白三叶(Trifolium repens)入侵对城市草坪生态系统土壤动物的影响

土壤动物是土壤生态系统中十分活跃的生物类群之一,也是城市草坪生态系统的重要组成部分。由于外来植物白三叶入侵,草坪土壤动物可能受到直接或间接的影响,进而改变城市草坪生态系统功能及过程。研究白三叶入侵对城市草坪生态系统土壤动物的影响,可为进一步了解外来植物入侵机制及城市草坪的建植养护提供理论依据。采用野外调查的试验方法对4种不同入侵程度下城市草坪的土壤动物群落特征和土壤理化性质进行了研究。结果如下：4种不同入侵程度的草坪样地共捕获土壤动物30099只,隶属于19目43科,其中小杆科、线蚓科和跳虫科为优势类群;土壤动物个体数量和类群数量整体表现为中度入侵 > 轻度入侵 > 对照 > 重度入侵,符合中度干扰假说;土壤动物个体数量和类群数量的季节动态主要表现为夏秋较高,冬春较低。CCA分析显示,土壤理化因子中铵态N对土壤动物影响相对较小,可能与白三叶改善了草坪土壤的供N水平并使其不成为土壤动物分布的限制因子有关;同时,土壤理化因子对土壤动物优势类群影响较小,而主要影响常见和稀有类群。总体而言,一定程度的白三叶入侵增加了土壤动物群落的多样性,但当达到重度入侵时,白三叶形成单优群落,减少了植物多样性,进而使土壤动物可获得的生活资源减少,土壤动物群落多样性呈下降趋势;白三叶入侵后通过改变土壤理化性质,影响到土壤动物常见和稀有类群,并最终改变草坪土壤动物的群落结构。     

豚草入侵对中小型土壤动物群落结构特征的影响

豚草(Ambrosia artemisiifolia)是菊科豚草属的恶性杂草,在我国华南地区已成功入侵并造成严重危害。为了解豚草入侵对中小型土壤动物群落的影响效应,采用野外样地试验法研究了豚草入侵地中小型土壤动物的群落特征。全年4次采样共获得中小型土壤动物4174头,隶属于4门11纲26类,其中线虫类为优势类群,蜱螨目和弹尾目是常见类群。结果显示,豚草入侵改变了中小型土壤动物群落的结构特征,入侵地中小型土壤动物的总个体数以及线虫类、弹尾目动物的个体数显著增加,但中小型土壤动物类群数的变化不明显;在群落的物种多样性方面,豚草入侵显著提高了群落的密度-类群指数,物种丰富度、优势度指数亦有所上升,但差异不明显,而均匀度、Shannon-Wiener指数则趋于下降;在群落相似性方面,入侵区与其它处理区的差异较小。豚草入侵所引起的局部气候环境、凋落物、根系分泌物与土壤理化性质的变化可能是造成中小型土壤动物群落结构特征改变的主要原因。     

不同入侵程度下飞机草对喀斯特地区土壤理化性质的影响

外来生物入侵威胁着全球的生物多样性和生态系统的功能,研究外来入侵植物对土壤理化性质的影响有助于理解外来入侵植物的入侵机制。以广西喀斯特地区飞机草（Chromolaena odorata）入侵生境为研究对象,比较分析了不同飞机草入侵程度下（对照、轻度入侵、中度入侵、重度入侵）土壤物理和化学指标变化特征。结果表明：随着飞机草的入侵程度加重,土壤容重显著增加,轻度入侵、中度入侵和重度入侵生境下土壤容重分别比对照增加了10.3%、16.7%、22.3%,土壤非毛管孔隙度、总孔隙度和土壤含水量显著降低,土壤毛管孔隙度无显著变化;飞机草入侵对土壤pH值无显著影响,随着入侵程度加重,土壤速效磷和速效钾的含量显著升高,在重度入侵生境下土壤速效磷和速效钾含量分别比对照增加了4.3倍、3.8倍,土壤全氮和有机质含量显著降低。飞机草入侵显著改变了土壤理化性状,导致喀斯特地区土壤物理结构退化,生态系统水土保持能力下降,同时飞机草通过改变土壤养分循环提高了土壤可直接利用养分的水平,创造对自身有利的土壤环境,进而促进其生长和扩散。     

薇甘菊入侵对中小型土壤动物群落 结构特征的影响

薇甘菊是菊科假泽兰属的恶性杂草,在我国华南地区已成功入侵并造成严重危害.为了解薇甘菊入侵对土壤动物的影响效应,采用野外样地试验法对广州市火炉山森林公园薇甘菊入侵群落的土壤动物进行了调查.2009年4次采样共获得中小型土壤动物5206头,隶属于4门10纲19类,其中线虫类为优势类群,蜱螨目、弹尾目和轮虫是常见类群.薇甘菊入侵地中小型土壤动物群落结构特征发生了改变,与其他样区相比,入侵区中小型土壤动物的总个体数以及线虫类、蜱螨目的个体数显著增加,但中小型土壤动物类群数的变化不明显;在群落多样性方面,薇甘菊入侵区土壤动物群落的密度-类群指数显著提高,物种丰富度、优势度指数亦有所上升,但差异不明显,而均匀度、Shannon指数趋于下降;在群落相似性方面,薇甘菊入侵区与群落交错区的相似程度较高,与土著植物区的相似程度较低.薇甘菊入侵所引起的局部气候环境、凋落物、根系分泌物与土壤理化性质的变化可能是造成中小型土壤动物群落结构特征改变的主要原因.     

薇甘菊入侵对中小型土壤动物群落结构特征的影响

薇甘菊是菊科假泽兰属的恶性杂草,在我国华南地区已成功入侵并造成严重危害.为了解薇甘菊入侵对土壤动物的影响效应,采用野外样地试验法对广州市火炉山森林公园薇甘菊入侵群落的土壤动物进行了调查.2009年4次采样共获得中小型土壤动物5206头,隶属于4门10纲19类,其中线虫类为优势类群,蜱螨目、弹尾目和轮虫是常见类群.薇甘菊入侵地中小型土壤动物群落结构特征发生了改变,与其他样区相比,入侵区中小型土壤动物的总个体数以及线虫类、蜱螨目的个体数显著增加,但中小型土壤动物类群数的变化不明显;在群落多样性方面,薇甘菊入侵区土壤动物群落的密度-类群指数显著提高,物种丰富度、优势度指数亦有所上升,但差异不明显,而均匀度、Shannon指数趋于下降;在群落相似性方面,薇甘菊入侵区与群落交错区的相似程度较高,与土著植物区的相似程度较低.薇甘菊入侵所引起的局部气候环境、凋落物、根系分泌物与土壤理化性质的变化可能是造成中小型土壤动物群落结构特征改变的主要原因.     

黄顶菊入侵对土壤养分和酶活性的影响

比较了外来植物黄顶菊不同入侵程度土壤养分和土壤酶活性变化规律,探讨了外来植物入侵对土壤生态的影响机制。结果表明,与裸土和本地植物土壤相比,黄顶菊入侵显著提高了有机质、全氮、硝态氮和铵态氮的含量,而全磷和速效磷的含量有所下降,且随着入侵程度增强趋势更为明显。重度入侵土壤有机质较本地植物土壤提高5.7%,全氮提高23.4%;而重度入侵土壤全磷含量只有本地植物的85%,土壤速效磷含量则下降了50%。黄顶菊重度入侵土壤和轻度入侵土壤脲酶含量分别为0.04和0.03mg.g-1.24h-1,均显著高于裸土和本地植物土壤,土壤磷酸酶活性变化规律与之类似,而多酚氧化酶无明显的变化。黄顶菊入侵可以改变土壤养分和土壤酶活性,创造对自身生长有利的土壤环境,并借此增强其竞争能力,实现种群的进一步扩张。     

红火蚁入侵对荔枝园土壤性理化质的影响

对红火蚁Solenopsis invicta Buren荔枝园发生区土壤理化性质进行了研究,结果表明:3类红火蚁不同发生区之间、红火蚁蚁巢4个不同方位之间以及距红火蚁蚁巢中心不同距离之间的土壤各项理化性质均有显著的差异(p＜0.01).从对照区、轻度发生区到重度发生区土壤粘粒变得越来越细,酸性逐渐升高.土壤有机质、碱解N、有效P的含量在轻、重度发生区较对照区分别下降了29.0%与39.9%、42.1%与53.6%、43.3%与52.6%,但速效K的含量在轻、重度发生区较对照区则分别升高了8.5%与34.0%.此外,在红火蚁发生区,蚁巢上的土壤理化性质与蚁巢以外的土壤理化性质明显不同(p＜0.01).蚁巢上的土壤明显比蚁巢以外的土壤酸性强.土壤有机质、碱解N、有效P的含量在蚁巢上明显最小,但土壤速效K的含量在蚁巢上明显最高.蚁巢不同方位上土壤的理化性质相对差异较小,红火蚁在荔枝园的分布存在一定的空间随机性.     

中度石漠化生态修复初期土壤螨类群落结构变化

土壤螨类群落监测是生态恢复监测的重要内容之一,土壤螨类群落结构的变化可以反映退化环境生态恢复的效果.2014年按季节对贵州省毕节市朝营小流域中度石漠化生态修复区的土壤螨类进行调查,并与原生栓皮栎林和中度石漠化区的土壤螨类群落结构进行对比.结果表明: 中度石漠化生态修复区土壤螨类计有3目35科58属,以单翼甲螨属和威单翼甲螨属为优势属;土壤上层具有较高的属数、个体数量和个体密度;捕食性革螨群落以r对策者为主,甲螨群落结构以有翅孔背甲螨型(P型)为主.中度石漠化生态恢复区土壤螨类个体数量与个体密度分别高于中度石漠化区和栓皮栎林,属数、多样性指数和丰富度指数分别高于中度石漠化区,但低于栓皮栎林;该区在对照区基础上恢复和新增螨类共32属,占该区螨类总属数的55.2%.在研究区,螨类属数与土壤速效钾含量、丰富度指数与有机碳含量具有显著相关性,个体数量、个体密度、多样性指数、丰富度指数分别与速效钾含量以及丰富度指数与有机质含量具有显著相关性.表明石漠化的植被修复有利于土壤螨类群落结构的恢复和土壤环境的改善,且螨类优势类群、新增类群和恢复类群能对环境变化进行较好地指示,但土壤螨类群落结构与修复区植被和土壤理化因子变化的关系还有待进一步研究.     

台州市路桥区重金属污染对土壤动物群落结构的影响

通过对台州市路桥区峰江再生园区及周围的8个样地采样调查,分析了重金属污染对土壤动物多样性的影响。对研究区域进行大量取样调查并测定土样中Cu、Zn、Pb、Cd 的含量。计算出重金属综合污染指数,运用地理信息系统(GIS)技术中的插值法根据重金属综合污染指数制作重金属综合污染趋势图。结合重金属综合污染趋势图与实地勘察,共设置8个采样地。研究方法为大型土壤动物采用野外分层手捡计数,中小型土壤动物用取土器分层取土,带回室内分别用干湿漏斗分离并镜检。结果表明:(1)8个样地共获得土壤动物844只,分属于4门9纲20个类群,其中优势类群为蜱螨目、弹尾目和线虫纲,其他为常见类群和稀有类群。(2)8个样地中土壤动物的个体数量存在极显著性差异(P<0.01),类群数存在极显著性差异(P<0.01)。在严重污染区,土壤动物的数量和类群数稀少,而轻度污染区土壤动物的密度大、群落多样性高,稀有类群大量出现。表明重金属污染能对土壤动物多样性构成严重影响,土壤动物群落的类群和个体数量随污染程度的加重而减少(3)用群落多样性指标DG衡量重金属污染对土壤动物多样性的影响,结果为随着综合污染指数的减小,DG指数逐渐增大。(4)用简单相关性分析表明土壤动物数量变化与综合污染指数具有极显著负相关性(相关系数-0.86),与有机质含量有一定相关性(相关系数0.42)。在轻度污染的样地中,土壤动物的数量随有机质含量的升高而上升,表明在严重的污染区,重金属的毒性对土壤动物的影响是主要的,而在轻度污染区,有机质含量可能成为影响土壤动物多样性的主要因素。(5)从弹尾目的等节跳科和球角跳科在8个样地中的分布特点可知,本地区的等节跳科对重金属污染的耐受力最强。球角跳科数量大、密度高,对重金属污染较敏感,可作为重金属污染的指示生物。     

紫茎泽兰(Ageratina adenophora)入侵对土壤微生物群落和理化性质的影响

外来入侵植物与入侵地土壤微生物群落的互作关系是影响外来植物入侵力和生态系统可入侵性的一个重要领域。因此,研究外来植物入侵对入侵地土壤微生物群落及其理化性质的影响不仅可以全面地评估入侵植物对生态系统的影响,而且对于探索外来植物入侵的土壤微生物学机制尤为重要。采用磷脂脂肪酸(PLFAs)和传统培养相结合的方法研究了外来入侵植物紫茎泽兰对入侵地土壤微生物群落结构的影响;同时研究了紫茎泽兰入侵对11种土壤理化因子的影响。结果表明紫茎泽兰入侵改变了土壤微生物群落结构,提高了土壤自生固氮菌、氨氧化细菌和真菌的数量;同时,显著地提高了土壤的有效磷、速效钾、硝态氮、氨态氮和土壤有机碳含量,降低了土壤总钾含量和pH值。土壤微生物不同生理类群的变化与土壤中植物可直接吸收利用养分的变化显著相关。紫茎泽兰在入侵地成功定殖后,可能通过改变土壤微生物群落结构,特别是增加了与土壤养分循环相关的微生物功能类群数量,进而提高了土壤可利用的养分水平,创造对自身生长有利的土壤环境。紫茎泽兰改变土壤微生物群落是其入侵的一部分,这种改变进而加速了土壤养分循环,可能增强了紫茎泽兰的养分吸收,进而促进其生长、竞争和扩张。     

Different Degrees of Plant Invasion Significantly Affect the Richness of the Soil Fungal Community

Several studies have shown that soil microorganisms play a key role in the success of plant invasion. Thus, ecologists have become increasingly interested in understanding the ecological effects of biological invasion on soil microbial communities given continuing increase in the effects of invasive plants on native ecosystems. This paper aims to provide a relatively complete depiction of the characteristics of soil microbial communities under different degrees of plant invasion. Rhizospheric soils of the notorious invasive plant Wedelia trilobata with different degrees of invasion (uninvaded, low-degree, and high-degree using its coverage in the invaded ecosystems) were collected from five discrete areas in Hainan Province, P. R. China. Soil physicochemical properties and community structure of soil microorganisms were assessed. Low degrees of W. trilobata invasion significantly increased soil pH values whereas high degrees of invasion did not significantly affected soil pH values. Moreover, the degree of W. trilobata invasion exerted significant effects on soil Ca concentration but did not significantly change other indices of soil physicochemical properties. Low and high degrees of W. trilobata invasion increased the richness of the soil fungal community but did not pose obvious effects on the soil bacterial community. W. trilobata invasion also exerted obvious effects on the community structure of soil microorganisms that take part in soil nitrogen cycling. These changes in soil physicochemical properties and community structure of soil microbial communities mediated by different degrees of W. trilobata invasion may present significant functions in further facilitating the invasion process.     

Dissecting Solidago canadensis–soil feedback in its real invasion

The importance of plant–soil feedback (PSF) has long been recognized, but the current knowledge on PSF patterns and the related mechanisms mainly stems from laboratory experiments. We aimed at addressing PSF effects on community performance and their determinants using an invasive forb Solidago canadensis. To do so, we surveyed 81 pairs of invaded versus uninvaded plots, collected soil samples from these pairwise plots, and performed an experiment with microcosm plant communities. The magnitudes of conditioning soil abiotic properties and soil biotic properties by S. canadensis were similar, but the direction was opposite; altered abiotic and biotic properties influenced the production of subsequent S. canadensis communities and its abundance similarly. These processes shaped neutral S. canadensis–soil feedback effects at the community level. Additionally, the relative dominance of S. canadensis increased with its ability of competitive suppression in the absence and presence of S. canadensis–soil feedbacks, and S. canadensis‐induced decreases in native plant species did not alter soil properties directly. These findings provide a basis for understanding PSF effects and the related mechanisms in the field conditions and also highlight the importance of considering PSFs holistically.     

Responses of the soil fungal communities to the co‐invasion of two invasive species with different cover classes

• Soil fungal communities play an important role in the successful invasion of non‐native species. It is common for two or more invasive plant species to co‐occur in invaded ecosystems.
• This study aimed to determine the effects of co‐invasion of two invasive species (Erigeron annuus and Solidago canadensis) with different cover classes on soil fungal communities using high‐throughput sequencing.
• Invasion of E. annuus and/or S. canadensis had positive effects on the sequence number, operational taxonomic unit (OTU) richness, Shannon diversity, abundance‐based cover estimator (ACE index) and Chao1 index of soil fungal communities, but negative effects on the Simpson index. Thus, invasion of E. annuus and/or S. canadensis could increase diversity and richness of soil fungal communities but decrease dominance of some members of these communities, in part to facilitate plant further invasion, because high soil microbial diversity could increase soil functions and plant nutrient acquisition. Some soil fungal species grow well, whereas others tend to extinction after non‐native plant invasion with increasing invasion degree and presumably time. The sequence number, OTU richness, Shannon diversity, ACE index and Chao1 index of soil fungal communities were higher under co‐invasion of E. annuus and S. canadensis than under independent invasion of either individual species.
• The co‐invasion of the two invasive species had a positive synergistic effect on diversity and abundance of soil fungal communities, partly to build a soil microenvironment to enhance competitiveness of the invaders. The changed diversity and community under co‐invasion could modify resource availability and niche differentiation within the soil fungal communities, mediated by differences in leaf litter quality and quantity, which can support different fungal/microbial species in the soil.

     

An Invasive Plant Promotes Its Arbuscular Mycorrhizal Symbioses and Competitiveness through Its Secondary Metabolites: Indirect Evidence from Activated Carbon

Secondary metabolites released by invasive plants can increase their competitive ability by affecting native plants, herbivores, and pathogens at the invaded land. Whether these secondary metabolites affect the invasive plant itself, directly or indirectly through microorganisms, however, has not been well documented. Here we tested whether activated carbon (AC), a well-known absorbent for secondary metabolites, affect arbuscular mycorrhizal (AM) symbioses and competitive ability in an invasive plant. We conducted three experiments (experiments 1–3) with the invasive forb Solidago canadensis and the native Kummerowia striata. Experiment 1 determined whether AC altered soil properties, levels of the main secondary metabolites in the soil, plant growth, and AMF communities associated with S. canadensis and K. striata. Experiment 2 determined whether AC affected colonization of S. canadensis by five AMF, which were added to sterilized soil. Experiment 3 determined the competitive ability of S. canadensis in the presence and absence of AMF and AC. In experiment 1, AC greatly decreased the concentrations of the main secondary metabolites in soil, and the changes in concentrations were closely related with the changes of AMF in S. canadensis roots. In experiment 2, AC inhibited the AMF Glomus versiforme and G. geosporum but promoted G. mosseae and G. diaphanum in the soil and also in S. canadensis roots. In experiment 3, AC reduced S. canadensis competitive ability in the presence but not in the absence of AMF. Our results provided indirect evidence that the secondary metabolites (which can be absorbed by AC) of the invasive plant S. canadensis may promote S. canadensis competitiveness by enhancing its own AMF symbionts.     

Impact of Parthenium hysterophorus L. invasion on plant species composition and soil properties of grassland communities in Nepal

Parthenium hysterophorus (Asteraceae) is a noxious plant that is considered one of the most invasive species in the world. We studied changes in the composition of plant species and soil properties related to the invasion of P. hysterophorus in three grassland communities of central Nepal. We collected vegetation and soil data along transects that were established in densely invaded to non-invaded areas within homogenous grassland stands. We found significant differences between invaded, transitional and non-invaded plots in species composition and soil properties. There were fewer species in non-invaded than transitional and invaded plots. By P. hysterophorus invasion both native and non-native species were supported or replaced, respectively. The concentrations of soil nitrogen and organic matter were significantly higher in transitional and invaded plots than in non-invaded plots. Soil pH, phosphorus and potassium were highest in the invaded plots, lowest in the non-invaded and intermediate in the transitional plots. Due to changes in above-ground vegetation and below-ground soil nutrient contents, P. hysterophorus invasion is likely to have an overall negative effect on the functioning of the entire ecosystem. Therefore, management of noxious P. hysterophorus is necessary to prevent future problems.     

Ant communities and Solidago plant invasion: Environmental properties and food sources

The invasion of Solidago is one of the main threats to the biodiversity of natural meadows, leading to changes in animal and plant communities, as well as soil features. We compared effects of soil microclimatic conditions (temperature and moisture) and the availability of potential protein sources (dry mass of epigean invertebrates) on ants between meadows invaded by Solidago altissima and S. canadensis and those uninvaded. Our results showed that the ant communities were different between the uninvaded and invaded meadows, with reduction of ant abundance and species richness in the latter. Myrmica spp. were abundant in the uninvaded meadows, whereas Lasius niger was the dominant species in the invaded ones. We found that the lower moisture negatively influenced the abundance of Myrmica species in the Solidago‐invaded meadows. Moreover, the epigean invertebrate dry mass, as an estimation of the availability of protein sources, varied between the two types of meadows, with a higher abundance in the uninvaded ones. The abundance of Myrmica ants with narrower ecological requirements showed a positive correlation with the invertebrate biomass in the invaded meadows. In contrast, the abundance of L. niger with broad ecological requirements was negatively correlated with the invertebrate biomass in the invaded meadows, possibly as a strategy to reduce interspecific competition. Our study showed that the invasion of Solidago plants caused changes in the abundance and species composition of ant communities through modification in microhabitat conditions, that is, decreasing soil moisture, reducing biomass and changing distribution of prey invertebrates.     

Soil characteristics of Rocky Mountain National Park grasslands invaded by Melilotus officinalis and M. alba

Aim Invasion of nitrogen‐fixing non‐native plant species may alter soil resources and impact native plant communities. Altered soils may be the driving mechanism that provides a suitable environment to facilitate future invasions and decrease native biodiversity. We hypothesized that Melilotus invasion would increase nitrogen availability and produce soil microclimate and biochemical changes, which could in turn alter plant species composition in a montane grassland community. Location Our research addressed the effects of white and yellow sweet clover (Melilotus officinalis and M. alba) invasion on soil characteristics and nitrogen processes in the montane grasslands in Rocky Mountain National Park. Methods We sampled soil in replicate sites of Melilotus‐invaded and control (non‐invaded) patches within disturbed areas in montane grassland habitats. Soil composites were analysed for available nitrogen, net nitrogen mineralization, moisture, carbon/nitrogen (C : N ratio), texture, organic matter and pH. Data were recorded at three sample dates during the growing seasons of 1998 and 1999. Results Contrary to our expectations, we observed lower nitrogen availability and mineralization in invaded patches, and differences in soil moisture content and soil C : N. Soil C : N ratios were higher in invaded plots, in spite of the fact that Melilotus had the lowest C : N ratios of other plant tissue analysed in this study. Main conclusions These findings provide land managers of natural areas with a better perspective on the possibilities of nitrogen‐fixing species impact on soil nutrient levels.     

Invasion by Solidago species has limited impacts on soil seed bank communities

Increasing attention in invasion biology is being paid to measuring and understanding the impacts of invasive species. For plant invasions, however, the impact of invasion on soil seed bank communities has been under-studied. At six sites in southern Germany, we investigated whether areas invaded by Solidago gigantea and Solidago canadensis experienced a reduction in seed bank species richness, size and diversity, and a change in species composition compared to adjacent uninvaded areas. We found no overall effect of invasion on seed bank size, or on species richness and diversity. Seed bank size significantly decreased from 0–5 cm to 5–10 cm depth in both invaded and uninvaded areas. A significant amount of variation in species composition was explained by invasion, but it was only one-tenth of that explained solely by site effects. Our study suggests that invasion by Solidago species may not have the same impacts on the soil seed banks of native species as other invasive perennial forbs that have so far been studied.     

加拿大一枝黄花的入侵机理研究进展

我国是世界上遭受外来生物入侵最为严重的国家之一。加拿大一枝黄花(Solidago canadensis L.)是一种世界性的入侵植物,侵入我国已有70多年的历史,目前国内外对其入侵机理已有许多研究。结合近年来的研究热点和重要进展,从加拿大一枝黄花的生物学特点、入侵地环境的可入侵性、化感效应以及植物与土壤的反馈作用等4个方面综述了其成功入侵的机理,分析和评价了各种机理之间的内在联系。认为生物入侵是一个涉及多个营养级的复杂生物学过程,是多种机制综合作用的结果,人类活动的干扰是入侵事件频发的深层次原因。加拿大一枝黄花强大的入侵能力、高度的抗逆性、对本地植物和土传病原菌的化感抑制、入侵地环境的脆弱性以及菌根真菌等有益微生物的协助在入侵过程中起着关键作用,但天敌释放并未使其获得更大竞争优势。