植物间正相互作用对种群动态和群落结构的影响:基于个体模型的研究进展

植物间的相互作用对种群动态和群落结构有着重要的影响。大量的野外实验已经揭示了正相互作用(互利)在群落中的普遍存在及其重要性。为了弥补野外实验方法的不足, 模型方法被越来越多地应用于正相互作用及其生态学效应的研究中。该文基于个体模型研究, 探讨了植物间正相互作用对种群动态和群落结构的影响。介绍了植物间正相互作用的定义和发生机制、植物间相互作用与环境梯度的关系。正相互作用是指发生在相邻的植物个体之间, 至少对其中一个个体有益的相互作用。植物通过直接(生境改善或资源富集)或间接(协同防御等)作用使局部环境有利于邻体而发生正相互作用。胁迫梯度假说认为互利的强度或重要性随着环境胁迫度的增加而增加, 但是越来越多的经验研究认为胁迫梯度假说需要改进。以网格模型和影响域模型为例, 介绍了基于个体的植物间相互作用模型方法。基于个体模型, 对近年来国内外正相互作用对种群时间动态(如生物量-密度关系)、空间分布格局和群落结构(如群落生物量-物种丰富度关系)影响的研究进行了总结。指出未来的研究应集中在对正相互作用概念和机制的理解, 新的模型, 新的种群、群落, 甚至生态系统问题, 以及在全球变化背景下进行相关的研究。     

气候变暖对植物、传粉者及其相互作用的影响

植物和传粉者之间的相互作用,构成了错综复杂的传粉网络。近20年来,以全球变暖为主要特征的气候变化对开花植物、传粉昆虫和植物-传粉者相互作用的影响已成为研究热点。这些研究有助于更好地预测气候变暖对传粉网络乃至整个生态系统功能的影响。本文综述了气候变暖对开花植物个体的繁殖过程(物候、花吸引、报酬特征)和传粉昆虫活动及其相互作用产生的影响,以及在群落水平上对植物-传粉者网络产生的影响。气候变暖影响开花植物的花报酬(花蜜量、花粉的数量与质量)以及繁殖成功;温度升高对传粉昆虫的影响主要包括决定花粉流的传粉行为和传粉成功率。因此,气候变暖背景下植物、传粉者及其相互作用的改变会导致传粉网络结构发生变化,而传粉网络通过缓冲机制减少植物-传粉者间的错配而产生的负面效应,以维持自身稳定性。总体上,目前对气候变暖背景下植物-传粉者网络的研究较为薄弱,今后的研究应进一步关注以下问题:(1)气候变暖影响植物与传粉者物候匹配程度的原因、机制以及影响因素;(2)进一步探究不同生态系统中植物-传粉者的相互作用;(3)从群落水平或生态系统水平对植物-传粉者相互作用网络开展长期性研究。     

海草床中的海草-草食动物相互作用

海草及草食动物间的相互作用是影响海草生态系统结构及功能的重要因素。本文综述了海草床中植物-草食动物相互作用的研究进展,重点聚焦于以下几方面:(1)海草对草食动物的防御作用:抵抗策略和忍受策略;(2)草食动物对海草防御的适应,如补偿摄食;(3)海草与草食动物间积极的相互作用包括:草食动物可以调控海草群落的组成、调节年龄结构、控制附着生物和增强海草对水体富营养化的耐受力及海草床为草食动物提供良好的庇护条件。对海草床中海草-草食动物相互作用提出了研究展望,包括营养质量和化学防御对不同类型草食者草食模式的影响,草食者对植物化学、形态学、繁殖生物学和种类组成的影响以及植物和草食动物间直接和间接的相互作用。加强和完善海草床中海草-草食动物相互作用的研究,将有利于我们更加深入全面地认识了解海草床生态系统,使其更好地发挥生态服务功能。     

植物——土壤反馈研究进展

该文综述了植物—土壤反馈研究的定义、途经、方法和国内外的研究现状以及存在的问题。植物—土壤反馈是指植物改变根际土壤的生物和非生物特征,同时被改变的也能提高或降低该植物的生长,形成正的或负的反馈,从而影响植物群落组成及植物间相互作用。植物—土壤反馈研究对于理解植物群落演替、生态系统多样性与生产力形成与维持机制,认识生态系统对气候变化和生物入侵等全球生态事件的响应具有重要的理论意义。外来物种快速生长和繁殖及其可能的负反馈可能会导致本地种被竞争排除,未来气候变化可能导致物种组成发生变化及生物多样性丢失,但资源互补和植物—土壤反馈效应则可能使植物群落具有较高的生产力和多样性。因此,未来植物—土壤反馈关系应该加强以下几方面研究:(1)开展不同生态系统植物—土壤反馈关系的比较研究;(2)植物—土壤及土壤—植物等群落水平的反馈研究;(3)特别是要加强分子和基因工具在植物土壤—反馈关系中的应用,揭示植物—土壤反馈关系的分子机理。     

干旱半干旱草地生态系统与土壤水分关系研究进展

研究干旱半干旱草地生态系统与土壤水分关系和相互作用机理对于揭示草地生态系统稳定性及其水土关键要素的变化过程具有重要意义。从不同界面、不同尺度综述了草地生态系统对土壤水分的影响及草地生态系统的响应与适应机制,总结了草地生态系统与土壤水分关系模型研究的相关进展,并分析了气候变化对草地生态系统和土壤水分关系的影响。草地生态系统通过影响水文过程和生态过程来影响土壤水分,土壤水分在植物生长发育、形态、生理生态过程、种间关系、群落组成和结构以及草地生态系统功能等方面对草地生态系统产生影响;充分揭示草地生态系统-土壤水分相互作用机理是模型研究的关键;气候变化对草地生态系统植物与土壤水分关系具有重要影响。今后应加强以下研究:1)开展草地不同优势种和植物功能型与土壤水分关系的研究,找出能反映植物对土壤水分响应的性状指标,阈值响应点及适应机制;2)注重对不同时间和空间尺度上的转换和比较;3)加强个体、群体和生态系统尺度草地植物生长模型的研究及其与土壤-植被-大气水分传输模型的耦合;4)加强草地生态系统与土壤水分关系对气候变化响应的研究。     

不同利用方式下内蒙古典型草原群落物种相互关系的年际变化

放牧和刈割作为内蒙古草原主要利用方式,对大针茅群落演替以及建群种更替有着重要影响。以内蒙古典型草原大针茅群落为研究对象,2013—2015年连续3年对放牧和刈割两种利用方式下大针茅群落小尺度格局进行定点观测,其中,2014年气候极端干旱且温度较高;运用Ecosim7.72软件计算C-score和V-ratio来反映群落物种间相互作用关系,并分析相互作用关系的年际变化以及影响因素。结果表明:(1)2013年和2015年温度和降水与该区域年均值比较接近,放牧和刈割两种利用方式下群落植物间相互作用关系均为竞争;(2)2014年为极端干旱和高温的年份,放牧和刈割两种利用方式下群落植物间相互作用的值或方向发生了很大变化,其中,放牧利用加剧植物间的竞争,而刈割利用方式下植物间相互作用为促进。(3)所得结果表明气候波动是影响群落物种间相互作用的主要因素,而在极端年份,利用方式会影响群落物种间相互作用的方向和大小,这不仅为胁迫梯度假说的完善提供了实验数据,而且可为该区域草原的合理利用和保护措施的制定提供理论指导。     

基于功能性状的毛乌素沙地不同演替阶段适生植物筛选

沙地生态系统修复是恢复生态学研究的热点问题,适生植物筛选是修复的关键。植物功能性状反映了植物在不同环境中的生存策略,探究沙地植物功能性状及其与环境之间的关系,有助于筛选用于植被恢复的物种,为保护沙地生态系统提供理论依据。以毛乌素沙地为研究区,分析了1983-2015年间沙地典型飞播样地群落演替特征及其对环境因子的响应,建立基于10个植物功能性状的毛乌素沙地潜在种库,进一步筛选飞播恢复下沙地不同演替阶段的适生植物。研究表明:(1)飞播恢复下的毛乌素沙地植物群落分为三个演替阶段:固沙先锋物种群落、沙生植物为主的杂类草群落、中生植物为主的杂类草群落。(2)土壤因子是群落演替的主要驱动力,其中土壤全氮、土壤总有机碳、土壤硝态氮是影响群落演替的关键因素。(3)基于功能性状筛选出29种适生物种用于植被恢复,演替第一阶段可用雾冰藜、猪毛菜等,演替第二阶段可用拂子茅、无芒隐子草等,演替第三阶段可用草地风毛菊、猪毛蒿等。通过物种功能性状特征可以快速选择适合沙地退化生态系统修复的候选物种,为植被恢复提供了一定的理论支持。     

种间互作网络的结构、生态系统功能及稳定性机制研究

生态群落中不同物种间发生多样化的相互作用, 形成了复杂的种间互作网络。复杂生态网络的结构如何影响群落的生态系统功能及稳定性是群落生态学的核心问题之一。种间互作直接影响到物质和能量在生态系统不同组分之间的流动和循环以及群落构建过程, 使得网络结构与生态系统功能和群落稳定性密切相关。在群落及生态系统水平上开展种间互作网络研究将为群落的构建机制、生物多样性维持、生态系统稳定性、物种协同进化和性状分化等领域提供新的视野。当前生物多样性及生态系统功能受到全球变化的极大影响, 研究种间互作网络的拓扑结构、构建机制、稳定性和生态功能也可为生物多样性的保护和管理提供依据。该文从网络结构、构建机制、网络结构和稳定性关系、种间互作对生态系统功能的影响等4个方面综述当前种间网络研究进展, 并提出在今后的研究中利用机器学习和多层网络等来探究环境变化对种间互作网络结构和功能的影响, 并实现理论和实证研究的有效整合。     

生态系统恢复后干热河谷植物叶片N、P、K含量及物种优势度的变化

以金沙江干热河谷生态系统为对象,通过对比恢复区与对照区(干扰区)的植物叶片N、P、K含量、比值及其与优势度的关系,研究了生态系统恢复对植物叶片化学计量特征的影响.结果表明：生态系统恢复显著降低了植物叶片的N、P含量以及P/K,对K含量无显著影响.其中,恢复区内植物叶片N、P、K含量均值分别为10.405、0.604和9.619 g·kg^-1,比对照区分别下降了16.9%、34.9%和4.7%.恢复区中植物优势度与叶片P含量间呈极显著负相关,而对照区植物优势度与叶片K含量的负相关关系最显著.生态系统恢复改变了N、P、K之间标度关系的斜率和截距.而恢复区与对照区同一物种间叶片N、P含量差别不显著.研究区植物叶片化学计量特征的改变主要是由群落物种替代引起的.     

植物群落中物种小尺度空间结构研究

植物群落中, 物种小尺度空间结构影响着种群或群落的动态及有关的生态学过程。植物主要是和它同种或异种的邻近个体相互作用,植物个体周围的局部环境和大空间尺度下群落的平均水平是完全不同的。群落中的许多过程都影响小尺度空间结构的形成和动态,同样,局部空间结构反过来影响着植物的生长、更新和凋亡等重要过程。鉴于目前对小尺度空间结构进行的大量研究以及其重要性,有必要对其研究进展进行适当的总结,以期明确将来进一步的研究方向。该文以此为出发点,首先介绍了植物群落中物种小尺度空间结构产生的6个原因:1)生境的空间异质性;2)植物繁殖体的传播;3)植物之间的相互作用;4)生物环境(动物和微生物)的作用;5)外界干扰的作用;6)多因子综合作用。然后阐述了小尺度空间结构意义及对生物多样性、植物种群遗传学和恢复生态学研究的影响。最后对目前物种小尺度空间结构研究存在的几点问题及将来的研究方向作以下归纳:1)大尺度植被动态的研究应该整合小尺度空间结构的信息;2)不论从生物学还是生态学上来讲, 植被小尺度空间结构的研究应该把植物作为中心,确定适当的尺度和采取合理的空间统计方法;3)充分重视小尺度空间结构在退化生态系统恢复中的应用意义;4)注重从小尺度的局部格局研究入手对群落总体特征进行整合;5)植物群落动态研究中,物种小尺度空间结构与平均场假说相结合的必要性。     

Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

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.     

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or intensity of competition and facilitation will change inversely along abiotic stress gradients. It was originally postulated that increasing environmental stress can induce a monotonic increase in facilitation. However, more recent models predicted that the relationship between severity and interaction exhibits a hump‐shaped pattern, in which positive interactions prevail under moderate stress but decline at the extreme ends of stress gradients. In the present study, we conducted a field experiment along a temporal rainfall gradient for five consecutive years, in order to investigate interactions in a shrub‐herbaceous plant community at the southern edge of the Badain Jaran Desert, and, more specifically, investigated the effects of Calligonum mongolicum, a dominant shrub species, on both abiotic environmental variables and the performance of sub‐canopy plant species. We found that shrubs can improve sub‐canopy water regimes, soil properties, plant biomass, density, cover, and richness and, more importantly, that the positive effect of shrubs on sub‐canopy soil moisture during the summer diminishes as rainfall decreases, a pattern that partly explains the collapse of the positive interaction between shrubs and their understory plants. These results provide empirical evidence that the positive effect of shrubs on understory plant communities in extreme arid environments may decline and become neutral with increasing drought stress.     

Extending the stress‐gradient hypothesis – is competition among animals less common in harsh environments?

The role of positive interactions has become widely accepted as a mechanism shaping community dynamics. Most empirical evidence comes from plant communities and sessile marine organisms. However, evidence for the relative role of positive interactions in organizing terrestrial animal communities is more limited, and a general framework that includes positive interactions among animals is lacking. The ‘stress gradient hypothesis’ (SGH) developed by plant ecologists predicts that the balance between positive and negative interactions will vary along gradients of biotic and abiotic stress, with positive interactions being more important in stressful environments. Paralleling the SGH, stress gradients for terrestrial herbivores could be equated to inverse primary productivity gradients, so we would expect positive interactions to prevail in more stressful, low productivity environments. However, this contradicts the typical view of terrestrial animal ecology that low primary productivity systems will foster intense competition for resources among consumers. Here we use alpine herbivores as a case study to test one of the predictions of the SGH in animal communities, namely the prevalence of positive interactions in low productivity environments. We identify potential mechanisms of facilitation and review the limited number of examples of interspecific interactions among alpine herbivores to assess the role of positive and negative interactions in structuring their communities. A meta‐analysis showed no clear trend in the strength and direction of interactions among alpine herbivores. Although studies were biased towards reporting significant negative inter actions, we found no evidence of competition dominating in harsh environments. Thus, our results only partially support the SGH, but directly challenge the dominant view among animal ecologists. Clearly, a sound theoretical framework is needed to include competition, positive and neutral interactions as potential mechanisms determining the structure of animal communities under differing environmental conditions, and the stress‐gradient hypothesis can provide a solid starting point.     

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.     

What does the stress‐gradient hypothesis predict? Resolving the discrepancies

In recent years the importance of facilitative interactions in ecological communities is increasingly recognized. This phenomenon has been observed repeatedly, particularly in vegetation communities, in a wide range of environmental conditions. The current hypothesis predicts that the role of facilitation becomes increasingly important in conjunction with increasing stress. Several empirical studies, however, failed to detect such patterns, particularly at the extreme ends of the stress gradients. Herein, we present a conceptual model that may resolve discrepancies between expected and observed and provides a more precise framework of the existing hypotheses. By relaxing two common assumptions commonly used by the stress‐gradient hypothesis (SGH) we are able to demonstrate that under some circumstances the importance of facilitation may be less at the extreme ends of these gradients. Namely, we first re‐emphasize the notion that physiological response is not linear with respect to environmental changes along stress gradients. Second, it is argued that the net outcome of facilitative and competitive interactions is reflected in the fitness of individuals as a product of these two processes, in contrast to the commonly applied assumption of additivity. Accordingly, a synthesis of the concepts of population biology (measures of fitness) and plant physiology (nonlinear responses) with the stress gradient hypothesis while retaining the original simplicity of the SGH model contributes to a better specification of the predictions of the stress‐gradient hypothesis and the resolution of observed contradictions.     

A dynamic model of facilitation on environmental stress gradients

Theories based on competition for resources in animals and other non‐sessile organisms rarely consider the role of facilitative interactions. Yet these interactions are important for community assembly, especially under stressful environments (e.g. the stress‐gradient hypothesis, SGH). To make an explicit link between species interaction theory and SGH patterns, I used a classic resource competition model promoting coexistence between a beneficiary and its facilitator sharing a common resource along a stress gradient. I compared model outcomes for two fundamentally different mechanisms of facilitation (alleviation of resource versus non‐resource stress), and also tested the effect of a reciprocal cost of facilitation from the beneficiary. I then tested model's biological relevance using experimental data from two tuber moth species (Lepidoptera, Gelechiidae) for which facilitation in resource access was previously established. Simulation outcomes revealed that both the mode of facilitation and the incorporation of facilitation costs affected the shape of the facilitation–stress relationship. These predictions are in line with current SGH observations and experiments on both plants and animals and reconcile the frequently reported variability of this relationship in nature. Moreover, a sensitivity analysis of model's parameters confirmed the robustness of the modelling framework to uncover the mechanisms responsible for observed species interaction–stress patterns. Finally, when parameterized with tuber moth demographic data, model's results corresponded to observed interaction outcomes along resource stress gradients. Overall, having a common model for plants and animals may simplify assumptions in SGH studies, allow contrasting the shapes of different consumer–resource relationships and specifying the conditions that favour one type of interaction outcome over another.     

Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size

Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xeric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure.     

Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.