外来植物入侵的化感作用机制探讨

在最近的几十年,在全球范围内的外来植物入侵的频率达到了前所未有的水平,所以对其入侵机制的研究显得尤为重要。目前,已有几种假说从不同的侧面解释外来植物的成功入侵。本文在综述前人工作基础上,从化感作用的角度解释外来植物入侵,并给出解释模式。由于本土动植物对某些外来植物释放的化感物质比较敏感,外来植物可以利用它的“化学武器”与本土植物竞争,抵御植食性动物的取食和病源菌的感染,从而在与本土物种的相互干扰中占据优势,实现成功入侵。进一步从理论和应用的角度探讨了化感作用作为外来植物入侵的机制给我们的启示,并对今后这方面研究提出几点建议和看法。     

表型可塑性与外来植物的成功入侵

生物入侵是导致生物多样性丧失的第二大因素,掌握入侵机制可提高外来入侵物种防控管理能力。表型可塑性被认为是外来植物成功入侵的重要机制之一,具有确定的遗传基础。近30年来表型可塑性的研究发展迅猛,野外调查与移植实验相结合,宏观生态与分子生态交叉等多种方法被用来探索外来植物成功入侵的表型可塑性机制。本文从表型可塑性概念、研究发展历程及其在外来植物入侵中的作用等方面进行了综述,并对今后外来入侵植物表型可塑性的研究方向进行了展望。表型可塑性的生态作用是目前研究的关注重点,表型可塑性增强了外来入侵植物的适应性和入侵能力,与遗传分化一起共同促进了外来植物的成功入侵。未来多学科相结合的综合研究方法将成为表型可塑性研究发展趋势,气候变化、土地利用等因素与入侵植物表型可塑性相结合,以及运用基因组学和分子生物学等方法,定量分析比较个体、种群或物种间的表型可塑性是未来研究外来植物入侵机制的方向之一。     

表型可塑性与外来植物的入侵能力

外来植物的入侵能力与其性状之间的关系是入侵生态学中的基本问题之一。成功的入侵种常常能占据多样化的生境,并以广幅的环境耐受性为特征。遗传分化(包括生态型分化)和表型可塑性是广布性物种适应变化、异质性生境的两种不同但并不矛盾和排斥的策略。越来越多的实验证据表明,表型可塑性具有确定的遗传基础,本身是一种可以独立进化的性状。许多入侵种遗传多样性比较低,但同时又占据了广阔的地理分布区和多样化的生境,表型可塑性可能在这些物种的入侵成功和随后的扩散中起到了关键作用。本文首先介绍表型可塑性的含义,简述表型可塑性和生物适应的关系,然后从理论分析和实验证据两个方面论述了表型可塑性与外来植物入侵能力的相关性,最后针对进一步的研究工作进行了讨论。当然,并非所有入侵种的成功都能归因于表型可塑性,作者认为对于那些遗传多样性比较低同时又占据多样化生境的入侵种,表型可塑性和入侵能力的正相关可能是一条普遍法则,而非特例。     

化感——外来入侵植物的“Novel Weapons”

综述了与外来植物入侵相关的理论假说。并主要介绍了从化感角度提出的“N ove lW eapons”假说(NW假说)。某些外来入侵植物能产生一些化学物质。这些物质可以发挥较强的化感作用,或者成为植物和土壤微生物之间相互作用的调节者。由于不同区域的植物群落共同进化的轨迹不同,被入侵群落的植物对这些化学物质缺乏适应性,因此,这些物质成为了外来种入侵的利器。而对外来种发源地的群落来说,由于长期的共存关系,这些物质对其伴生植物的影响相对较小。NW假说为化感作为外来植物入侵的一种机制奠定了理论基础,为外来入侵植物的防治提供了理论依据,但作为一种较新的理论假说,还需要更多的实验研究加以支持。     

广州外来入侵植物

根据野外调查及相关文献资料,初步探讨了广州市外来入侵植物的种类组成、分布生境、生长特性、危害以及来源.结果表明:广州市现有外来人侵植物73种,隶属于27科59属,其中菊科(19种)、禾本科(8种)、豆科(7种)和苋科(7种)植物占了入侵植物总种数的56.2%.生活型分析发现,在广州有分布的外来入侵植物中,陆生草本植物最多(58种),其次为灌木(10种)、水生草本(3种),乔木种类最少(2种),它们主要生长在人类干扰比较频繁的低海拔地区.另外,基于对外来入侵植物目前在野外的生长和分布状况等生态学特性进行分析,可大致将它们对本地生物多样性危害程度分为3级,其中危害严重的有11种,危害中等的有25种,危害较轻的有37种.从原产地分析来看,61种来自于热带美洲地区,占广州外来入侵植物总种数的83.6%.与国内其他地区外来入侵植物进行比较发现,广州和广西共有种数最多,有62种,然而,对已经造成严重危害的外来植物种类,在数量上广州与香港、深圳和广西相同种类较多,分别有8种、7种和6种.对广州市目前外来植物的入侵风险(潜在威胁)的评估结果表明,对一些在其它地区已经造成严重危害的外来植物,应当加强监控,防止入侵扩散并造成重大生态灾难.还对广州市外来植物入侵的主要途径及防治策略做了简要分析和说明.     

外来杂草入侵的化学机制

由外来杂草入侵引发的严重生态和经济问题已倍受关注,外来杂草在新生境成功入侵,除了具备一些基本的生物生态学特征外,还应具备一些特有的入侵机制,阐明外来杂草的各种入侵机制可以为入侵杂草的预测和控制提供科学依据。外来杂草只有在新生境中与原产地生物种间的相互作用中取得优势,才能定植并扩增种群而成功入侵．在这些外来杂草和原产地生物种间的相互作用关系中,化学关系是不可忽视的方面．目前研究已经证实：植物的化感作用在外来杂草成功入侵中发挥着重要的作用．事实上,植物也可以通过合成和释放特定的化学物质防御或抑制新生境的动物、植物和微生物．外来杂草入侵的化学机制涉及到植物化学生态学的各个方面。因此,外来杂草的化学生态学特征应作为入侵种预测的重要指标之一,外来杂草入侵的化学机制应是今后重要的研究方向。     

外来入侵植物薇甘菊(Mikania micrantha)对温度升高的响应

为探讨全球变化温度升高对外来入侵植物薇甘菊(Mikania micrantha)化感作用和入侵能力的影响,研究不同温度(22、26和30℃)处理对其种子萌发、幼苗生长、生物量分配、挥发物成分和化感作用的影响。结果表明,薇甘菊种子在温度为22、26和30℃时的萌发率分别为29.2%、52.4%和75.2%。30℃条件下薇甘菊种子萌发率最高,萌发速度快。温度升高显著增加薇甘菊茎的生长、增加对茎的生物量分配。GC-MS和GC测定显示,温度升高改变了薇甘菊挥发物的化学成分。同时,生物测定表明,温度升高增强薇甘菊挥发物对萝卜(Raphanus sativus)和油菜(Brassica campestris)的化感作用。说明温度升高促进了薇甘菊种子的萌发和生长,同时增强了该植物的化感作用,温度升高可能加速薇甘菊的生物入侵。     

红河流域的外来入侵植物

红河流域是中国生物多样性最为丰富的地区,植物区系起源古老,生态系统类型复杂多样.本文论述了红河流域外来入侵植物的现状,列举了73种红河流域外来入侵物的种类,对危害比较严重的紫茎泽兰、飞机草、马樱丹、肿柄菊、凤眼莲和空心莲子草6种植物作了具体介绍.对外来入侵植物的控制对策,入侵植物本身的生物学特性的研究、保护生物多样性和退化生态系统的恢复以及路域生态系统管理等几方面作了论述.结果表明,减少人类对生态系统的干扰和破坏,恢复本地植被是防止外来入侵植物入侵的有效途径.     

京津冀外来入侵植物

基于文献报道和分类学考证, 首次全面、系统而又完整地整理了京津冀近20年报道的46科141属210种外来入侵植物种类及信息。经过分析并校正分为国产种和入侵种两大类。国产种又包括一般国产种22科39属39种、国产逸生类9科13属13种、国产归化类3科3属4种、国产入侵类1科1属1种; 外来种包括实则在京津冀没有达到入侵的温室及露地栽培类8科9属9种、外来逸生类12科32属36种、外来归化类10科19属20种、外来有待观察类6科6属6种, 真正对京津冀构成入侵的外来植物20科48属82种。根据京津冀目前标本数目及采集时间、京津冀报道文献次数及具体内容描述, 结合国家入侵等级及京津冀部分地区的实地考察情况, 对82个入侵种进行入侵级别划分, 分析并讨论了京津冀外来入侵物种的组成、原产地、分布特征、生活型和入侵途径以及京津冀外来入侵植物的危害现状, 并提出防治及管理措施。     

南四湖湿地外来入侵植物

南四湖是我国重要的淡水湖泊湿地之一。为阐明南四湖湿地外来植物的现状及入侵规律, 为湿地生态系统的重建、生物多样性的保护和湿地资源开发等提供指导, 通过野外调查和参考有关文献资料, 对南四湖湿地外来植物进行了较为深入的研究, 并与青岛、昆嵛山、滨州沿海三地作了比较。结果表明, 南四湖湿地外来入侵植物有41种, 隶属19科35属; 以菊科和豆科为优势科; 外来入侵物种中热带来源与温带来源比例相当, 人为引入导致的外来入侵植物高达39%。南四湖与沿海三地相比, 沿海三地外来入侵植物的种类均比南四湖湿地多, 其中以经济最发达的青岛种类最多; 青岛外来入侵植物中有35种在南四湖湿地未发现; 南四湖湿地的特有外来入侵植物有5种, 体现出湿地外来入侵植物水生特性。群落相似性分析表明, Jaccard指数和Sorenson指数呈现的规律一致, 南四湖与滨州的相似性最高, 其次为昆嵛山, 与青岛的差异最大。通过分析南四湖湿地外来入侵植物种类和分布规律, 提出了相应的防治对策。     

基于竞争和扩散能力的非捕食-被捕食集合种群系统的竞争机制

对于非捕食 被捕食(食饵)生态系统,强弱物种之间存在一定的竞争影响.在不考虑栖息地毁坏的情况下,引进双向竞争机制,将Tilman的单向竞争模式推广为n集合种群双向竞争模型,并对6-集合种群的竞争动态进行了计算机模拟研究.结果表明,在平衡态,种群竞争共存的条件是其竞争能力与扩散能力呈现指数型负相关关系,竞争的结果使物种的强弱序列发生变化;物种竞争排除与共存受迁移扩散能力和竞争能力影响很大,在局域斑块上竞争排斥的集合种群在广域尺度上可以竞争共存,即逃亡共存.     

Latitudinal variation of life-history traits of an exotic and a native impatiens species in Europe

Understanding the responses of invasive and native populations to environmental change is crucial for reliable predictions of invasions in the face of global change. While comparisons of responses across invasive species with different life histories have been performed before, comparing functional traits of congeneric native and invasive species may help to reveal driving factors associated with invasion. Here we compared morphological functional trait patterns of an invasive species (Impatiens parviflora) with its congeneric native species (I. noli-tangere) along an approximately 1600 km European latitudinal gradient from France (49°34′N) to Norway (63°40′N). Soil nitrogen was recorded during six weeks of the growing season, and light, soil moisture, and nutrient availability were estimated for each sampled population using community weighted means of indicator values for co-occurring species. Temperature data were gathered from nearby weather stations.Both the native and invasive species are taller at higher latitudes and this response is strongest in the invasive species. Seed mass and number of seeds per capsule increase in I. noli-tangere but decrease in I. parviflora towards higher latitudes. Surprisingly, plant height in the invasive I. parviflora decreases with increasing soil nitrogen availability. The latitudinal pattern in seed mass is positively related to temperature in I. noli-tangere and negatively in I. parviflora. Leaf area of both species decreases with increasing Ellenberg indicator values for nitrogen and light but increases with increasing soil moisture. Soil nitrogen concentrations and Ellenberg indicator values for nitrogen have significant positive (I. noli-tangere) and negative (I. parviflora) effects on the number of seeds per capsule. Our results show that the native I. noli-tangere has efficient reproduction at its range edge while the invasive I. parviflora shows a marked decrease in seed size and seed number per capsule. These patterns are unrelated to the growth and obtained size of the plants: even low soil nitrogen availability in the north seemed not to limit plant growth and size. Our results suggest that the invasive I. parviflora tends to become more invasive at lower latitudes by producing heavier seeds and more seeds per capsule.     

All stressed out and nowhere to go: does evolvability limit adaptation in invasive species?

Introduced and invasive species are major threats native species and communities and, quite naturally, most scientists and managers think of them in terms of ecological problems. However, species introductions are also experiments in evolution, both for the alien species and for the community that they colonize. We focus here on the introduced species because these offer opportunities to study the properties that allow a species to succeed in a novel habitat and the constraints that limit range expansion. Moreover, an increasing body of evidence from diverse taxa suggests that the introduced species often undergo rapid and observable evolutionary change in their new habitat. Evolution requires genetic variation, which may be decreased or expanded during an invasion, and an evolutionary mechanism such as genetic drift or natural selection. In this volume, we seek to understand how natural selection produces adaptive evolution during invasions. Key questions include what is the role of biotic and abiotic stress in driving adaptation, and what is the source of genetic variation in introduced populations.     

Mahonia invasions in different habitats: local adaptation or general-purpose genotypes?

Rapid evolutionary adaptations and phenotypic plasticity have been suggested to be two important, but not mutually exclusive, mechanisms contributing to the spread of invasive species. Adaptive evolution in invasive plants has been shown to occur at large spatial scales to different climatic regions, but local adaptation at a smaller scale, e.g. to different habitats within a region, has rarely been studied. Therefore, we performed a case study on invasive Mahonia populations to investigate whether local adaptation may have contributed to their spread. We hypothesized that the invasion success of these populations is promoted by adaptive differentiation in response to local environmental conditions, in particular to the different soils in these habitats. To test this hypothesis, we carried out a reciprocal transplantation experiment in the field using seedlings from five Mahonia populations in Germany that are representative for the range of habitats invaded, and a greenhouse experiment that specifically compared the responses to the different soils of these habitats. We found no evidence for local adaptation of invasive Mahonia populations because seedlings from all populations responded similarly to different habitats and soils. In a second greenhouse experiment we examined genetic variation within populations, but seedlings from different maternal families did not vary in their responses to soil conditions. We therefore suggest that local adaptation of seedlings does not play a major role for the invasion success of Mahonia populations and that phenotypic plasticity, instead, could be an important trait in this stage of the life cycle.     

Resolving the genetic basis of invasiveness and predicting invasions

Considerable effort has been invested in determining traits underlying invasiveness. Yet, identifying a set of traits that commonly confers invasiveness in a range of species has proven elusive, and almost nothing is known about genetic loci affecting invasive success. Incorporating genetic model organisms into ecologically relevant studies is one promising avenue to begin dissecting the genetic underpinnings of invasiveness. Molecular biologists are rapidly characterizing genes mediating developmental responses to diverse environmental cues, i.e., genes for plasticity, as well as to environmental factors likely to impose strong selection on invading species, e.g., resistance to herbivores and competitors, coordination of life-history events with seasonal changes, and physiological tolerance of heat, drought, or cold. Here, we give an overview of molecular genetic tools increasingly used to characterize the genetic basis of adaptation and that may be used to begin identifying genetic mechanisms of invasiveness. Given the divergent traits that affect invasiveness, “invasiveness genes” common to many clades are unlikely, but the combination of developmental genetic advances with further evolutionary studies and modeling may provide a framework for identifying genes that account for invasiveness in related species.     

Is there any genetic differentiation among populations of Piptochaetium napostaense (Speg.) Hack (Poaceae) with different grazing histories?

Native populations of perennial grasses subjected to heavy grazing are typically shorter and more prostrate than ungrazed or lightly defoliated ones. However, it is often difficult to find out whether the morphological modifications are the result of genetic differentiation or phenotypic plasticity. Piptochaetium napostaense (Speg.) Hack. is a native perennial cool-season palatable grass with a dwarf form abundant in the areas subjected to heavy grazing. In this study, we tried to determine whether the populations with different grazing histories are genetically differentiated. We considered three different grazing conditions: enclosure (prevented from grazing during 20 years), livestock grazing, and burrow (heavily grazed by cattle and a wild rodent herbivore, the vizcacha). Isozyme analyses were carried out in order to assess the genetic variability of the populations under study. We further studied the progeny of plants with different grazing histories to determine whether the morphological differences are transmitted to the next generation. Seedlings obtained from seeds belonging to enclosure and burrow were grown in the greenhouse and their vegetative and reproductive response under different water and nutrient availability levels were recorded. From the isozymes analyses we found low levels of genetic variation in the populations studied, with an average of 20.5% polymorphic loci, 1.2 alleles per locus and 0.015 mean expected heterozygosity. From the total genetic diversity, only 1.4% was due to differences among population. In addition, either enclosure or burrow populations had the same growth and reproductive response over treatments differing in water and nutrient levels. The morphological differentiation among plants with different grazing histories appears to be the outcome of a phenotypically plastic response of adapted genotypes.     

Effects of seed traits variation on seedling performance of the invasive weed,Ambrosia artemisiifolia L.

Seedling performance can determine the survival of a juvenile plant and impact adult plant performance. Understanding the factors that may impact seedling performance is thus critical, especially for annuals, opportunists or invasive plant species. Seedling performance can vary among mothers or populations in response to environmental conditions or under the influence of seed traits. However, very few studies have investigated seed traits variations and their consequences on seedling performance. Specifically, the following questions have been addressed by this work: 1) How the seed traits of the invasive Ambrosia artemisiifolia L. vary among mothers and populations, as well as along the latitude; 2) How do seed traits influence seedling performance; 3) Is the influence on seedlings temperature dependent. With seeds from nine Western Europe ruderal populations, seed traits that can influence seedling development were measured. The seeds were sown into growth chambers with warmer or colder temperature treatments. During seedling growth, performance-related traits were measured. A high variability in seed traits was highlighted. Variation was determined by the mother identity and population, but not latitude. Together, the temperature, population and the identity of the mother had an effect on seedling performance. Seed traits had a relative impact on seedling performance, but this did not appear to be temperature dependent. Seedling performance exhibited a strong plastic response to the temperature, was shaped by the identity of the mother and the population, and was influenced by a number of seed traits.     

Genetic and environmental effects on growth of children from a subsistence agricultural community in southern Mexico

Sibling correlations for size attained in height, weight, sitting height, estimated leg length, the triceps skinfold, arm circumference, and estimated midarm muscle circumference were compared in 6- through 13-year-old schoolchildren grouped by household socioeconomic status. The children were residents of a Zapotec-speaking, subsistence agricultural community in the Valley of Oaxaca in southern Mexico. Sibling pairs were classified as being from high and low socioeconomic status (SES) households, and sibling correlations were computed within each SES group controlling for environmental effects derived from a factor analysis of information on household demography and land and livestock holdings. Like-sex siblings from lower SES households have significantly different correlations in four instances. Correlations are higher for leg length in lower SES brothers and higher for sitting height and weight in lower SES sisters, while the correlation for sitting height is higher in upper SES brothers. The sibling correlation results are not entirely consistent with observations on growth status by SES, particularly if the power and similarity of a common environment is the only cause of higher sibling correlations. Reduced body size under poorer socioeconomic and presumably nutritional circumstances is apparent, but it is not possible in this analysis to distinguish whether genotypic (developmental) plasticity or genetic adaptation, or both, are involved.     

Plasticity of seed output in response to soil nutrients and density in Abutilon theophrasti: implications for maintenance of genetic variation

 Seed output is determined by two processes: resource acquisition and the allocation of resources to seeds. In order to clarify how the reaction norm of seed output is controlled by the phenotypic expression of its two components, we examined the genetic components of plasticity of seed dry mass, plant size, and reproductive allocation under different conditions of soil nutrient availability and conspecific competition among eight families of Abutilon theophrasti. Without competition, the reaction norm of seed mass of the families crossed between the lowest and other nutrient levels, although neither of its components, plant size and reproductive allocation, showed such a response. The crossing reaction norm (i.e., reversal of relative fitnesses of different genotypes along the environmental gradient) of seed mass resulted from (1) a trade-off between plant size and reproductive allocation, and (2) changes in the relative magnitude of genetic variances in plant size and reproductive allocation with soil nutrient availability. While allocation was more important in determining seed mass under limiting nutrient conditions, plant size became more important under high-nutrient conditions. There were no significant genetic variances in seed mass, plant size, and reproductive allocation in the competition treatment, except at the highest nutrient level. The results show that plant competition mitigated the effects of genetic differences in plant performance among the families. We discuss the results in relation to maintenance of genetic variation within a population. Received: 16 August 1996 / Accepted 26 April 1997