植物群落的生物多样性及其可入侵性关系的实验研究

 生物入侵已经成为一个普遍性的环境问题,并为许多学者所关注。尽管一些理论研究和观察表明生物多样性丰富的群落不容易受到外来种的入侵,但后来有些实验研究并没能证实两者的负相关性,多样性 可入侵性假说仍然是入侵生态学领域争论比较多的一个焦点。人为构建不同物种多样性和物种功能群多样性（C3 禾本科植物、C4植物、非禾本科草本植物和豆科植物）梯度的小尺度群落,把其它影响可入侵性的外在因子和多样性效应隔离开来,研究入侵种喜旱莲子草(Alternanthera philoxeroides)在不同群落里的入侵过程来验证多样性 可入侵性及其相关假说。研究结果显示,物种功能群丰富的群落可入侵程度较低,功能群数目相同而物种多样性不同的群落可入侵性没有显著性差异,功能群特征不同的群落也表现出可入侵性的差异,生活史周期短的单一物种群落和有着生物固氮功能的豆科植物群落可入侵程度较高,与喜旱莲子草属于同一功能群且有着相似生态位的土著种莲子草(A. sessilis)对入侵的抵抗力最强。实验结果表明,物种多样性和群落可入侵性并没有很显著的负相关,而是与物种特性基础上的物种功能群多样性呈负相关,群落中留给入侵种生态位的机会很可能是决定群落可入侵性的一个关键因子。     

草地物种多样性对群落可入侵性影响的研究进展

综述草地群落入侵实验中物种多样性和群落可入侵性关系的研究进展。目前物种多样性与群落可入侵性主要出现了对立的关系模式,被普遍接受的解释机制为尺度依赖。但其它研究中出现了更为复杂的关系,提出物种特性、植物更新、种间关系变化和群落构建机制等其它因素可能是导致物种多样性与群落可入侵性出现复杂关系的原因。建议未来研究中应注意的几个问题,即物种多样性与群落可入侵性关系在不同营养级适用性,与群落构建机制变化间的联系和时间尺度对物种多样性与群落可入侵性关系的影响。     

四川攀西地区云南松群落物种多样性和谱系多样性对紫茎泽兰入侵的影响

外来植物入侵严重威胁着入侵地本土植物多样性和生态系统功能, 认识外来物种的入侵机制有助于提高对入侵植物的防控能力。本文以攀西地区云南松(Pinus yunnanensis)林下外来入侵植物紫茎泽兰(Ageratina adenophora)为研究对象, 基于大量野外群落调查, 从群落可入侵性入手, 分析了环境因子和群落物种多样性、谱系多样性等群落生态学特征对紫茎泽兰入侵的影响。结果表明: 海拔、坡向和火烧强度等环境因子和冠层郁闭度、灌木层盖度等生物因子对研究区紫茎泽兰入侵强度没有显著影响(P > 0.05); 但群落灌木层物种多样性和草本层组分种与紫茎泽兰的亲缘关系显著影响紫茎泽兰的入侵强度(P < 0.05), 说明灌木层对光照等环境资源的占用和草本层(同层)物种对相似资源的竞争能够在很大程度上抑制紫茎泽兰的入侵。     

可利用资源波动对外来种入侵的抵抗性

生物入侵带来的生态和经济危害引起了人们的广泛关注。在入侵生态学研究方面,生物多样性与生物入侵之间的关系长久以来成为群落可入侵性探讨的焦点。Elton经典假说认为,物种多样性越高对外来种入侵的抵抗能力越强,许多模型或野外试验都支持这一假说。但现在越来越多的试验对此提出了异议,各种假说纷纷出现。究竟生物多样性会不会影响外来种的入侵?假设两种不同的群落结构(功能群),设计6种外来种入侵土著群落的情景分析不同多样性及相同多样性下外来种的入侵状况。结果发现,在多样性相同的情况下,两种群落对外来种入侵的抵抗力不同。外来种成功入侵等比群落,却被倍数群落排斥在系统之外。进一步分析表明这主要是由于可利用资源的波动引起的,即Davis提出的"资源机遇假说"。在相同的物种多样性下,由于倍数群落的特殊结构,整个群落所占有资源远远大于等比群落资源比率。因此,外来种在等比群落中更易找到合适的入侵机会。而在物种多样性不同的情况下,由于物种多样性与已占有资源的变化是成正比的,因此,混淆了多样性与剩余资源可利用性对外来种入侵的影响。     

植物种多样性对生态系统功能的影响

采用8种1年生植物,利用室外盆栽方法构建的植物群落对物种多样性与群落生产力、杂草入侵性之间的关系及其潜在的作用机理进行了研究.结果表明,各物种单播时其生产力和抗杂草入侵能力都有较大差异.物种多样性与群落生产力之间不存在线性关系,而呈现单峰格局关系,即在一定范围内(本研究为4种),物种多样性对生产力增加有促进作用,超过该范围,物种多样性增加,群落生产力不再增加.但群落生产力和物种多样性均与杂草入侵量之间呈显著负相关关系.相对产量总和(RYT)分析表明,资源互补效应是本研究中所观察到的多样性-生产力、多样性-入侵性及生产力-入侵性关系的主要作用机理.     

历史因素对土壤微生物群落与外来细菌入侵间关系的影响

群落的组成和结构如何影响其可入侵性一直是入侵生态学的研究热点。然而关于群落可入侵性和群落特征间关系的认知却很不统一。采用交叉互换的试验方法,首先将野外采集的两种长期不同施肥土壤(有机肥和化肥)进行灭菌并回接已方和对方的土壤悬液,研究土壤环境(历史非生物因素)和土壤微生物群落(历史生物因素)对重建土壤微生物群落特征的相对贡献。随后将用红色荧光蛋白标记的青枯菌作为外来种接入重建的土壤中,探究不同土壤微生物群落特征对外来细菌存活数量(前期入侵潜力)和存活时间(后期入侵潜力)的影响。结果表明,历史生物因素对重建土壤的原生动物数量、革兰氏阴性与阳性细菌比等群落特征和外来细菌的存活数量有影响;历史非生物因素对土壤微生物活性、细菌物种多样性和功能多样性等群落特征以及外来细菌入侵土壤后总的存活时间有影响;外来细菌入侵前期状况仅与原生动物数量、革兰氏阴性与阳性细菌比相关,而入侵后期的状况则仅与微生物活性、细菌物种多样性和功能多样性相关。总之,外来细菌在土壤中各时期的入侵潜力和土著微生物群落特征的相关性主要取决于二者是否由同种历史影响因素控制。本研究对于阐明生物群落结构与群落可入侵性之间关系,及指导土壤外来病原菌的防控均具有重要意义。     

群落组成及物种间相互作用对外来植物入侵的影响

外来植物入侵已成为严重的环境和社会问题,了解外来植物的入侵机制是有效控制其入侵的前提。生物阻抗假说认为,入侵地本地植物群落中的许多生物因子及生物过程能够抵御外来植物入侵。但关于群落抵抗外来种入侵的主要机制,目前还没有确定的结论。本文综述了群落中物种功能特征的多样性以及与外来种功能特征的相似度、植物与动物、植物与植物以及植物与土壤微生物间的相互作用等因素对外来植物入侵的影响,以及以前研究存在的不足。未来研究应该注重不同条件下植物与植物间的相互作用;不同竞争强度下,植物与食草动物的相互作用;植物、动物及土壤微生物三者之间的相互作用对外来植物入侵的影响。这些研究不仅能够丰富和完善入侵生态学理论,而且对于预测外来植物未来的扩散范围,合理有效地管理生态系统,防止外来植物入侵,保护本地生物多样性具有重要的实际意义。     

Species turnover drives grassland community to phylogenetic clustering over long-term grazing disturbance

放牧通过改变草地群落物种组成和生物多样性,进而影响草地群落结构,对草地生态系统服务和功能产生深远的影响。然而,有关系统发育多样性和系统发育群落结构对长期放牧干扰的响应和适应的研究仍然很少,尤其是对于分布在极端环境中的生态系统。我们在青藏高原高寒草地上开展了多放牧强度的试验,探讨放牧干扰对植物系统发育多样性和群落结构的影响。研究发现,放牧干扰增加了植物群落的物种丰富度,促进了群落物种周转,从而改变了群落物种组成。低强度放牧对系统发育多样性和群落结构没有显著影响,而高强度放牧促使群落结构由分散向聚集变化。高强度放牧通过强烈的环境过滤作用,选择了一些耐牧的草地植物物种。在高强度放牧条件下,草地群落的聚集结构由近缘种的入侵和远缘种的丢失共同驱动。在植物功能性状水平上,我们发现与低强度放牧相比,高强度放牧通过改变根系深度对物种入侵产生影响,在很大程度上提升了物种的入侵性。我们的研究强调,仅仅利用物种丰富度和多样性并不能全面反映放牧干扰对草地群落的影响,而且在以后的放牧生态学研究中应该更加关注物种周转对群落系统发育多样性和群落结构的影响。     

植物群落多样性与可入侵性关系研究进展

综述了植物群落的多样性和可入侵性关系的研究进展.对模型研究、观察性研究和实验性研究得出植物群落的多样性和可入侵性关系不同结论的原因进行了分析,并提出了该方面进一步研究的建议.模型研究的群落多样性形成机制单一、群落状态达到平衡、物种特征差异不明显等简化前提假设,削弱了其结论的可信性.观察性研究的尺度不同导致不同结论,在小尺度上的主要影响因素可能是群落内部的生物因素,但在较大尺度上很多外部因素共同影响多样性和可入侵性,会导致两者出现正相关.实验性研究通常是在小尺度下进行,排除了外部因素的干扰,但是在人工组建群落多样性梯度时,会出现取样效应、相同物种多样性的群落丰富度和密度不同等问题.未来研究应重点考虑的几个方面,即观察性研究与实验相结合,分析不同尺度上入侵的机理和过程;平衡实验设计,避免抽样效应等误来源;模型与实验设计相耦合.     

近缘种比较研究在植物入侵生态学中的应用

为什么有些物种能够成功入侵而有些物种却入侵失败？这是一直困扰入侵生态学家的问题。人们已从不同角度开展了研究, 其中包括不同物种之间的比较。然而以往对不同物种进行比较时往往忽略了亲缘关系对比较研究的影响, 使得出的结果不具可比性。将入侵种与其近缘的土著种或入侵能力不强的外来种进行比较则能够排除亲缘关系带来的影响, 使结果更加具有说服力。现有资料表明近缘种比较已广泛应用于菊科、禾本科、松科等科入侵植物的研究, 探讨与这些植物入侵有关的生物学特性以及生境的可入侵性、入侵的预测和入侵控制等入侵生态学的核心问题。本文综述了入侵种与其近缘种比较在研究生物入侵几个重要问题中的应用, 包括物种的入侵性、群落的可入侵性以及入侵种预测研究, 并分析了近缘种比较研究的发展方向。     

Exotic plant species in a C4-dominated grassland: invasibility, disturbance, and community structure

We used data from a 15-year experiment in a C₄-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C₃ plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C₄ grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C₄-dominated grassland can also be directly related to community structure independent of disturbance. Received: 9 February 1999 / Accepted: 12 May 1999     

Modeling Hawaiian Ecosystem Degradation due to Invasive Plants under Current and Future Climates

Occupation of native ecosystems by invasive plant species alters their structure and/or function. In Hawaii, a subset of introduced plants is regarded as extremely harmful due to competitive ability, ecosystem modification, and biogeochemical habitat degradation. By controlling this subset of highly invasive ecosystem modifiers, conservation managers could significantly reduce native ecosystem degradation. To assess the invasibility of vulnerable native ecosystems, we selected a proxy subset of these invasive plants and developed robust ensemble species distribution models to define their respective potential distributions. The combinations of all species models using both binary and continuous habitat suitability projections resulted in estimates of species richness and diversity that were subsequently used to define an invasibility metric. The invasibility metric was defined from species distribution models with <0.7 niche overlap (Warrens I) and relatively discriminative distributions (Area Under the Curve >0.8; True Skill Statistic >0.75) as evaluated per species. Invasibility was further projected onto a 2100 Hawaii regional climate change scenario to assess the change in potential habitat degradation. The distribution defined by the invasibility metric delineates areas of known and potential invasibility under current climate conditions and, when projected into the future, estimates potential reductions in native ecosystem extent due to climate-driven invasive incursion. We have provided the code used to develop these metrics to facilitate their wider use (Code S1). This work will help determine the vulnerability of native-dominated ecosystems to the combined threats of climate change and invasive species, and thus help prioritize ecosystem and species management actions.     

Timing is everything: priority effects alter community invasibility after disturbance

Theory suggests that communities should be more open to the establishment of regional species following disturbance because disturbance may make more resources available to dispersers. However, after an initial period of high invasibility, growth of the resident community may lead to the monopolization of local resources and decreased probability of successful colonist establishment. During press disturbances (i.e., directional environmental change), it remains unclear what effect regional dispersal will have on local community structure if the establishment of later arriving species is affected by early arriving species (i.e., if priority effects are important). To determine the relationship between time‐since‐disturbance and invasibility, we conducted a fully factorial field mesocosm experiment that exposed tundra zooplankton communities to two emerging stressors – nutrient and salt addition, and manipulated the arrival timing of regional dispersers. Our results demonstrate that invasibility decreases with increasing time‐since‐disturbance as abundance (nutrient treatments) or species richness (salt treatments) increases in the resident community. Results suggest that the relative timing of dispersal and environmental change will modify the importance of priority effects in determining species composition after a press disturbance.     

Defining invasiveness and invasibility in ecological networks

The success of a biological invasion is context dependent, and yet two key concepts—the invasiveness of species and the invasibility of recipient ecosystems—are often defined and considered separately. We propose a framework that can elucidate the complex relationship between invasibility and invasiveness. It is based on trait-mediated interactions between species and depicts the response of an ecological network to the intrusion of an alien species, drawing on the concept of community saturation. Here, invasiveness of an introduced species with a particular trait is measured by its per capita population growth rate when the initial propagule pressure of the introduced species is very low. The invasibility of the recipient habitat or ecosystem is dependent on the structure of the resident ecological network and is defined as the total width of an opportunity niche in the trait space susceptible to invasion. Invasibility is thus a measure of network instability. We also correlate invasibility with the asymptotic stability of resident ecological network, measured by the leading eigenvalue of the interaction matrix that depicts trait-based interaction intensity multiplied by encounter rate (a pairwise product of propagule pressure of all members in a community). We further examine the relationship between invasibility and network architecture, including network connectance, nestedness and modularity. We exemplify this framework with a trait-based assembly model under perturbations in ways to emulate fluctuating resources and random trait composition in ecological networks. The maximum invasiveness of a potential invader (greatest intrinsic population growth rate) was found to be positively correlated with invasibility of the recipient ecological network. Additionally, ecosystems with high network modularity and high ecological stability tend to exhibit high invasibility. Where quantitative data are lacking we propose using a qualitative interaction matrix of the ecological network perceived by a potential invader so that the structural network stability and invasibility can be estimated from the literature or from expert opinion. This approach links network structure, invasiveness and invasibility in the context of trait-mediated interactions, such as the invasion of insects into mutualistic and antagonistic networks.     

Invasiveness,invasibility and the role of environmental stress in the spread of non-native plants

Invasion ecology, the study of how organisms spread in habitats to which they are not native, asks both about the invasiveness of species and the invasibility of habitats: Which species are most likely to become invasive? Which habitats are most susceptible to invasion? To set the stage for considering these questions with regard to plants, we offer a two-way classification of nativeness and invasiveness that distinguishes natives, non-invasive non-natives and invasive non-natives. We then consider the current state of knowledge about invasiveness and invasibility. Despite much investigation, it has proven difficult to identify traits that consistently predict invasiveness. This may be largely because different traits favour invasiveness in different habitats. It has proven easier to identify types of habitats that are relatively invasible, such as islands and riverbanks. Factors thought to render habitats invasible include low intensities of competition, altered disturbance regimes and low levels of environmental stress, especially high resource availability. These factors probably often interact; the combination of altered disturbance with high resource availability may particularly promote invasibility. When biotic factors control invasibility, non-natives that are unlike native species may prove more invasive; the converse may also be true. We end with a simple conceptual model for cases in which high levels of environmental stress should and should not reduce invasibility. In some cases, it may be possible to manipulate stress to control biological invasions by plants.     

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.     

Quantifying levels of biological invasion: towards the objective classification of invaded and invasible ecosystems

Biological invasions are a global phenomenon that threatens biodiversity, and few, if any, ecosystems are free from alien species. The outcome of human‐mediated introductions is affected by the invasiveness of species and invasibility of ecosystems, but research has primarily focused on defining, characterizing and identifying invasive species; ecosystem invasibility has received much less attention. A prerequisite for characterizing invasibility is the ability to compare levels of invasion across ecosystems. In this paper, we aim to identify the best way to quantify the level of invasion by nonnative animals and plants by reviewing the advantages and disadvantages of different metrics. We explore how interpretation and choice of these measures can depend on the objective of a study or management intervention. Based on our review, we recommend two invasion indices and illustrate their use by applying them to two case studies. Relative alien species richness and relative alien species abundance indicate the contribution that alien species make to a community. They are easy to measure, can be applied to various taxa, are independent of scale and are comparable across regions and ecosystems, and historical data are often available. The relationship between relative alien richness and abundance can indicate the presence of dominant alien species and the trajectory of invasion over time, and can highlight ecosystems and sites that are heavily invaded or especially susceptible to invasion. Splitting species into functional groups and examining invasion patterns of transformer species may be particularly instructive for gauging effects of alien invasion on ecosystem structure and function. Establishing standard, transparent ways to define and quantify invasion level will facilitate meaningful comparisons among studies, ecosystem types and regions. It is essential for progress in ecology and will help guide ecosystem restoration and management.     

Eco-Evolutionary Community Dynamics: Covariation between Diversity and Invasibility across Temperature Gradients *

Abstract Understanding biodiversity gradients is a long-standing challenge, and progress requires theory unifying ecology and evolution. Here, we unify concepts related to the speed of evolution, the influence of species richness on diversification, and niche-based coexistence. We focus on the dynamics, through evolutionary time, of community invasibility and species richness across a broad thermal gradient. In our framework, the evolution of body size influences the ecological structure and dynamics of a trophic network, and organismal metabolism ties temperature to eco-evolutionary processes. The framework distinguishes ecological invasibility (governed by ecological interactions) from evolutionary invasibility (governed by local ecology and constraints imposed by small phenotypic effects of mutation). The model yields four primary predictions: (1) ecological invasibility declines through time and with increasing temperature; (2) average evolutionary invasibility across communities increases and then decreases through time as the richness-temperature gradient flattens; (3) in the early stages of diversification, richness and evolutionary invasibility both increase with increasing temperature; and (4) at equilibrium, richness does not vary with temperature, yet evolutionary invasibility decreases with increasing temperature. These predictions emerge from the "evolutionary-speed" hypothesis, which attempts to account for latitudinal species richness gradients by invoking faster biological rates in warmer, tropical regions. The model contrasts with predictions from other richness-gradient hypotheses, such as "niche conservatism" and "species energy." Empirically testing our model's predictions should help distinguish among these hypotheses.     

Dominance not richness determines invasibility of tallgrass prairie

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis , in intact, native Kansas grassland. We manipulated dominance of C₄ grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment of Melilotus , was not affected by a 3-fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.     

Species-Rich Scandinavian Grasslands are Inherently Open to Invasion

Invasion of native habitats by alien or generalist species is recognized worldwide as one of the major causes behind species decline and extinction. One mechanism determining community invasibility, i.e. the susceptibility of a community to invasion, which has been supported by recent experimental studies, is species richness and functional diversity acting as barriers to invasion. We used Scandinavian semi-natural grasslands, exceptionally species-rich at small spatial scales, to examine this mechanism, using three grassland generalists and one alien species as experimental invaders. Removal of two putative functional groups, legumes and dominant non-legume forbs, had no effect on invasibility except a marginally insignificant effect of non-legume forb removal. The amount of removed biomass and original plot species richness had no effect on invasibility. Actually, invasibility was high already in the unmanipulated community, leading us to further examine the relationship between invasion and propagule pressure, i.e. the inflow of seeds into the community. Results from an additional experiment suggested that these species-rich grasslands are effectively open to invasion and that diversity may be immigration driven. Thus, species richness is no barrier to invasion. The high species diversity is probably in itself a result of the community being highly invasible, and species have accumulated at small scales during centuries of grassland management.