南方菟丝子寄生对喜旱莲子草生长及群落多样性的影响

菟丝子属植物是一种有潜力的入侵植物的生物防治剂。以南方菟丝子(Cuscuta australis)天然寄生的喜旱莲子草(Alternanthera philoxeroides)群落为研究对象,对比分析了南方菟丝子寄生对入侵植物喜旱莲子草生长的影响,同时采用群落调查的方法分析南方菟丝子寄生对入侵群落多样性的影响,以判断南方菟丝子是否具有防治喜旱莲子草的能力。结果表明南方菟丝子寄生可以降低喜旱莲子草的根生物量、茎生物量、叶生物量和总生物量,但与对照之间均不存在显著性差异;南方菟丝子寄生可以显著降低茎生物量比,显著增加根生物量比和根冠比。在无南方菟丝子寄生的喜旱莲子草群落中,除南方菟丝子和喜旱莲子草外,共有10科14属14种植物;而在南方菟丝子寄生的喜旱莲子草群落中,除南方菟丝子和喜旱莲子草外,共有16科27属28种植物,南方菟丝子能以产生吸器而寄生生长的植物共有19种,占样地植物种数的67.86%。南方菟丝子寄生可使群落物种丰富度显著性增加,也可使群落Simpson指数、Shannon-Wiener指数、McIntosh指数和均匀度指数增加,但是与对照之间不存在显著性差异。南方菟丝子寄生使喜旱莲子草的多度显著性下降,使喜旱莲子草的盖度和高度下降,但与未寄生的喜旱莲子草群落相比不存在显著性差异。但是南方菟丝子寄生可使喜旱莲子草在群落上的相对盖度、相对高度和相对多度均显著性下降,从而导致群落中喜旱莲子草的重要值显著性下降。南方菟丝子寄生可以在一定程度上抑制入侵植物喜旱莲子草的生长,促使群落多样性增加,促进本地群落的恢复。     

不同土壤水分和养分条件下喜旱莲子草与同属种生长状况的比较研究

喜旱莲子草(Alternanthera philoxeroides)入侵已在中国造成巨大的生态和经济损失。为揭示喜旱莲子草成功入侵的生态机制并预测其种群扩张趋势及其与环境因子的关系, 作者比较了喜旱莲子草与其同属的外来弱入侵种刺花莲子草(A. pungens)以及土著种莲子草(A. sessilis)在不同土壤水分、养分条件下的生长状况。结果显示: 在高水高肥条件下, 喜旱莲子草的生物量要高于刺花莲子草和莲子草, 而在低水低肥条件下却不如这两个同属种; 弱入侵种刺花莲子草在低水条件下的生物量要高于强入侵种喜旱莲子草和土著种莲子草, 说明植物的入侵性受环境条件的影响。另外, 强入侵种喜旱莲子草形态学性状的可塑性较高, 在各种条件下都具有较高的比叶面积, 暗示这两个指标可作为莲子草属外来植物入侵性的预测指标。     

喜旱莲子草入侵群落土壤对植物生长的影响

分别以受喜旱莲子草(Alternanthera philoxeroides)入侵和未受喜旱莲子草入侵的当地植物群落土壤为生长基质,比较不同基质上入侵植物喜旱莲子草和同属的土著植物莲子草(A.sessilis)的生长指标,探讨喜旱莲子草入侵群落土壤对喜旱莲子草及莲子草生长的影响机制。结果表明,喜旱莲子草入侵群落土壤抑制了莲子草的生长,显著降低了根生物量、茎生物量和总生物;改变了形态特征,显著降低了分枝数量、茎长度、根长、根体积;减少了对根的生物量分配,显著抑制了根质量比与根冠比。喜旱莲子草入侵群落土壤对入侵植物喜旱莲子草的生物量、分枝数量、茎长度、根长、根体积没有显著的抑制作用,而显著增加了其叶片数量和叶质量比。这种效应将有利于喜旱莲子草在入侵地形成单优群落,表明土壤在喜旱莲子草成功入侵中起了重要作用。     

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。     

喜旱莲子草茎叶解剖结构从原产地到入侵地的变异式样

长期以来人们一直认为,外来种入侵及其危害是由于一个物种从原产地到入侵地其环境因子改变(如天敌压力的减弱等)而导致的。近年来,越来越多的研究者开始认识到,生物入侵过程实际上是一个现代人类活动影响下的物种的快速进化过程,生物入侵的进化遗传学已成为入侵生物学研究中最活跃的分支之一。作者比较了来自原产地(阿根廷)和入侵地(中国和美国)的喜旱莲子草(Alternantheraphiloxeroides)的11个种群在茎、叶解剖结构方面的变异式样,发现所研究的19个性状在原产地(阿根廷)和入侵地(中国和美国)的变异情况明显不同:在原产地种群中,共有9个性状指标存在显著差异,遗传率在49–89%之间,这9个性状是气孔密度、气孔指数、茎直径、髓腔直径、维管柱直径、皮层厚度、维管柱面积比、髓腔面积比和叶形指数;而在入侵地种群间,19个性状指标均无明显差异。这表明喜旱莲子草从原产地到入侵地其遗传多样性降低;入侵地喜旱莲子草种群间的形态变异主要为表型可塑性。根据19个形态指标对喜旱莲子草11个种群进行主成分分析和聚类,结果显示:所有入侵地种群和原产地的Ar1种群(SantaFé,59°49′W,29°16′S)聚为一类,原产地的Ar4(Tandil,59°03′W,37°11′S)单独聚为一类,原产地的其他4个种群聚为一类。表明Ar1种群可能与入侵中国的喜旱莲子草在基因型上更为接近。这一结果为进一步揭示喜旱莲子草入侵机理(如杂交适应性)和在原产地寻求对应天敌的生物防治工作提供了基础数据。     

水生空心莲子草入侵群落数量分类及其物种分布的环境解释

生境异质性在不同生态层次上影响物种分布与扩散,进而可能影响外来植物的入侵态势。该研究通过在中国21°N～31°N范围内设置23个10m×10m的样地,利用双向聚类(TWINSPAN)和除趋势对应分析(DCA)等数量生态学方法,研究水生型空心莲子草(Alternanthera philoxeroides)入侵群落的结构特征并划分其群丛类型,并采用冗余分析法(RDA)分析空心莲子草种群特征、群落物种分布与环境因子(经度、纬度、海拔、氨态氮、硝态氮、电导率、溶解氧)的关系,以期为生物入侵防治及本土生物多样性保护提供参考。结果显示:(1)23个样地共记录植物48种,隶属于21科42属,空心莲子草优势地位显著,主要伴生种为双穗雀稗(Paspalum paspaloides)、李氏禾(Leersia hexandra)和水蓼(Polygonum hydropiper)。(2)23个样地群落可划分为6组群丛类型,其中空心莲子草+双穗雀稗+水芹(Oenanthe javanica)是最主要的类型。(3)48种植物可划分为4类功能群,分别为先锋种型、优势伴生种型、次要伴生种型和逃避型,功能群的DCA分异性高于群丛类型。(4)群落物种分布状况主要由氨态氮和电导率决定,而物种丰富度及空心莲子草种群特征则受到电导率、经度、海拔和溶解氧等多个环境变量的影响;水生空心莲子草的入侵重要值取决于其无性分枝数和种群盖度。研究表明,水生空心莲子草入侵群落的结构相对简单,应加大对其入侵监测力度并维持较高的生境异质性,以延缓水域生态系统的生物入侵进程。     

不同水陆生境下入侵种喜旱莲子草与土著种莲子草表型变异和细胞渗透势调节能力的比较研究

喜旱莲子草(Alternanthera philoxeroides(Mart.)Griseb.)能成功入侵不同水陆生境,而其同属近缘种莲子草(A.sessilis(Linn.)DC.)适于生长在潮湿的陆地环境中。为揭示两物种生态幅差异的机理及与其入侵潜力的关系,我们在模拟不同水陆生境的同质园环境下,比较了喜旱莲子草和莲子草的形态特征、细胞渗透势变异状况和细胞内溶质物质合成相关基因(蛋氨酸合成酶基因)的表达水平。结果表明:喜旱莲子草对环境变异更敏感,表型变异幅度更大,具有更有效的渗透势调节能力;蛋氨酸合成酶基因在喜旱莲子草进入水生环境的早期阶段表现出短暂上调的特殊表达趋势。这说明有效的细胞渗透势调节机制与喜旱莲子草对不同水陆生境的广泛适应性和较强的表型可塑性能力可能有关,从而帮助其在不同生境中成功入侵。     

松嫩草地草本植物生物多样性:物种多样性和功能群多样性

生物多样性对生态系统功能有重要的影响。作为生物多样性研究的重要方面,物种多样性和功能群多样性引起了生态学者的广泛关注。目前,有关松嫩草地的生物多样性一直缺乏全面而系统的研究。本研究通过对松嫩草地42种植物群落(涉及427个物种)的调查,全面探究了松嫩草地草本植物的生物多样性。从整体上看,松嫩草地主要以禾本科、菊科、豆科植物为主。松嫩草地优势种地上生物量在群落中占绝对优势,其地上生物量与群落地上生物量之间呈极显著正相关(R2=0.904)。不同群落之间物种多样性指数差异很大,其中最主要群落——羊草群落物种丰富度指数(2.6)最小。同时,松嫩草地不同功能群多样性指数之间差异明显,C4禾草为优势种的群落功能群多样性指数明显高于C3禾草为优势种的群落。松嫩草地功能群多样性与物种多样性之间呈极显著正相关(P0.01),功能群多样性在一定程度上能够指示松嫩草地生态系统的生物多样性。     

群落可入侵性及环境胁迫

群落的可入侵性研究是入侵生态学的中心内容。从群落进化历史,群落结构,繁殖体压力。干扰和胁迫等5个影响入侵的主要因素综述了近年来群落可入侵性研究的工作进展和成果。其中重点阐述了群落结构对入侵的影响。包括群落物种组成,物种多样性和物种功能群多样性的诸多实验研究和相关假说,并用理论模型结合具体实验研究探讨了环境胁迫和可入侵性的关系。针对目前群落可入侵性研究存在的问题提出了自己的见解。     

入侵植物喜旱莲子草——生物学、生态学及管理

喜旱莲子草Alternanthera philoxeroides原产于南美洲,属于苋科Amaranthaceae莲子草属Alternanthera。一百多年来,该物种通过压舱水或军马饲料被传播到北美洲、大洋洲、欧洲,以及东南亚和中国等地,成为暖温带-热带湿润气候区淡水生态系统的重要外来入侵种。本文结合对喜旱莲子草在原产地的分布状况的考察、原产地和入侵地喜旱莲子草的比较研究,以及入侵生态学研究的最新成果,对该物种在原产地的地理分布和种内变异、生活史特征、入侵机制和控制策略等方面进行评述,以提高我国科研人员对这一外来入侵种的认识,为科学管理和控制提供依据。     

Invasions: the perspective of diverse plant communities

Exotic plant invasions represent a threat to natural and managed ecosystems. Understanding of the mechanisms that determine why a given species may invade a given ecosystem, or why some biomes and regions seem more prone to invasions, is limited. One potential reason for this lack of progress may lie in how few studies have addressed invasion mechanisms from the point of view of the invaded community. On the other hand, the renewed debate about the relationship between ecological diversity and ecosystem stability offers the opportunity to revisit existing theory and empirical evidence, and to attempt to investigate which characteristics of plant communities, including their diversity, contribute to their invasibility. Empirical studies have shown both positive and negative relationships between species diversity of resident plant communities and their invasibility by external species. Rather than attempting to build a larger collection of case studies, research now needs to address the mechanisms underlying these relationships. Previous knowledge about the mechanisms favouring invasion needs to be coupled with community theory to form the basis of these new investigations. Modern community theory offers hypotheses and techniques to analyse the invasibility of communities depending on their diversity and other factors, such as species’ life histories and environmental variability. The body of knowledge accumulated in invasion ecology suggests that the role of disturbances, in interaction with fertility, and the importance of interactions with other trophic levels, are specific factors for consideration. In addition, it is essential for future studies to explicitly tease apart the effects of species richness per se from the effects of other components of ecological diversity, such as functional diversity (the number of functional groups) and trophic diversity (the number of interactions among trophic levels).     

Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants

Theoretical predictions and empirical studies suggest that resident species diversity is an important driver of community invasibility. Through trait-based processes, plants in communities with high resident species diversity occupy a wider range of ecological niches and are more productive than low diversity communities, potentially reducing the opportunities for invasion through niche preemption. In terrestrial plant communities, biotic ecosystem engineers such as earthworms can also affect invasibility by reducing leaf litter stocks and influencing soil conditions. In a greenhouse experiment, we simultaneously manipulated resident species diversity and earthworm presence to investigate independent and interactive effects of these two variables on the success of several invasive plants. Higher diversity of resident species was associated with lower biomass of invasives, with the effect mediated through resident species biomass. The presence of earthworms had a strong positive effect on the biomass of invasive species across all levels of resident species diversity and a weaker indirect negative effect via decreased soil moisture. Earthworms also weakened the positive correlation between resident species diversity and productivity. We did not observe any interactive effects of resident species biomass and earthworms on invasive species success. Partitioning the net biodiversity effect indicated that selection effects increased with resident species diversity whereas complementarity effects did not. Results suggest that managing for diverse forest communities may decrease the susceptibility of these communities to invasions. However, the presence of introduced earthworms in previously earthworm-free sites may undermine these efforts. Furthermore, future studies of plant community invasibility should account for the effects of introduced earthworms.     

Productivity gradients cause positive diversity–invasibility relationships in microbial communities

Models predict that community invasibility generally declines with species diversity, a prediction confirmed by small‐scale experiments. Large‐scale observations and experiments, however, find that diverse communities tend to be more heavily invaded than simple communities. One hypothesis states that large‐scale environmental heterogeneity, which similarly influences native and invasive species, can cause a positive correlation between diversity and invasibility, overriding the local negative effects of diversity on invasibility. We tested this hypothesis using aquatic microbial communities consisting of protists and rotifers consuming bacteria and nanoflagellates. We constructed a productivity gradient to simulate large‐scale environmental heterogeneity, started communities with the same number of species along this gradient, and subjected equilibrial communities to invasion by non‐resident consumer species. Both invaders and most resident species increased their abundances with resource enrichment, resulting in a positive correlation between diversity and invasibility. Intraspecific interference competition within resident species and the positive effect of enrichment on the number of available resources probably accounted for the higher invasibility with enrichment. Our results provide direct experimental evidence that environmental heterogeneity in productivity can cause a positive diversity–invasibility relationship.     

Plant community diversity and invasibility by exotics: invasion of Mediterranean old fields by Conyza bonariensis and Conyza canadensis

A series of communities were established in situ to differentiate the effects of species richness, functional richness and functional group identity on invasibility of Mediterranean annual old fields. We monitored the demographic and vegetative parameters of two exotic annuals introduced as seedlings, Conyza bonariensis and C. canadensis . Community species richness and functional composition determined resistance to invasion by Conyza. Conyza bonariensis biomass decreased with increasing species richness. Legumes increased the biomass and consequently the net fecundity of both Conyza , while survival was favoured by Asteraceae . Communities with fewer Asteraceae and grasses increased the reproductive effort of C. bonariensis . A separate glasshouse experiment using the same species mixes revealed that establishment of Conyza decreased with increasing species richness or when grasses were present. Patterns of Conyza performance are interpreted in the light of measurements of ecosystem functional parameters, making it possible to formulate hypotheses about mechanisms limiting community invasibility.     

Biodiversity and invasibility in grassland microcosms

In the years since Charles Elton proposed that more diverse communities should be less susceptible to invasion by exotic species, empirical studies have both supported and refuted Elton's hypothesis. Here, I use grassland community microcosms to test the effect of functional diversity on the success of an invasive annual weed (Centaurea solstitialis L.). I found that high functional diversity reduced the success of Centaurea by reducing resource availability. An equally important, but unstudied, question is whether diversity can buffer a community against the impacts of invasive species. In this experiment, although species diversity (independent of functional diversity) did not affect the success of the invader, the invader suppressed growth of species-poor communities more strongly. Invasion of Centaurea also increased summer evapotranspiration in species-poor communities. These results suggest that loss of species diversity alone does not affect community invasibility, but that communities with fewer species may be more likely to decline as a consequence of invasion.     

Relative abundance of invasive plants more effectively explains the response of wetland communities to different invasion degrees than phylogenetic evenness

入侵植物的相对多度比群落系统发育均匀度更能解释湿地群落对不同入侵程度的响应 本地植物群落普遍受到入侵植物不同程度的入侵。然而,入侵植物相对多度与群落系统发育均匀度对不同入侵程度下湿地植物群落响应入侵的相对贡献尚不明确。此外,这种贡献是否随淹水等环境 条件的变化而变化也不清楚。为了探讨这些问题,我们选择空心莲子草(Alternanthera philoxeroides)作为入侵植物,通过改变植物群落物种组成,构建了4个不同的入侵程度,并且设置了水淹和无水淹两种处理。 改变群落入侵程度的同时改变了空心莲子草的相对多度和群落的系统发育均匀度。研究结果表明,不同的入侵程度显著影响了空心莲子草和一些本地物种的单株生物量。变异分割结果表明,无论淹水情况结果如何,空心莲子草相对多度对植物群落指标变异的贡献都大于系统发育均匀度。斯皮尔曼等级相关检验结果表明,空心莲子草的相对多度与空心莲子草和部分本地物种的单株生物量显著负相关;群落系统发育均匀度仅与少数本地种性状显著正相关。其相关强度和显著性均受特定的物种和水淹环境的影响。总之,这些研究结果表明：无论淹水情况如何,入侵植物(空心莲子草)的相对多度都比群落的系统发育均匀度更能有效地解释湿地植物群落对不同入侵程度的响应。     

Phylogenetic diversity is a better predictor of wetland community resistance to Alternanthera philoxeroides invasion than species richness

• Highly biodiversity communities have been shown to better resist plant invasions through complementarity effects. Species richness (SR) is a widely used biodiversity metric but lacks explanatory power when there are only a few species. Communities with low SR can have a wide variety of phylogenetic diversities (PD), which might allow for a better prediction of invasibility.
• We assessed the effect of diversity reduction of a wetland community assemblage typical of the Beijing area on biotic resistance to invasion of the exotic weed Alternanthera philoxeroides and compared the reduction in SR and PD in predicting community invasibility.
• The eight studied resident species performed similarly when grown alone and when grown in eight‐species communities together with the invasive A. philoxeroides. Variation partitioning showed that PD contributed more to variation in both A. philoxeroides traits and community indicators than SR. All A. philoxeroides traits and community indicators, except for evenness index, showed a linear relationship with PD. However, only stem length of A. philoxeroides differed between the one‐ and two‐species treatments, and the diversity index of the communities differed between the one‐ and two‐species treatments and between the one‐ and four‐species treatments.
• Our results showed that in natural or semi‐natural wetlands with relatively low SR, PD may be a better predictor of invasibility than SR. When designing management strategies for mitigating A. philoxeroides invasion, deliberately raising PD is expected to be more efficient than simply increasing species number.

     

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.     

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.     

Invasion by Non-Native Annual Grasses: The Importance of Species Biomass, Composition, and Time Among California Native Grasses of the Central Valley

The Central Valley of California is noted for its dearth of remnant native grass populations and for low native grass seedling establishment within grasslands now dominated by non‐native annual species. In contrast, remnant populations are common along the coast, and studies have shown an ability for seedlings and adults to compete with non‐native annual grasses. The invasibility of well‐established populations of native grasses in the Central Valley remains unclear. The objectives of this study were to compare the invasibility of native grasses differing in density and species composition and, given the species in this study, to assess the ability of mixes with greater species richness to resist invasion relative to their abilities in monoculture. In the Sacramento Valley of California, six species of native grasses were planted at three densities in monospecific and mixed‐species plots. Percent cover of native perennial and non‐native annual grasses was measured in years 2 and 3, and biomass was sampled in year 5. Native grass biomass and, to a lesser extent, species composition were important in explaining variation in non‐native grass invasibility in the fifth year. Species‐rich treatments did not experience less invasion than would be expected by the proportional invasibility of each species in monoculture. However, invasibility of plots consisting of slower growing, shorter statured species decreased over time, suggesting a successional benefit to diverse communities. This study demonstrates that established stands of native grasses in the Sacramento Valley can resist invasion by non‐native annual grasses and that stand biomass is a particularly important factor in determining invasibility.