草地群落物种多样性维持机制的研究Ⅱ物种实现的生态位

尽管为解释种类丰富的植物群落物种共存和多样性维持机制,生态学家位做了大量的努力并提出了许多假说和模型。但这一问题仍处在争议之中,需要更多的证据支持他们的观点或提出新的看法,使这一生物多样性难题不断地向前推进。以松赖平原物种丰富度较高的羊草－杂类草群落为对象,在土壤C、N、P、K和H2O等5个资源轴上,探讨了物种多样性与实现生态位的关系。结果表明：尽管物种生态位存在一定程度的分化,但多数物种的生态位是高度重叠的,物种生态位的分化在草地群落物种共存和多样性维持中,不是唯一的途径,认为应更加重视的物种在长期协同进化中所形成的生物学特性。     

草地群落物种多样性维持机制的研究（Ⅱ）：物种实现生态位

尽管为解释种类丰富的植物群落物种共存和多样性维持机制,生态学家们做了大量的努力并提出了许多假说和模型,但这一问题仍处在争议之中,需要更多的证据支持他们的观点或提出新的看法,使这一生物多样性难题不断地向前推进。以松嫩平原物种丰富度较高的羊草—杂类草群落为对象,在土壤C、N、P、K和H2O等5个资源轴上,探讨了物种多样性与实现生态位的关系。结果表明：尽管物种生态位存在一定程度的分化,但多数物种的生态位是高度重叠的,物种生态位的分化在草地群落物种共存和多样性维持中,不是唯一的途径,认为应更加重视物种在长期协同进化中所形成的生物学特性。     

群落构建的中性理论和生态位理论

物种共存和生物多样性维持一直是生态学研究的中心论题。基于物种生态位分化的群落构建理论已经发展了近一个世纪, 但我们对群落构建和生物多样性维持的机理仍然不清楚。近年来, 群落中性理论以其简约性和预测能力成为群落生态学研究的焦点, 但由于其“物种在生态功能上等价”的假设与大量研究结果相悖, 同时对自然群落结构的准确预测也只限于少数的生态系统, 因而饱受质疑。如今, 越来越多的生态学家认为群落构建的生态位理论与中性理论之争的最终归宿应该是二者的整合。 在本文中, 我们在简要回顾生态位理论和群落中性理论发展的基础上, 分析二者之间的主要分歧和互补性, 试图梳理二者整合的途径。我们认为, 尽管中性理论的发展极大地丰富了群落构建理论, 但二者的整合尚处于初级阶段; 群落构建零模型假说、中性—生态位连续体假说、随机生态位假说等都不失为有价值的尝试, 今后需要在其他类型的生态系统中进行实验验证, 以更好地理解确定性过程和随机过程在决定群落构建和生物多样性维持中的作用。     

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。     

森林生态系统中植食性昆虫与寄主的互作机制、假说与证据

围绕"多样性稳定性"假说、"联合抗性假说"、"生长势假说"、"胁迫假说"、以及下调、上调和推拉等机制与假说提出的背景与实验验证的证据,力图辨析其概念以及它们之间的相互关系。作者认为,多样性-稳定性机制关注森林生态系统的功能,是基于群落甚至景观层次。多样性条件下的联合抗性机制和联合易感性应属于稳定性中的抵抗力范畴。联合抗性机制的主要基础是基于资源集中假说和天敌假说,这些观点在种群层次上更易理解;上调力和下调力机制是以食物网底部的资源与顶端的天敌来探讨这种互作关系。因此,资源集中与上调力有着对应关系,而天敌假说只是下调力机制中的一个层面而已。植物生长势假说和植物胁迫假说力图从植物个体或种的群体的生长状态出发解析植食性动物的对寄主的选择趋势。上述有关植食性昆虫与寄主互作的机制、假说与证据是基于不同的层面提出的,因而在解析研究目标时,由于基本面的差异有可能会得出不同的结论。以近年来的研究进展和研究成果为依据有针对性地阐述这些理论对森林有害生物生态调控技术的指导作用,其中,联合抗性和联合易感性理论对指导森林有害生物生态控制具有更直接的指导作用。进一步提出了相应的亟待解决的科学问题。     

病原菌及菌根菌维持森林植物多样性机制的研究进展

森林植物物种与土壤微生物之间的相互作用决定森林生物多样性。尤其是土壤中的病原菌、菌根菌等微生物在森林物种多样性保护中起到重要作用。Janzen-Connell假说和植物-土壤反馈模型是利用植物与土壤微生物之间的相互作用关系解释森林物种多样性机制的重要模型。本文针对土壤真菌解释森林物种多样性机制进行了综合评述，介绍了病原菌和菌根菌各自在森林实生苗更新的作用机制，探讨病原菌及其菌根菌两者对森林物种多样性的相对重要性，森林实生苗与土壤微生物之间相互作用的变化规律，总结了近年来国际森林物种多样性机制的研究。提出了病原菌和菌根菌两者共同在生态系统中的作用，并探讨未来的研究方向。     

云南高黎贡山两种同域分布雉类冬季的食物分析及取食策略

本文采用粪便显微分析法对高黎贡山同域分布的白尾梢虹雉(Lophophorus sclateri)与血雉(Ithaginis cruentus)进行食物组成研究,旨在弄清这两种濒危雉类越冬期间(12月—翌年2月)的食谱以及它们对食物资源的竞争状况。结果表明:白尾梢虹雉取食48种植物,血雉取食43种植物,两种雉类的食物多样性指数分别为3.21和3.35,食物生态位重叠指数达0.598,有27种植物被两种雉类共同取食,且在各自食谱中所占比例甚高;较高的食物多样性和食物生态位重叠度表明两种雉类冬季食谱拓宽,并暗示这两种雉类在冬季可能存在食物竞争;食物种类和取食部位的分化是它们得以同域分布和长期共存的基础;结合这两种雉类的食谱特征和野外取食行为,提出它们冬季的取食策略假说,白尾梢虹雉采取以植物根为食和雪后向低海拔迁移寻求替代食物这两种策略来应对冬季食物不足,而血雉则采取集群游荡以扩大种群取食范围的策略。     

植物对昆虫多样性的影响

昆虫多样性是生物多样性的重要组成部分,植物及植物群落对昆虫多样性有极为重要的影响。本文从植物不同方面对昆虫多样性影响进行阐述：1）植物高度与盖度的增大为昆虫提供更多的生存空间,增加了昆虫多样性;2）基因多样性高的植物种群能提供更大生态位,使昆虫多样性增加,个体数量增大;3）植物物种和功能群多样性增加,使得有更多的物种数量可以支持更多昆虫;4）植物生产力增加使昆虫需要的食物资源增加,昆虫多样性随之增大。已有相应假说和理论,解释植物对昆虫多样性影响机制。同时,指出了植物对昆虫多样影响存在问题以及今后发展方向。     

热带森林植物功能群及其动态研究进展

热带森林极高的物种多样性和结构复杂性给生态学研究带来了很多挑战。植物功能群是对特定环境响应相似或对主要的生态过程具有相似作用的物种组合。应用植物功能群的方法是有效减少热带森林群落复杂性,并揭示其格局和过程的良好途径。在介绍植物功能群概念和划分途径的基础上,分析了热带森林植物功能群的时空动态规律。一般来讲,划分植物功能群通常有3种途径,并可通过5个步骤来完成。热带森林植物功能群的空间分布常受景观格局的制约,而环境异质性往往是影响不同植物功能群组配比例变化的直接原因。不同类型的植物功能群随演替过程发生显著的替代,而干扰体系和全球气候变化对功能群的动态过程具有重要的驱动作用。以功能群为基础的动态模型在模拟热带林群落动态和预测植被潜在分布等方面具有广阔的发展前景。探索有效的植物功能分类方法、构建完善的植物功能性状数据库、开发更为精确的功能群动态模型以及加强基于景观水平的植物功能群动态机制的认识等是未来热带森林植物功能群及其动态研究的重要方向。     

丛枝菌根真菌多样性对植物群落构建和稀有种维持的研究进展

丛枝菌根(AM)是植物与微生物联系中最为古老的共生体,全球范围内约80%的陆生植物与AM真菌共生形成丛枝菌根。这一共生关系在气候稳定和土壤磷贫瘠的热带、亚热带森林中更为普遍。以往的研究表明AM真菌通过提高植物对磷的吸收促进植物生长和定植,即产生植物-土壤正反馈。植物-土壤正反馈可降低由土壤病原菌引起的植物-土壤负反馈,进而降低植物-土壤负反馈维持植物多样性的能力,这与热带、亚热带森林中极高的植物多样性以及占比惊人的稀有种相悖。随着对热带、亚热带森林中AM真菌多样性研究的不断深入,越来越多的研究发现AM真菌多样性在不同的生境条件下以及不同的宿主植物间存在较大差异,这些差异可引起植物适合度的不同,进而影响植物群落构建。该文整合了AM真菌在宿主植物群落构建、宿主植物共存及稀有种维持等方面的研究进展,以期为验证“稀有种优势”假说提出新的研究思路,进而更有效地保护稀有植物。     

Plant diversity in tropical forests: a review of mechanisms of species coexistence

Evidence concerning mechanisms hypothesized to explain species coexistence in hyper-diverse communities is reviewed for tropical forest plants. Three hypotheses receive strong support. Niche differences are evident from non-random spatial distributions along micro-topographic gradients and from a survivorship-growth tradeoff during regeneration. Host-specific pests reduce recruitment near reproductive adults (the Janzen-Connell effect), and, negative density dependence occurs over larger spatial scales among the more abundant species and may regulate their populations. A fourth hypothesis, that suppressed understory plants rarely come into competition with one another, has not been considered before and has profound implications for species coexistence. These hypotheses are mutually compatible. Infrequent competition among suppressed understory plants, niche differences, and Janzen-Connell effects may facilitate the coexistence of the many rare plant species found in tropical forests while negative density dependence regulates the few most successful and abundant species.     

Seedling interactions in a tropical forest in Panama

Competition is believed to be a central force limiting local diversity and controlling the structure of plant communities. However, it has been proposed that the stressed understory environment limits total understory plant density to such low levels that competitive exclusion cannot be an important factor limiting the local diversity of understory plants. To evaluate the importance of inter-seedling competition, we performed a seedling competition experiment with five shade-tolerant species in a tropical moist forest in Panama. Three-month-old seedlings were transplanted into the forest singly or with their roots intertwined with a single conspecific or heterospecific seedling in all pairwise species combinations. If competition is important, performance (survival, stem height, and number of leaves after one and six years) would be expected to be lowest with a conspecific neighbor and greatest without a neighbor. The experiment was replicated in five 0.24-m² plots at each of 20 sites in tall secondary forest. To test whether seedling performance differed among treatments we fitted linear mixed models (LMM) and generalized linear mixed models (GLMM), treating species identity and microsite (site and plot) as random effects. The five shade-tolerant study species all experienced good establishment with relatively high survival and growth rates. The neighbor treatment consistently affected seedling performance, but the effect was always very small, both in absolute terms and relative to the much stronger species and microsite effects. Seedlings with a conspecific neighbor consistently performed worse than seedlings with a heterospecific neighbor, but having no neighbor generally did not cause superior performance relative to the other treatments. We conclude that direct competitive interactions are relatively unimportant among understory plants in humid tropical forests.     

Phytochemical diversity impacts herbivory in a tropical rainforest tree community

Metabolomics provides an unprecedented window into diverse plant secondary metabolites that represent a potentially critical niche dimension in tropical forests underlying species coexistence. Here, we used untargeted metabolomics to evaluate chemical composition of 358 tree species and its relationship with phylogeny and variation in light environment, soil nutrients, and insect herbivore leaf damage in a tropical rainforest plot. We report no phylogenetic signal in most compound classes, indicating rapid diversification in tree metabolomes. We found that locally co-occurring species were more chemically dissimilar than random and that local chemical dispersion and metabolite diversity were associated with lower herbivory, especially that of specialist insect herbivores. Our results highlight the role of secondary metabolites in mediating plant–herbivore interactions and their potential to facilitate niche differentiation in a manner that contributes to species coexistence. Furthermore, our findings suggest that specialist herbivore pressure is an important mechanism promoting phytochemical diversity in tropical forests.     

Small Canopy Gaps Influence Plant Distributions in the Rain Forest Understory1

This study tested three hypotheses regarding how plants respond to the spatial heterogeneity in light availability in the rain forest understory: (1) understory plants occur preferentially in the lighter parts of the understory; (2) under–story palms are more shade tolerant than other understory plants; (3) rain forest plants differ in their ontogenetic response to understory light conditions. The study was carried out in old–growth rain forest in the Yasuní National Park, Amazonian Ecuador. The hypotheses were tested by comparing the distributions of 20 plant species (1454 individuals) over microsites with differing degrees of exposure to canopy gaps to the background distribution of these microsites in the forest. The gap exposure of a given microsite was described by an index based on the number and size of gaps in the canopy to which the site was exposed. Two plant height classes were studied: 0.80–2.49 and 2.50–5 m. The first and third hypotheses were accepted, while the second hypothesis was rejected. The results for the individual species corresponded well with what is known from earlier studies about the ecology of these species or close relatives, suggesting that the patterns observed can be generalized for Neotropical rain forests. Notably, the most abundant species in the study represent several different life history strategies. Thus, abundance in the rain forest understory can be achieved by several different strategies. This suggests that niche differentiation in terms of response to small changes in understory light conditions may be an important factor in the maintenance of the high local plant species richness of tropical rain forests.     

Effects of experimental nitrogen additions on plant diversity in an old‐growth tropical forest

Response of plant biodiversity to increased availability of nitrogen (N) has been investigated in temperate and boreal forests, which are typically N‐limited, but little is known in tropical forests. We examined the effects of artificial N additions on plant diversity (species richness, density and cover) of the understory layer in an N saturated old‐growth tropical forest in southern China to test the following hypothesis: N additions decrease plant diversity in N saturated tropical forests primarily from N‐mediated changes in soil properties. Experimental additions of N were administered at the following levels from July 2003 to July 2008: no addition (Control); 50 kg N ha^?1 yr^?1 (Low‐N); 100 kg N ha^?1 yr^?1 (Medium‐N), and 150 kg N ha^?1 yr^?1 (High‐N). Results showed that no understory species exhibited positive growth response to any level of N addition during the study period. Although low‐to‐medium levels of N addition (≤100 kg N ha^?1 yr^?1) generally did not alter plant diversity through time, high levels of N addition significantly reduced species diversity. This decrease was most closely related to declines within tree seedling and fern functional groups, as well as to significant increases in soil acidity and Al mobility, and decreases in Ca availability and fine‐root biomass. This mechanism for loss of biodiversity provides sharp contrast to competition‐based mechanisms suggested in studies of understory communities in other forests. Our results suggest that high‐N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition.     

Does plant diversity increase top–down control of herbivorous insects in tropical forest?

Higher trophic level interactions are key mediators of ecosystem functioning in tropical forests. A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underlying such effects. The 'enemies hypothesis’ states that predators exert more effective top–down control of herbivorous insects with increasing plant diversity. Support for this hypothesis has been found in temperate forests and agroecosystems, but remains understudied in tropical forests. We compared incidence of attacks of different natural enemies using artificial caterpillars in a tropical forest landscape and investigated the role of plant community structure (i.e. species richness, composition and density), and the role of forest fragmentation (i.e. patch size, edge distance and canopy openness) on predation intensity. Plant community effects were tested with respect to three vegetation strata: trees, saplings and herbs. Observed predation was substantially due to ants. Predation rates increased with plant species richness for trees and herbs. Density of saplings, herb cover and herb species composition were important factors for predation. No significant patterns were found for fragmentation parameters, suggesting that forest fragmentation has not altered predation intensity. We conclude that in tropical forests, top–down control of herbivorous insects in the understory vegetation is affected by a combination of plant diversity, plant species composition and structural features of the plant community.     

Seasonal drought, soil fertility and the species density of tropical forest plant communities

Recent comparisons of plant species densities in tropical forest make it possible to evaluate factors that govern species richness. Contrary to earlier predictions, plant species densities are not greater on soils of relatively low fertility. In fact, the opposite trend is often observed, although the relationship between species densities and soil fertility is highly variable. However, tropical forest plant species densities consistently increase with rainfall. Species coexistence in wetter tropical forests may be facilitated by the absence of competition for moisture combined with year-round pest pressure and low understory light levels, which reduce growth rates and the potential for competition for other resources.     

Canopy Openness Enhances Diversity of Ant–Plant Interactions in the Brazilian Amazon Rain Forest

In closed‐canopy tropical forest understory, light availability is a significant determinant of habitat diversity because canopy structure is highly variable in most tropical forests. Consequently, variation in canopy cover affects the composition and distribution of plant species via creating variable light environments. Nevertheless, little is known about how variation in canopy openness structures patterns of plant–animal interactions. Because of the great diversity and dominance of ants in tropical environments, we used ant–plant interactions as a focal network to evaluate how variation in canopy cover influences patterns of plant–insect interactions in the Brazilian Amazon rain forest. We observed that small increases in canopy openness are associated with increased diversity of ant–plant interactions in our study area, and this change is independent of plant or ant species richness. Additionally, we found smaller niche overlap for both ants and plants associated with greater canopy openness. We hypothesize that enhanced light availability increases the breadth of ant foraging sources because variation in light availability gives rise to plant resources of different quality and amounts. Moreover, greater light availability promotes vegetative growth in plants, creating ant foraging ‘bridges’ between plants. In sum, our results highlight the importance of environmental heterogeneity as a determinant of ant–plant interaction diversity in tropical environments.     

Floristic diversity of sabal palmetto woodland: an endemic and endangered vegetation type from Mexico

The sabal palmetto woodland is a tropical plant formation dominated by Sabal mexicana, with restricted distribution to southeast Mexico. Sabal palms grow on poor soils but accumulate large quantities of organic substrate in their crowns, harboring a contingent of plants that use it as phorophyte. Although it is a threatened formation, basic information on its biodiversity is scant. We examined the floristic diversity of recruited (diameter at breast height, DBH, ≥1 cm) and understory (DBH ≤ 1 cm) plants, and its variation with anthropogenic disturbance. We also examined the floristic diversity of plants that use the sabal palms as phorophytes, and assessed its variation with human impact. All plants present in transects within a conserved and an adjacent perturbed area were sampled. The list of observed taxa shows that this vegetation has a clear affinity with tropical dry and wet forests of Mexico, with a small representation of taxa from desert ecosystems. The floristic contingent included a total of 81 species in 2000 m². Richness, composition and diversity were affected by disturbance. Recruited and understory vegetation in the disturbed site were 5- and 1.6-times less diverse than in the conserved site, and species of mature, conserved vegetation were substituted by heliophytes in the disturbed site. In contrast, abundance of palms and diversity and identity of epiphytic/hemiepiphytic plants were not affected by disturbance. We show that even monodominated tropical ecosystems growing on poor soils have a high floristic diversity and that current anthropogenic impact threatens not only species and populations but also entire plant formations.     

Diversity and composition of understory vegetation in the tropical seasonal rain forest of Xishuangbanna, SW China

Tropical forests vegetation and community research have tended to focus on the tree component, and limited attention has been paid to understory vegetation. Species diversity and composition of the understory of tropical seasonal rain forest were inventoried in a 625 m2 area (for sapling layer) and a 100 m2 area (for herb/seedling layer) in three 1 ha plots. We found 3068 individuals belonging to 309 species, 192 genera and 89 families. The most important family as determined by the Family Importance Value (FIV) was Rubiaceae in both sapling and herb/seedling layers. In terms of Importance Value Index (IVI), the shrub Mycetia gracilis (Rubiaceae) was the most important species in the sapling layer and the pteridophyte Selaginella delicatula (Selaginellaceae) was the most ecological significant species in the herb/seedling layer. Much more vascular plant species were registered in the understory than in the tree layer totaled among the three plots. The species diversity did not differ significantly among the tree layer, sapling layer and herb/seedling layer. Given that we still know little about the understory plant community for growth forms other than trees, the results from the present study indicate that more attention should be paid to the understory vegetation during the decision-making process for biodiversity conservation in the tropical forests.