Petroleum hydrocarbon contamination in boreal forest soils: a mycorrhizal ecosystems perspective

The importance of developing multi-disciplinary approaches to solving problems relating to anthropogenic pollution is now clearly appreciated by the scientific community, and this is especially evident in boreal ecosystems exposed to escalating threats of petroleum hydrocarbon (PHC) contamination through expanded natural resource extraction activities. This review aims to synthesize information regarding the fate and behaviour of PHCs in boreal forest soils in both ecological and sustainable management contexts. From this, we hope to evaluate potential management strategies, identify gaps in knowledge and guide future research. Our central premise is that mycorrhizal systems, the ubiquitous root symbiotic fungi and associated food-web communities, occupy the structural and functional interface between decomposition and primary production in northern forest ecosystems (i.e. underpin survival and productivity of the ecosystem as a whole), and, as such, are an appropriate focal point for such a synthesis. We provide pertinent basic information about mycorrhizas, followed by insights into the ecology of ecto- and ericoid mycorrhizal systems. Next, we review the fate and behaviour of PHCs in forest soils, with an emphasis on interactions with mycorrhizal fungi and associated bacteria. Finally, we summarize implications for ecosystem management. Although we have gained tremendous insights into understanding linkages between ecosystem functions and the various aspects of mycorrhizal diversity, very little is known regarding rhizosphere communities in PHC-contaminated soils. This makes it difficult to translate ecological knowledge into environmental management strategies. Further research is required to determine which fungal symbionts are likely to survive and compete in various ecosystems, whether certain fungal - plant associations gain in ecological importance following contamination events, and how PHC contamination may interfere with processes of nutrient acquisition and exchange and metabolic processes. Research is also needed to assess whether the metabolic capacity for intrinsic decomposition exists in these ecosystems, taking into account ecological variables such as presence of other organisms (and their involvement in syntrophic biodegradation), bioavailability and toxicity of mixtures of PHCs, and physical changes to the soil environment.     

A meta-analysis of biotic resistance to exotic plant invasions

Biotic resistance describes the ability of resident species in a community to reduce the success of exotic invasions. Although resistance is a well‐accepted phenomenon, less clear are the processes that contribute most to it, and whether those processes are strong enough to completely repel invaders. Current perceptions of strong, competition‐driven biotic resistance stem from classic ecological theory, Elton's formulation of ecological resistance, and the general acceptance of the enemies‐release hypothesis. We conducted a meta‐analysis of the plant invasions literature to quantify the contribution of resident competitors, diversity, herbivores and soil fungal communities to biotic resistance. Results indicated large negative effects of all factors except fungal communities on invader establishment and performance. Contrary to predictions derived from the natural enemies hypothesis, resident herbivores reduced invasion success as effectively as resident competitors. Although biotic resistance significantly reduced the establishment of individual invaders, we found little evidence that species interactions completely repelled invasions. We conclude that ecological interactions rarely enable communities to resist invasion, but instead constrain the abundance of invasive species once they have successfully established.     

Patterns in potassium dynamics in forest ecosystems

The biotic cycling of potassium (K) in forest systems has been relatively understudied in comparison with nitrogen (N) and phosphorus (P) despite its critical roles in maintaining the nutrition of primary production in forests. We investigated the ecological significance of K in forests from a literature review and data synthesis. We focused on (1) describing patterns of the effects of K availability on aboveground growth and change in foliar tissue of tree species from a variety of forests; and (2) documenting previously unreported relationships between hydrologic losses of K and N in forested watersheds from the Americas. In a review of studies examining tree growth under K manipulations/fertilizations, a high percentage (69% of studies) showed a positive response to increases in K availability in forest soils. In addition, 76% of the tree studies reviewed showed a positive and significant increase in K concentrations in plant tissue after soil K manipulation/fertilization. A meta-analysis on a subset of the reviewed studies was found to provide further evidence that potassium effects tree growth and increased tissue [K] with an effect size of 0.709 for growth and an overall effect size of 0.56. In our review of watershed studies, we observed that concentrations of K typically decreased during growing seasons in streams draining forested areas in the Temperate Zones and were responsive to vegetation disturbance in both temperate and tropical regions. We found a strong relationship (r2 = 0.42-0.99) between concentrations of K and N (another critical plant nutrient) in stream water, suggesting that similar mechanisms of biotic retention may control the flow of these nutrients. Furthermore, K dynamics appear to be unique among the base cations, e.g. calcium, magnesium, and sodium, because the others do not show similar seasonal patterns to K. We suggest that K may be important to the productivity and sustenance of many forests, and its dynamics and ecological significance warrant further study. We suggest that knowledge about the dynamics of this understudied element is imperative for our understanding patterns and processes in forest ecosystems.     

城市菌根真菌多样性、变化机制及功能应用

菌根真菌能够与大多数陆生植物的根系形成菌根共生体,具有改善宿主植物矿质营养、增强抗逆性、改良土壤结构等重要生态功能。城市化过程中气候、土壤、植被、土地利用等因素的改变,对菌根真菌的多样性产生了直接或间接的影响。目前城市菌根真菌的研究多侧重对其空间分布及群落组成的简单描述,缺乏针对城市典型生态现象及生态问题系统性的探讨。分别从城市菌根真菌的多样性变化、影响机制及功能应用等3方面进行了综述,全面揭示城市菌根真菌的研究现状及研究的复杂性,发现当前研究存在多样性评估简单化、研究层次单一化、内在机制现象化及功能应用停滞化等问题,认为今后应建立更为系统、综合、标准的研究体系以深刻而准确地认识与理解城市化对菌根真菌多样性的影响,为城市微生物资源的保存及绿地系统维持提供理论依据。     

Heterogeneity of ecological patterns,processes, and funding of marine manipulative field experiments conducted in Southeastern Pacific coastal ecosystems

Ecological manipulative experiments conducted in marine coastal ecosystems have substantially improved ecological theory during the last decades and have provided useful knowledge for the management and conservation of coastal ecosystems. Although different studies report global trends in ecological patterns worldwide, Southeastern Pacific coastal ecosystems have been poorly considered. Given that the SE Pacific coast encompasses diverse coastal ecosystems, consideration of studies conducted along this range can shed light on the heterogeneity of processes regulating coastal communities. We reviewed the biotic interactions and habitat type considered, as well as the complexity in terms of spatial and temporal extent of manipulative field experimental studies conducted along the SE Pacific coast from 0°S to 56°S (Ecuador to Chile). We test the effect of funding reported by different studies as a main factor limiting experimental complexity. From field ecological studies published from 1970 to 2016, we found that 81 studies were truly manipulative, in which one or multiple factors were “manipulated.” Around 77% of these studies were located between 21°S and 40°S, and conducted in intertidal rocky habitats. An increase in experimental studies was observed between 2010 and 2015, especially focused on herbivore–alga interactions, although we found that both the temporal extent and spatial extent of these studies have shown a decrease in recent decades. Funding grant amount reported had a positive effect on elapsed time of field experiments, but no effect was observed on spatial extent or in the biotic interactions considered. Elapsed time of experiments was different among the main biotic interactions considered, that is, herbivory, predation, and competition. We suggest that to further progress in applied ecological knowledge, it will be necessary to consider pollution and urbanization processes explicitly using a field experimental framework. This information could improve our understanding of how ecosystems present along the SE Pacific coast respond to climate change and increased levels of human interventions.     

Management of invasive plants through ecological resistance

Despite debates on the real impact of plant invasion on native biodiversity, there remain many situations where exotic invasive plants must be managed and habitats restored. Restoration practices that build on plant community assembly principles could be useful to delay or prevent re-invasion after control, but there are still few syntheses of the biodiversity theory, ecological mechanisms and experimental evidence relevant to invasive plant management, possibly delaying applications. To provide such a synthesis, we review current knowledge on three key determinants of invasion success: biotic resistance, abiotic constraints, and propagule pressure. We elaborate on the ecological mechanisms at play for each determinant and emphasize, using case studies, their relevance for invasive plant management and ecological restoration. We find evidence that restoring a plant cover can enhance invasion resistance, but the challenge for both research and field applications is to understand how multiple determinants interact in relation to species traits in the fields. Failure to recognize these interactions and their effect on community assembly processes may explain some of the mixed species responses observed. While we need control and restoration case studies with local species at different sites, the development of a coherent, dynamic and adaptive framework around biotic/ecological resistance will have to go beyond the idiosyncrasy of the many species and systems being tested. Emphasizing the functional diversity of the restored community seems a promising approach when facing potentially multiple invaders and/or fluctuating abiotic conditions.     

Unraveling negative biotic interactions determining soil microbial community assembly and functioning

Microbial communities play important roles in all ecosystems and yet a comprehensive understanding of the ecological processes governing the assembly of these communities is missing. To address the role of biotic interactions between microorganisms in assembly and for functioning of the soil microbiota, we used a top-down manipulation approach based on the removal of various populations in a natural soil microbial community. We hypothesized that removal of certain microbial groups will strongly affect the relative fitness of many others, therefore unraveling the contribution of biotic interactions in shaping the soil microbiome. Here we show that 39% of the dominant bacterial taxa across treatments were subjected to competitive interactions during soil recolonization, highlighting the importance of biotic interactions in the assembly of microbial communities in soil. Moreover, our approach allowed the identification of microbial community assembly rule as exemplified by the competitive exclusion between members of Bacillales and Proteobacteriales. Modified biotic interactions resulted in greater changes in activities related to N- than to C-cycling. Our approach can provide a new and promising avenue to study microbial interactions in complex ecosystems as well as the links between microbial community composition and ecosystem function.Subject terms: Soil microbiology, Ecology     

Food web structure provides biotic resistance against plankton invasion attempts

It is generally accepted that native communities provide resistance against invaders through biotic interactions. However, much remains uncertain about the types of ecological processes and community attributes that contribute to biotic resistance. We used experimental mesocosms to examine how zooplankton community structure, invertebrate predation, and nutrient supply jointly affected the establishment of the exotic Daphnia lumholtzi. We predicted that establishment would increase with declining biomass and diversity of native zooplankton communities and that an invertebrate predator (IP) would indirectly facilitate the establishment of D. lumholtzi due to its relatively long predator-deterring spines. Furthermore, we hypothesized that elevated nutrient supply would increase algal food availability and facilitate establishment. Only when the biomass and diversity of native zooplankton were significantly reduced, was D.␣lumholtzi able to successfully invade mesocosms. Although invertebrate predation and resource supply modified attributes of native zooplankton communities, they did not influence the establishment of D. lumholtzi. Overall, our␣results are consistent with observed population dynamics in invaded reservoirs where D.␣lumholtzi tends to be present only during the late summer, coinciding with historic mid-summer declines in native zooplankton populations. Lakes and reservoirs may be more susceptible to invasion not only by D. lumholtzi, but also by other planktonic species, in the late summer when native communities exhibit characteristics associated with lower levels of biotic resistance.     

Linking human impacts to community processes in terrestrial and freshwater ecosystems

Human impacts such as habitat loss, climate change and biological invasions are radically altering biodiversity, with greater effects projected into the future. Evidence suggests human impacts may differ substantially between terrestrial and freshwater ecosystems, but the reasons for these differences are poorly understood. We propose an integrative approach to explain these differences by linking impacts to four fundamental processes that structure communities: dispersal, speciation, species-level selection and ecological drift. Our goal is to provide process-based insights into why human impacts, and responses to impacts, may differ across ecosystem types using a mechanistic, eco-evolutionary comparative framework. To enable these insights, we review and synthesise (i) how the four processes influence diversity and dynamics in terrestrial versus freshwater communities, specifically whether the relative importance of each process differs among ecosystems, and (ii) the pathways by which human impacts can produce divergent responses across ecosystems, due to differences in the strength of processes among ecosystems we identify. Finally, we highlight research gaps and next steps, and discuss how this approach can provide new insights for conservation. By focusing on the processes that shape diversity in communities, we aim to mechanistically link human impacts to ongoing and future changes in ecosystems.     

Primary and secondary causes and consequences of contemporary forest decline

Realization that forest decline (Waldsterben) has become an ecological crisis throughout the developed world has resulted in massive research efforts to determine the causes of declines. It is now recognized that no single causal factor is responsible, but that there are a variety of anthropogenic causal factor complexes interacting with natural events and processes that, together, induce stresses in forests that culminate in declines of individual plants and of ecosystems. It is the thesis of this article that forest declines involve all biotic and abiotic facets and parameters of forested ecosystems and that the declines are themselves new causal factor complexes that continue to affect the stability of forested ecosystems independently of the initial causal factor complexes. Lacking direct field or laboratory studies on these cascades of causes and effects, this article attempts to utilize the growing body of information on plant physiological ecology to provide a heuristic framework for evaluating long-term forest declines.     

氮有效性增加对细根解剖、形态特征和菌根侵染的影响

氮(N)有效性增加对森林生态系统结构和功能有重要影响.细根作为树木地下最为活跃的组分,其对N有效性增加的响应已成为森林生态学研究的热点.本文对N有效性增加条件下细根解剖构造、形态和菌根侵染的响应趋势及潜在机制进行了综述.N有效性增加导致细根皮层厚度、皮层层数和皮层细胞直径下降,外皮层上通道细胞数量减少,而中柱和木质部横截面积,以及导管(或管胞)直径、数量和壁厚度均增加,根解剖构造的响应与植物激素浓度变化有关;N有效性增加后菌根侵染率下降,比根长(单位根干质量的长度)在外生菌根树种中普遍下降,而在内生菌根树种中增加;根直径和组织密度的变化趋势则表现出较大的种间差异.这些个体根和树种水平上的结果对于理解森林生态系统水平上的碳和养分循环具有重要意义.最后,根据目前研究中存在的不足提出了今后的研究方向与问题.     

Resource heterogeneity and persistence of exotic annuals in long-ungrazed Mediterranean-climate woodlands

Processes that promote weed invasion are often well-demonstrated, but mechanisms that facilitate ecological resistance to weed invasion in non-invaded communities, or promote weed persistence in invaded communities, are poorly understood. Yet it is these processes that must be addressed to achieve sustainable ecological restoration. We surveyed soil heterogeneity in 25 long-ungrazed, unfertilized York gum (Eucalyptus loxophleba Benth. subsp. loxophleba)—jam (Acacia acuminata Benth.) woodlands of the Western Australian wheatbelt to investigate differences in soil characteristics between patches locally-invaded or non-invaded by widespread exotic annuals. Based on studies in other ecosystems, we hypothesized that (1) weed persistence is associated with elevated soil resource levels, and (2) of these soil resources, phosphorus is the key contributor to weed persistence in Western Australian woodlands, that typically occur on phosphorus-impoverished soils. Our first hypothesis was partly supported, with soil nutrients associated with up to 40% of the variation in cover of exotic annuals. In particular, low concentrations of total nitrogen, nitrate and available phosphorus are likely to contribute to resistance to invasion in many non-invaded woodland patches, especially in gaps between trees. However, other non-invaded patches had comparable nutrient concentrations to invaded patches, suggesting this resistance may be weak at more productive sites or that patches have not reached a stable equilibrium. Inconsistent with our second hypothesis, exotic annuals were as strongly correlated with elevated total nitrogen and associated variables as they were with available phosphorus, probably reflecting a history of grazing without fertilization. We conclude that effectiveness of ‘bottom-up’ approaches to weed control is likely to differ among ecosystems according to interactions with disturbance history and attributes of the non-invaded community, even where the weed species or functional types are the same.     

Abiotic conditions shape the relationship between indigenous and exotic species richness in a montane biodiversity hotspot

Montane ecosystems are more prone to invasions by exotic plant species than previously thought. Besides abiotic factors, such as climate and soil properties, plant-plant interactions within communities are likely to affect the performance of potential invaders in their exotic range. The biotic resistance hypothesis predicts that high indigenous species richness hampers plant invasions. The biotic acceptance hypothesis, on the other hand, predicts a positive relationship between indigenous and exotic species richness. We tested these two hypotheses using observational data along an elevational gradient in a southern African biodiversity hotspot. Species composition data of indigenous and exotic plants were recorded in 20 road verge plots along a gradient of 1775–2775 m a.s.l. in the Drakensberg, South Africa. Plots were 2?×?50 m in size and positioned at 50 m elevational intervals. We found a negative correlation between indigenous and exotic richness for locations with poorly developed mineral soils, suggesting biotic resistance through competitive interactions. A strong positive correlation for plots with very shallow soils at high elevations indicated a lack of biotic resistance and the possibility of facilitating interactions in harsher environments. These results suggest that biotic resistance is restricted to the lower and mid elevations while biotic acceptance prevails in presence of severe abiotic stress, potentially increasing the risk of plant invasions into montane biodiversity hotspots.

     

Impacts of recent climate change on terrestrial flora and fauna: Some emerging Australian examples

The effects of anthropogenic climate change on biodiversity are well known for some high‐profile Australian marine systems, including coral bleaching and kelp forest devastation. Less well‐published are the impacts of climate change being observed in terrestrial ecosystems, although ecological models have predicted substantial changes are likely. Detecting and attributing terrestrial changes to anthropogenic factors is difficult due to the ecological importance of extreme conditions, the noisy nature of short‐term data collected with limited resources, and complexities introduced by biotic interactions. Here, we provide a suite of case studies that have considered possible impacts of anthropogenic climate change on Australian terrestrial systems. Our intention is to provide a diverse collection of stories illustrating how Australian flora and fauna are likely responding to direct and indirect effects of anthropogenic climate change. We aim to raise awareness rather than be comprehensive. We include case studies covering canopy dieback in forests, compositional shifts in vegetation, positive feedbacks between climate, vegetation and disturbance regimes, local extinctions in plants, size changes in birds, phenological shifts in reproduction and shifting biotic interactions that threaten communities and endangered species. Some of these changes are direct and clear cut, others are indirect and less clearly connected to climate change; however, all are important in providing insights into the future state of terrestrial ecosystems. We also highlight some of the management issues relevant to conserving terrestrial communities and ecosystems in the face of anthropogenic climate change.     

Ecological impacts of feral pigs in the Hawaiian Islands

The foraging habits of exotic ungulate species can directly and indirectly affect native plant and animal distribution and abundance patterns. Most of the studies on feral pig interactions with other biota in the Hawaiian Islands have been published as difficult to access reports to governmental and nongovernmental organizations, graduate student theses, and a few in peer reviewed journals. In this paper we discuss the origins of pig introductions to Hawaii, their feralization process, population expansion, and interactions with native and non-native biota. We also consider the environmental effects triggered by pigs on local ecosystems and biotic communities. Feral pig activities can reduce the abundance of native plant species, enhance conditions for the establishment of invasive non-indigenous plants, and perhaps indirectly negatively impact native forest bird species. Pig foraging and traveling patterns may also lead to physical alteration of ecosystems by increasing soil erosion that may lead to watershed degradation. However, much remains to be learned about the strength and significance of aforementioned interactions and their long-term effects on Hawaiian biota and ecosystems due to some confounding events. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions.     

Trophic cascades in an invaded ecosystem: native keystone predators facilitate a dominant invader in an estuarine community

Determining how various factors contribute to the invasibility of systems is essential for both understanding community formation and informing management of natural areas. Research demonstrating that predators can provide biotic resistance to invasions by consuming invasive species has led to the presence of healthy predator populations being associated with reduced invasion potential of ecosystems. However, predators structure communities in many ways and their presence could also potentially facilitate invasions if they decrease populations of native species that compete with or consume an invader. We considered these two impacts of predators on invasion by analyzing the effects of two keystone predators (Pisaster spp. and Enhydra lutris nereis) on two foundation species (a native mussel Mytilus californianus and the invasive exotic bryozoan Watersipora subtorquata, a putative competitor for space with Mytilus californianus). Both native predators were found to facilitate the invasion of the exotic bryozoan, and the rate of invasion was highest when both predators were present. Facilitation of W. subtorquata occurred via indirect mechanisms that partly involved the removal of a competitor (mussels) via predation. These results illustrate that although predators can provide biotic resistance to invasion, healthy predator populations do not always confer this advantage and in fact may facilitate invasions. Therefore, implementation of management actions to enhance populations of top predators could also potentially increase the invasibility of some ecosystems.     

Spatially designed revegetation—why the spatial arrangement of plants should be as important to revegetation as they are to natural systems

The spatial arrangements of plants, both within and between species, play a key role in natural systems and influence many fundamental ecological processes (e.g. survival, competition, facilitation, pollination, and seed dispersal) and ecosystem functions (e.g. habitat value, erosion, water, and nutrient capture). Despite this knowledge, fine‐scale planting arrangements are rarely considered during restoration plantings, yet manipulation of planting designs has the potential to aid the development of resilient and self‐sustaining ecosystems. Here, we outline how the spatial arrangement of plants can influence processes both at the vegetation level and more broadly at the ecosystem level. The review is focused on woodland systems, but also draws on key examples from grassland ecosystems. Following this synthesis, we identify research gaps in the revegetation literature that could usefully be addressed to help develop this understudied field of research. Finally, we outline components of population and community level arrangements (e.g. spacing, aggregation, community composition) that can be considered during restoration plantings—spatially designed revegetation—which are likely to lead to improved ecological outcomes of woodland and grassy woodland revegetation.     

Mussel disturbance dynamics: signatures of oceanographic forcing from local interactions

Local interactions, biotic and abiotic, can have a strong influence on the large-scale properties of ecosystems. However, ecological models often explore the influence of local biotic interactions where physical disturbance is included as a large-scale and imposed source of variability but is not allowed to interact with biotic processes at the local scale. In marine intertidal communities dominated by mussels, wave disturbances create gaps in the mussel bed that recover through a successional sequence. We present a lattice model of mussel disturbance dynamics that allows local interactions between wave disturbance and mussel recolonization, in which each cell of the lattice can be empty, occupied by a mussel bed element, or disturbed (which corresponds to a newly disturbed cell that has unstable edges). As in natural ecosystems, wave disturbance can also spread from disturbed to adjacent occupied cells, and recolonization can also spread from occupied to adjacent empty cells. We first validate the local rules from artificial gap experiments and from natural gap monitoring along the Oregon coast. We analyze the properties of the model system as a function of different oceanographic forcings of productivity and disturbance. We show that the mussel bed can go through phase transitions characterized by a large sensitivity of mussel cover and patterns to oceanographic forcings but also that criticality (scale invariance) is observed over wide ranges of parameters, which suggests self-organization. We also show that spatial patterns in the intertidal can provide a robust signature of local processes and can inform about oceanographic regimes. We do so by comparing the large-scale patterns of the simulation (scaling exponents) with field data, which suggest that some experimental sites are close to criticality. Our results suggest that regional patterns in disturbed populations can be explained by local biotic and abiotic processes submitted to oceanographic forcing.     

Community ecology of mesophotic coral reef ecosystems

Given the global degradation of shallow-water coral reef ecosystems resulting from anthropogenic activities, mesophotic coral reef ecosystems (MCEs) are gaining attention because they are generally considered a de facto refuge for shallow-water species. Despite their inferred importance, MCEs remain one of the most understudied reef habitats, and basic information on the taxonomic composition, depth range, habitat preferences, and abundance and distribution of MCE taxa is scarce. The processes that structure these communities are virtually unknown. Here, we provide a review of what is known about MCEs community ecology and outline essential gaps in our knowledge of these deeper water coral reef ecosystems. The primary findings of this review are as follows: (1) many dominant shallow-water species are absent from MCEs; (2) compared to shallow reefs, herbivores are relatively scarce, perhaps due to limited habitat complexity at depth; (3) changes in the dominant photosynthetic taxa with depth suggest adaptation and specialization to depth; (4) evidence regarding the importance of heterotrophy for zooxanthellate corals at depth is conflicting and inconclusive; and (5) decreased light with depth, but not temperature, appears to be the primary factor limiting the depth of MCEs. The majority of research done to date has been performed in the Caribbean, where some generalization can be made about the community structure and distribution of MCEs. The larger and more diverse Indo-Pacific remains largely unexplored with no apparent generalizations from the few sites that have been comparatively well studied. For MCEs, large gaps in knowledge remain on fundamental aspects of ecology. Advanced technologies must be harnessed and logistical challenges overcome to close this knowledge gap and empower resource managers to make informed decisions on conserving shallow-water and mesophotic coral reef ecosystems.     

农业生态系统中的AM真菌多样性

农业生态系统复杂庞大,是由如麦田生态系统、水稻田生态系统、果园生态系统、草地生态系统、保护地生态系统等组成的一个复合生态系统。重点介绍农业生态系统中丛枝菌根(AM)和AM真菌多样性,探讨农业生态系统中调控AM真菌多样性的途径以及今后研究的动向。