We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.

Most plants form root symbioses with arbuscular mycorrhizal (AM) fungi, which provide them with phosphate and other nutrients. High soil phosphate levels are known to affect AM symbiosis negatively, but the underlying mechanisms are not understood. This report describes experimental conditions which triggered a novel mycorrhizal phenotype under high phosphate supply: the interaction between pea and two different AM fungi was almost completely abolished at a very early stage, prior to the formation of hyphopodia. As demonstrated by split-root experiments, down-regulation of AM symbiosis occurred at least partly in response to plant-derived signals. Early signalling events were examined with a focus on strigolactones, compounds which stimulate pre-symbiotic fungal growth and metabolism. Strigolactones were also recently identified as novel plant hormones contributing to the control of shoot branching. Root exudates of plants grown under high phosphate lost their ability to stimulate AM fungi and lacked strigolactones. In addition, a systemic down-regulation of strigolactone release by high phosphate supply was demonstrated using split-root systems. Nevertheless, supplementation with exogenous strigolactones failed to restore root colonization under high phosphate. This observation does not exclude a contribution of strigolactones to the regulation of AM symbiosis by phosphate, but indicates that they are not the only factor involved. Together, the results suggest the existence of additional early signals that may control the differentiation of hyphopodia.

In forests, negative density/distance-dependent seedling mortality (NDD) caused by natural enemies plays a key role in maintaining species diversity [Janzen-Connell (J-C) model]. However, the relative importance of natural enemies in mediating NDD under heterogeneous light conditions has remained unclear. We examined the relative importance of pathogens (i.e., soil pathogens, leaf diseases) on seedling performance in forest understories (FUs) and gaps (gaps) during a 3-year period (results of first year of our study have been previously reported). For the hardwood, Prunus grayana, we investigated seedling mortality, morbidity agents, growth, and root infection by arbuscular mycorrhizal fungi (AMF) beneath conspecific and heterospecific adults in FUs and gaps. Seedling mortality was higher beneath conspecific than heterospecific adults throughout 3 years at both sites, mainly due to continuous leaf disease (i.e., angular leaf spot), whereas damping-off diseases caused mortality only in the first year. Beneath each adult, seedling mortality was higher in FUs than in gaps until second year, but it did not differ between two habitat types in the third year, because leaf diseases caused severe damage even in gaps. Seedling mass was significantly lower beneath conspecific adults. AMF infection of seedlings was also lower beneath conspecific adults, while it was higher in gaps than in FUs beneath both adults. This study demonstrates that the J-C model in a hardwood tree, P. grayana is mainly driven by high NDD seedling mortality caused by airborne leaf diseases, which continuously attack seedlings in a NDD manner regardless of environmental light conditions.

Many studies in recent years have investigated the effects of climate change on the future of biodiversity. In this review, we first examine the different possible effects of climate change that can operate at individual, population, species, community, ecosystem and biome scales, notably showing that species can respond to climate change challenges by shifting their climatic niche along three non-exclusive axes: time (e.g. phenology), space (e.g. range) and self (e.g. physiology). Then, we present the principal specificities and caveats of the most common approaches used to estimate future biodiversity at global and sub-continental scales and we synthesise their results. Finally, we highlight several challenges for future research both in theoretical and applied realms. Overall, our review shows that current estimates are very variable, depending on the method, taxonomic group, biodiversity loss metrics, spatial scales and time periods considered. Yet, the majority of models indicate alarming consequences for biodiversity, with the worst-case scenarios leading to extinction rates that would qualify as the sixth mass extinction in the history of the earth.© 2012 Blackwell Publishing Ltd/CNRS.

Plants can develop an enhanced defensive capacity in response to infection by arbuscular mycorrhizal fungi (AMF). This 'mycorrhiza-induced resistance' (MIR) provides systemic protection against a wide range of attackers and shares characteristics with systemic acquired resistance (SAR) after pathogen infection and induced systemic resistance (ISR) following root colonisation by non-pathogenic rhizobacteria. It is commonly assumed that fungal stimulation of the plant immune system is solely responsible for MIR. In this opinion article, we present a novel model of MIR that integrates different aspects of the induced resistance phenomenon. We propose that MIR is a cumulative effect of direct plant responses to mycorrhizal infection and indirect immune responses to ISR-eliciting rhizobacteria in the mycorrhizosphere. Crown Copyright © 2013. Published by Elsevier Ltd. All rights reserved.

The mechanisms underlying interspecific variation in conspecific negative density dependence (CNDD) are poorly understood. Using a multilevel modeling approach, we combined long-term seedling demographic data from a subtropical forest plot with soil fungal community data by means of DNA sequencing to address the feedback of various guilds of soil fungi on the density dependence of trees. We show that mycorrhizal type mediates tree neighborhood interactions at the community level, and much of the interspecific variation in CNDD is explained by how tree species differ in their fungal density accumulation rates as they grow. Species with higher accumulation rates of pathogenic fungi suffered more from CNDD, whereas species with lower CNDD had higher accumulation rates of ectomycorrhizal fungi, suggesting that mutualistic and pathogenic fungi play important but opposing roles.Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

群落内的多物种如何共存是群落生态学的核心研究内容之一。经典的物种共存理论强调物种之间的生态位分化, 注重具体共存机制的研究。这种以具体共存机制为研究对象的方法一定程度上促进了当代物种共存理论框架的形成。在当代物种共存理论框架下, 物种间的差异被划分为两类综合性的抽象差异——生态位差异和平均适合度差异, 前者促进物种共存, 对应稳定化机制; 后者导致竞争排除, 对应均等化机制。本文在简要回顾经典物种共存理论的基础上, 介绍了当代物种共存理论的框架(包括理论的形成和定义)、基于该理论的部分实验验证工作及其在一些重要生态学问题中的应用。当代物种共存理论不仅揭示了群落内物种是如何共存的这一基本理论问题, 更重要的是在全球变化的背景下该理论对生物多样性的保护和管理具有重要的应用价值。期望本文的介绍有助于国内生态学和生物多样性工作者了解当代物种共存理论, 并将其应用于群落构建和生物多样性维持机制等方面的研究。

The factors determining species commonness and rarity are poorly understood, particularly in highly diverse communities. Theory predicts that interactions with neighbors of the same (conspecific) and other (heterospecific) species can influence a species' relative abundance, but empirical tests are lacking. By using a hierarchical model of survival for more than 30,000 seedlings of 180 tropical tree species on Barro Colorado Island, Panama, we tested whether species' sensitivity to neighboring individuals relates to their relative abundance in the community. We found wide variation among species in the effect of conspecific, but not heterospecific, neighbors on survival, and we found a significant relationship between the strength of conspecific neighbor effects and species abundance. Specifically, rare species suffered more from the presence of conspecific neighbors than common species did, suggesting that conspecific density dependence shapes species abundances in diverse communities.

Tropical forests are renowned for their high diversity, yet in many sites a single tree species accounts for the majority of the individuals in a stand. An explanation for these monodominant forests remains elusive, but may be linked to mycorrhizal symbioses. We tested three hypotheses by which ectomycorrhizas might facilitate the dominance of the tree, Oreomunnea mexicana, in montane tropical forest in Panama. We tested whether access to ectomycorrhizal networks improved growth and survival of seedlings, evaluated whether ectomycorrhizal fungi promote seedling growth via positive plant-soil feedback, and measured whether Oreomunnea reduced inorganic nitrogen availability. We found no evidence that Oreomunnea benefits from ectomycorrhizal networks or plant-soil feedback. However, we found three-fold higher soil nitrate and ammonium concentrations outside than inside Oreomunnea-dominated forest and a correlation between soil nitrate and Oreomunnea abundance in plots. Ectomycorrhizal effects on nitrogen cycling might therefore provide an explanation for the monodominance of ectomycorrhizal tree species worldwide. © 2016 John Wiley & Sons Ltd/CNRS.

Plant traits associated with alien invasiveness may also distinguish rare from common native species. To test this, we grew 23 native (9 common, 14 rare) and 18 alien (8 common, 10 rare) herbaceous species in Switzerland from six plant families under nutrient-addition and competition treatments. Alien and common species achieved greater biomass than native and rare species did overall respectively. Across alien and native origins, common species increased total biomass more strongly in response to nutrient addition than rare species did and this difference was not confounded by habitat dissimilarities. There was a weak tendency for common species to survive competition better than rare species, which was also independent of origin. Overall, our study suggests that common alien and native plant species are not fundamentally different in their responses to nutrient addition and competition.© 2012 Blackwell Publishing Ltd/CNRS.

Ecological theory predicts that the high local diversity observed in tropical forests is maintained by negative density-dependent interactions within and between closely related plant species. By using long-term data on tree growth and survival for coexisting (Fabaceae, Mimosoideae) congeners, we tested two mechanisms thought to underlie negative density dependence (NDD): competition for resources and attack by herbivores. We quantified the similarity of neighbors in terms of key ecological traits that mediate these interactions, as well as the similarity of herbivore communities. We show that phytochemical similarity and shared herbivore communities are associated with decreased growth and survival at the sapling stage, a key bottleneck in the life cycle of tropical trees. None of the traits associated with resource acquisition affect plant performance, indicating that competition between neighbors may not shape local tree diversity. These results suggest that herbivore pressure is the primary mechanism driving NDD at the sapling stage.Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Background and aimsThe concept of plant-soil feedback is increasingly used to explain plant community assembly processes. Soil nutrient availability can be expected to play a critical role on these processes. However, little is known about the effects of nutrient availability on feedback direction and strength.MethodsA plant-soil feedback experiment was performed with the grasses Anthoxanthum odoratum and Festuca rubra, and the forbs Leontodon hispidus and Plantago lanceolata, on soil with either low or high nutrient availability. Additionally, we tested if plant-soil feedback of the two forbs under these conditions changed by inoculation of the soil with spores of an arbuscular mycorrhizal fungus.ResultsIncreased nutrient availability neutralised plant-soil feedback based on shoot biomass independent of its negative or positive direction, whereas the effects on root biomass were either not altered or turned negative. Mycorrhizal fungi spore addition decreased negative feedback and increased positive feedback.ConclusionsOur results suggest that negative plant-soil feedback on low nutrient soil can be overcome with nutrient addition, and that positive soil biota associations on low nutrient soil may become superfluous with nutrient increase. We hypothesize that species-specific, microbial mediated plant community assembly processes occur in low rather than high nutrient environments.

Conspecific negative density-dependent establishment, in which local abundance negatively affects establishment of conspecific seedlings through host-specific enemies, can influence species diversity of plant communities, but the generality of this process is not well understood. We tested the strength of density dependence using the United States Forest Service's Forest Inventory and Analysis database containing 151 species from more than 200,000 forest plots spanning 4,000,000 square kilometers. We found that most species experienced conspecific negative density dependence (CNDD), but there was little effect of heterospecific density. Additionally, abundant species exhibited weaker CNDD than rarer species, and species-rich regions exhibited stronger CNDD than species-poor regions. Collectively, our results provide evidence that CNDD is a pervasive mechanism driving diversity across a gradient from boreal to subtropical forests.

Understanding why some species are rare while others are common remains a central and fascinating question in ecology. Recently, interactions with soil organisms have been shown to affect local abundances of plant species within communities, however, it is not known whether they might also drive patterns of rarity at large scales. Further, little is known about the specificity of soil-feedback effects, and whether closely related plants share more soil pathogens than more distantly related plants. In a multi-species soil-feedback experiment (using 19 species) we tested whether regionally and locally rare species differed in their response to soil biota. Regional rarity was measured using range size or IUCN status and local rarity by typical abundance within an area. All species were grown on soils trained by a variety of regionally and locally rare and common species, which also varied in their degree of relatedness to the target. We found that, in general, regionally rare species suffered more than twice as much from soil biota than regionally common species. Soil cultured by regionally rare species also had a more negative effect on subsequent plant growth, suggesting they may have also accumulated more pathogens. Local rarity did not predict feedback strength. Further, soil trained by closely related plants had a more negative effect on growth than soil trained by distant relatives, which indicates a phylogenetic signal in the host range of soil biota. We conclude that soil biota may well contribute to plant rarity at large spatial scales, which offers a novel explanation for plant rarity and commonness. Moreover, our results show that phylogenetic relatedness between plants was a good predictor of the likelihood that two plant species interacted negatively via soil biota, which might mean that soil pathogens could prevent the coexistence of closely related plants and could drive patterns of phylogenetic overdispersion. Our results suggest that soil pathogens could restrict the ability of rare species to shift their ranges and might need to be considered by conservation biologists seeking to protect populations of rare plants.© 2018 by the Ecological Society of America.

Hyperdiverse forests occur in the lowland tropics, whereas the most species-rich shrublands are found in regions such as south-western Australia (kwongan) and South Africa (fynbos). Despite large differences, these ecosystems share an important characteristic: their soils are strongly weathered and phosphorus (P) is a key growth-limiting nutrient. Soil-borne pathogens are increasingly being recognized as drivers of plant diversity in lowland tropical rainforests, but have received little attention in species-rich shrublands. We suggest a trade-off in which the species most proficient at acquiring P have ephemeral roots that are particularly susceptible to soil-borne pathogens. This could equalize out the differences in competitive ability among co-occurring species in these ecosystems, thus contributing to coexistence. Moreover, effective protection against soil-borne pathogens by ectomycorrhizal (ECM) fungi might explain the occurrence of monodominant stands of ECM trees and shrubs amongst otherwise species-rich communities. We identify gaps in our knowledge which need to be filled in order to evaluate a possible link between P limitation, fine root traits, soil-borne pathogens and local plant species diversity. Such a link may help to explain how numerous plant species can coexist in hyperdiverse rainforests and shrublands, and, conversely, how monodominant stands can develop in these ecosystems.© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.

Liu, Yu; Fang, Suqin; He, Fangliang Sun Yat Sen Univ, SYSU Alberta Joint Lab Biodivers Conservat, Guangzhou 510275, Guangdong, Peoples R China. Liu, Yu; Fang, Suqin Sun Yat Sen Univ, Sch Life Sci, Guangdong Key Lab Biodivers Dynam & Conservat, Guangzhou 510275, Guangdong, Peoples R China. Liu, Yu; Fang, Suqin Sun Yat Sen Univ, Sch Life Sci, State Key Lab Biocontrol, Guangzhou 510275, Guangdong, Peoples R China. Chesson, Peter Univ Arizona, Dept Ecol & Evolutionary Biol, Tucson, AZ 85721 USA. Chesson, Peter Natl Chung Hsing Univ, Dept Life Sci, Taichung 402, Taiwan. He, Fangliang Univ Alberta, Dept Renewable Resources, Edmonton, AB T6G 2H1, Canada.

1. The enemy-induced Janzen-Connell ( JC) effect, a classic model invoking conspecific negative density dependence (CNDD) and distance dependence, is a primary biodiversity maintenance hypothesis. Yet, conflicting evidence for the JC effect leads to disagreement about its role in maintaining forest diversity. 2. We focus this review on soil-borne pathogens, which are the primary agent inducing the JC effect in many forest ecosystems. Although the test of the pathogeninduced JC effect in ecology critically rests on the seedling mortality caused by soil pathogens, what has not been explicitly explored in the early literature but has increasingly received attention is the long-recognized fact that the environment can alter virulence of pathogens and host susceptibility (thus pathogen-host interactions), as predicted by the classic disease triangle framework enlightened by pathology research in agricultural systems. 3. Here, following the disease triangle framework we review evidence on how the pathogen-induced JC effect may be contingent on context (e.g. environmental conditions, pathogen inoculum load and genetic divergence in host and pathogen populations). The reviewed evidence reveals and clarifies the conditions where pathogens may or may not cause disease to hosts, thus contributing to reconciling the inconsistent results about the pathogen-induced JC effect in the literature. The context dependence of the disease triangle predicts that the pathogen-induced JC effect would change under global change. 4. Gaining insights from evidence that the pathogen-induced JC effect is context-dependent, we suggest that future tests on the JC hypothesis be conducted under the framework of disease triangle, and we stress the necessity by controlling the effect of context factors on plant-pathogen interactions when testing for the JC effect. We conclude the review by proposing three lines of future research for testing the importance of the JC effect in maintaining global forest tree species diversity, with a particular emphasis on testing the effect of global warming on the strength of pathogen-host interactions for better predicting changes of forest biodiversity under climate change.

In tropical forests, rarer species show increased sensitivity to species-specific soil pathogens and more negative effects of conspecific density on seedling survival (NDD). These patterns suggest a connection between ecology and immunity, perhaps because small population size disproportionately reduces genetic diversity of hyperdiverse loci such as immunity genes. In an experiment examining seedling roots from six species in one tropical tree community, we found that smaller populations have reduced amino acid diversity in pathogen resistance (R) genes but not the transcriptome in general. Normalized R gene amino acid diversity varied with local abundance and prior measures of differences in sensitivity to conspecific soil and NDD. After exposure to live soil, species with lower R gene diversity had reduced defence gene induction, more cosusceptibility of maternal cohorts to colonization by potentially pathogenic fungi, reduced root growth arrest (an R gene-mediated response) and their root-associated fungi showed lower induction of self-defence (antioxidants). Local abundance was not related to the ability to induce immune responses when pathogen recognition was bypassed by application of salicylic acid, a phytohormone that activates defence responses downstream of R gene signalling. These initial results support the hypothesis that smaller local tree populations have reduced R gene diversity and recognition-dependent immune responses, along with greater cosusceptibility to species-specific pathogens that may facilitate disease transmission and NDD. Locally rare species may be less able to increase their equilibrium abundance without genetic boosts to defence via immigration of novel R gene alleles from a larger and more diverse regional population.© 2017 John Wiley & Sons Ltd.

Plant-soil feedbacks and interspecific competition are ubiquitous interactions that strongly influence the performance of plants. Yet few studies have examined whether the strength of these interactions corresponds with the abundance of plant species in the field, or whether feedbacks and competition interact in ways that either ameliorate or exacerbate their effects in isolation. We sampled soil from two intermountain grassland communities where we also measured the relative abundance of plant species. In greenhouse experiments, we quantified the direction and magnitude of plant-soil feedbacks for 10 target species that spanned a range of abundances in the field. In soil from both sites, plant-soil feedbacks were mostly negative, with more abundant species suffering greater negative feedbacks than rare species. In contrast, the average response to competition for each species was unrelated with its abundance in the field. We also determined how competitive response varied among our target species when plants competed in live vs. sterile soil. Interspecific competition reduced plant size, but the strength of this negative effect was unchanged by plant-soil feedbacks. Finally, when plants competed interspecifically, we asked how conspecific-trained, heterospecific-trained, and sterile soil influenced the competitive responses of our target species and how this varied depending on whether target species were abundant or rare in the field. Here, we found that both abundant and rare species were not as harmed by competition when they grew in heterospecific-trained soil compared to when they grew in conspecific-cultured soil. Abundant species were also not as harmed by competition when growing in sterile vs. conspecific-trained soil, but this was not the case for rare species. Our results suggest that abundant plants accrue species-specific soil pathogens to a greater extent than rare species. Thus, negative feedbacks may be critical for preventing abundant species from becoming even more abundant than rare species.© 2016 by the Ecological Society of America.

Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.

Most tropical rain forests contain diverse arrays of tree species that form arbuscular mycorrhizae. In contrast, the less common monodominant rain forests, in which one tree species comprises more than 50% of the canopy, frequently contain ectomycorrhizal (ECM) associates. In this study, I explored the potential for common ECM networks, created by aggregations of ECM trees, to enhance seedling survivorship near parent trees. I determined the benefit conferred by the common ECM network on seedling growth and survivorship of an ECM monodominant species in Guyana. Seedlings with access to an ECM network had greater growth (73% greater), leaf number (55% more), and survivorship (47% greater) than seedlings without such access, suggesting that the ECM network provides a survivorship advantage. A survey of wild seedlings showed positive distance-dependent distribution and survival with respect to conspecific adults. These experimental and survey results suggest that the negative distance-dependent mechanisms at the seedling stage thought to maintain tropical rain forest diversity are reversed for ECM seedlings, which experience positive feedbacks from the ECM network. These results may in part explain the local monodominance of an ECM tree species within the matrix of high-diversity, tropical rain forest.

Crop rotation schemes are believed to work by preventing specialist soil-borne pests from depressing the future yields of similar crops. In ecology, such negative plant-soil feedbacks may be viewed as a type of Janzen-Connell effect, which promotes species coexistence and diversity by preventing the same species from repeatedly occupying a particular site. In a controlled greenhouse experiment with 24 plant species and using soils from established field monocultures, we reveal community-wide soil-based Janzen-Connell effects between the three major functional groups of plants in temperate European grasslands. The effects are much stronger and more prevalent if plants are grown in interspecific competition. Using several soil treatments (gamma irradiation, activated carbon, fungicide, fertilizer) we show that the mechanism of the negative feedback is the buildup of soil pathogens which reduce the competitive ability of nearly all species when grown on soils they have formerly occupied. We further show that the magnitude of the change in competitive outcome is sufficient to stabilize observed fitness differences between functional groups in reasonably large communities. The generality and strength of this negative feedback suggests that Janzen-Connell effects have been underestimated as drivers of plant diversity in temperate ecosystems.

Recent extinction rates are 100 to 1000 times their pre-human levels in well-known, but taxonomically diverse groups from widely different environments. If all species currently deemed "threatened" become extinct in the next century, then future extinction rates will be 10 times recent rates. Some threatened species will survive the century, but many species not now threatened will succumb. Regions rich in species found only within them (endemics) dominate the global patterns of extinction. Although new technology provides details of habitat losses, estimates of future extinctions are hampered by our limited knowledge of which areas are rich in endemics.

Globally, phosphorus (P) limits productivity of trees in many forests and plantations especially in highly weathered, acidic or calcareous profiles. Most trees form mycorrhizal associations which are prevalent in the organic and mineral soil horizons. This review critically examines mechanisms that enhance the acquisition of P by tree roots. Mycorrhizal roots have a greater capacity to take up phosphate (Pi) from the soil solution than non-mycorrhizal root tips. Factors that contribute to this include the extent of extraradical hyphal penetration of soil and the physiology and biochemistry of the fungal/soil and fungal/plant interfaces. Ectomycorrhizal (ECM) trees are likely to benefit from association with basidiomycetes that possess several high-affinity Pi transporters that are expressed in extraradical hyphae and whose expression is enhanced by P deficiency. To understand fully the role of these putative transporters in the symbiosis, data regarding their localization, Pi transport capacities and regulation are required. Some ECM fungi are able to effect release of Pi from insoluble mineral P through excretion of low-molecular-weight organic anions such as oxalate, but the relative contribution of insoluble P dissolution in situ remains to be quantified. How the production of oxalate is regulated by nitrogen remains a key question to be answered. Lastly, phosphatase release from mycorrhizas is likely to play a significant role in the acquisition of Pi from labile organic forms of P (Po). As labile forms of Po can constitute the major fraction of the total P in some tropical and temperate soils, a greater understanding of the forms of Po available to the phosphatases is warranted.

Empirical studies show that plant-soil feedbacks (PSF) can generate negative density dependent (NDD) recruitment capable of maintaining plant community diversity at landscape scales. However, the observation that common plants often exhibit relatively weaker NDD than rare plants at local scales is difficult to reconcile with the maintenance of overall plant diversity. We develop a spatially explicit simulation model that tracks the community dynamics of microbial mutualists, pathogens, and their plant hosts. We find that net PSF effects vary as a function of both host abundance and key microbial traits (e.g., host affinity) in ways that are compatible with both common plants exhibiting relatively weaker local NDD, while promoting overall species diversity. The model generates a series of testable predictions linking key microbial traits and the relative abundance of host species, to the strength and scale of PSF and overall plant community diversity.

A major challenge in ecology is to understand how the millions of species on Earth are organized into biological communities. Mechanisms promoting coexistence are one such class of organizing processes, which allow multiple species to persist in the same trophic level of a given web of species interactions. If some mechanism promotes the coexistence of two or more species, each species must be able to increase when it is rare and the others are at their typical abundances; this invasibility criterion is fundamental evidence for species coexistence regardless of the mechanism. In an attempt to evaluate the level of empirical support for coexistence mechanisms in nature, we surveyed the literature for empirical studies of coexistence at a local scale (i.e., species found living together in one place) to determine whether these studies satisfied the invasibility criterion. In our survey, only seven of 323 studies that drew conclusions about species coexistence evaluated invasibility in some way in either observational or experimental studies. In addition, only three other studies evaluated necessary but not sufficient conditions for invasibility (i.e., negative density dependence and a trade-off in performance that influences population regulation). These results indicate that, while species coexistence is a prevalent assumption for why species are able to live together in one place, critical empirical tests of this fundamental assumption of community structure are rarely performed. These tests are central to developing a more robust understanding of the relative contributions of both deterministic and stochastic processes structuring biological communities.

Rarity is a population characteristic that is usually associated with a high risk of extinction. We argue here, however, that chronically rare species (those with low population densities over many generations across their entire ranges) may have individual-level traits that make populations more resistant to extinction. The major obstacle to persistence at low density is successful fertilisation (union between egg and sperm), and chronically rare species are more likely to survive when (1) fertilisation occurs inside or close to an adult, (2) mate choice involves long-distance signals, (3) adults or their surrogate gamete dispersers are highly mobile, or (4) the two sexes are combined in a single individual. In contrast, external fertilisation and wind- or water-driven passive dispersal of gametes, or sluggish or sedentary adult life habits in the absence of gamete vectors, appear to be incompatible with sustained rarity. We suggest that the documented increase in frequency of these traits among marine genera over geological time could explain observed secular decreases in rates of background extinction. Unanswered questions remain about how common chronic rarity actually is, which traits are consistently associated with chronic rarity, and how chronically rare species are distributed among taxa, and among the world's ecosystems and regions.© 2017 John Wiley & Sons Ltd/CNRS.

负密度制约假说主要描述由于资源竞争、有害生物侵害(比如病原微生物、食草动物捕食)等, 同种个体之间发生的相互损害行为; 它主要强调同种个体之间的相互作用, 解释自然群落物种共存的机理; 负密度制约机制主要在小尺度上降低群落内同种个体生长率, 同时提高个体死亡率, 从而为其他物种的生存提供空间和资源, 促进物种共存。目前负密度制约假说的检验研究主要侧重密度制约、距离制约、群落补偿效应等三个方面。最近, 研究者又探讨了近缘物种之间由于对相似资源的竞争所产生的负效应, 扩展了负密度制约假说, 进而提出异群保护假说和谱系多样性制约假说。负密度制约假说引起生态学家长久的探讨和关注, 世界范围内大尺度森林动态样地的建立, 又为探索不同尺度上密度制约效应的研究提供了条件。然而目前的研究仍然存在不足, 比如负密度制约假说的检验受到其他因素的干扰、区域研究不平衡等。因此, 生态学家们仍然怀疑负密度制约效应调节群落物种共存的重要性, 但是目前的研究还没有发现否定负密度制约假说的充分证据。