upon refill. In the 4 years without drawdown, many lakes returned to a macrophyte-poor turbid state. Although some lakes turned turbid again quickly, others took several years to shift into the turbid state. A model analysis suggests that such prolonged transient vegetated states may be explained by the fact that the system dynamics slow down in the vicinity of the “almost stable” macrophyte-dominated state. Such a “ghost” of an equilibrium causes the system to stick around that state relatively long before slipping into the only true stable state. Our results support the idea that transient dynamics rather than equilibrium may be the key to understanding the overall state of some ecosystems. A practical implication of our findings is that artificial stabilization of the water level in shallow lakes may have been an important factor aggravating the permanent loss of submerged macrophytes due to cultural eutrophication.     

An idealized model for tree–grass coexistence in savannas: the role of life stage structure and fire disturbances

1.  We discuss a simple implicit-space model for the competition of trees and grasses in an idealized savanna environment. The model represents patch occupancy dynamics within the habitat and introduces life stage structure in the tree population, namely adults and seedlings. A tree can be out-competed by grasses only as long as it is a seedling.
2.  The model is able to predict grassland, forest, savanna and bistability between forest and grassland, depending on the different characteristics of the ecosystem, represented by the model's parameters.
3.  The inclusion of stochastic fire disturbances significantly widens the parameter range where coexistence of trees and grasses is found. At the same time, grass-fire feedback can induce bistability between forest and grassland.
4.   Synthesis . These results suggest that tree–grass coexistence in savannas can be either deterministically stable or stabilized by random disturbances, depending on prevailing environmental conditions and on the types of plant species present in the ecosystem.