Arbuscular mycorrhizal fungal communities change among three stages of primary sand dune succession but do not alter plant growth

Plant interactions with soil biota could have a significant impact on plant successional trajectory by benefiting plants in a particular successional stage over others. The influence of soil mutualists such as mycorrhizal fungi is thought to be an important feedback component, yet they have shown benefits to both early and late successional plants that could either retard or accelerate succession. Here we first determine if arbuscular mycorrhizal (AM) fungi differ among three stages of primary sand dune succession and then if they alter growth of plants from particular successional stages. We isolated AM fungal inoculum from early, intermediate or late stages of a primary dune succession and compared them using cloning and sequencing. We then grew eight plant species that dominate within each of these successional stages with each AM fungal inoculum. We measured fungal growth to assess potential AM functional differences and plant growth to determine if AM fungi positively or negatively affect plants. AM fungi isolated from early succession were more phylogenetically diverse relative to intermediate and late succession while late successional fungi consistently produced more soil hyphae and arbuscules. Despite these differences, inocula from different successional stages had similar effects on the growth of all plant species. Host plant biomass was not affected by mycorrhizal inoculation relative to un‐inoculated controls. Although mycorrhizal communities differ among primary dune successional stages and formed different fungal structures, these differences did not directly affect the growth of plants from different dune successional stages in our experiment and therefore may be less likely to directly contribute to plant succession in sand dunes.     

Arbuscular mycorrhizal fungal community divergence within a common host plant in two different soils in a subarctic Aeolian sand area

There is rising awareness that different arbuscular mycorrhizal (AM) fungi have different autoecology and occupy different soil niches and that the benefits they provide to the host plant are dependent on plant-AM fungus combination. However, the role and community composition of AM fungi in succession are not well known and the northern latitudes remain poorly investigated ecosystems. We studied AM fungal communities in the roots of the grass Deschampsia flexuosa in two different, closely located, successional stages in a northern Aeolian sand area. The AM fungal taxa richness in planta was estimated by cloning and sequencing small subunit ribosomal RNA genes. AM colonization, shoot δ ¹³C signature, and %N and %C were measured. Soil microbial community structure and AM fungal mycelium abundance were estimated using phospholipid (PLFA) and neutral lipid (NLFA) analyses. The two successional stages were characterized by distinct plant, microbial, and fungal communities. AM fungal species richness was very low in both the early and late successional stages. AM frequency in D. flexuosa roots was higher in the early successional stage than in the late one. The AM fungal taxa retrieved belonged to the genera generally adapted to Arctic or extreme environments. AM fungi seemed to be important in the early stage of the succession, suggesting that AM fungi may help plants to better cope with the harsh environmental conditions, especially in an early successional stage with more extreme environmental fluctuations.     

Mycorrhizal fungal growth responds to soil characteristics,but not host plant identity,during a primary lacustrine dune succession

Soil factors and host plant identity can both affect the growth and functioning of mycorrhizal fungi. Both components change during primary succession, but it is unknown if their relative importance to mycorrhizas also changes. This research tested how soil type and host plant differences among primary successional stages determine the growth and plant effects of arbuscular mycorrhizal (AM) fungal communities. Mycorrhizal fungal community, plant identity, and soil conditions were manipulated among three stages of a lacustrine sand dune successional series in a fully factorial greenhouse experiment. Late succession AM fungi produced more arbuscules and soil hyphae when grown in late succession soils, although the community was from the same narrow phylogenetic group as those in intermediate succession. AM fungal growth did not differ between host species, and plant growth was similarly unaffected by different AM fungal communities. These results indicate that though ecological filtering and/or adaptation of AM fungi occurs during this primary dune succession, it more strongly reflects matching between fungi and soils, rather than interactions between fungi and host plants. Thus, AM fungal performance during this succession may not depend directly on the sequence of plant community succession.     

Do bacterial and fungal communities assemble differently during primary succession?

High‐throughput sequencing technologies are now allowing us to study patterns of community assembly for diverse microbial assemblages across environmental gradients and during succession. Here we discuss potential explanations for similarities and differences in bacterial and fungal community assembly patterns along a soil chronosequence in the foreland of a receding glacier. Although the data are not entirely conclusive, they do indicate that successional trajectories for bacteria and fungi may be quite different. Recent empirical and theoretical studies indicate that smaller microbes (like most bacteria) are less likely to be dispersal limited than are larger microbes – which could result in a more deterministic community assembly pattern for bacteria during primary succession. Many bacteria are also better adapted (than are fungi) to life in barren, early‐successional sediments in that some can fix nitrogen and carbon from the atmosphere – traits not possessed by any fungi. Other differences between bacteria and fungi are discussed, but it is apparent from this and other recent studies of microbial succession that we are a long way from understanding the mechanistic underpinnings of microbial community assembly during ecosystem succession. We especially need a better understanding of global and regional patterns of microbial dispersal and what environmental factors control the development of microbial communities in complex natural systems.     

云南松林次生演替阶段土壤细菌群落的变化

土壤细菌多样性是维持森林生态系统功能的关键因子,森林演替是影响其动态变化的重要因素。研究云南松林不同演替阶段土壤细菌群落结构及其多样性的变化规律,有助于深入理解森林生态系统恢复过程的驱动机制。本研究以云南省永仁县皆伐后形成的针叶林、针阔混交林和常绿阔叶林为对象,基于Illumina Hiseq高通量测序技术,分析森林演替过程中土壤细菌群落组成、结构、多样性及其影响因子的变化。结果表明: 土壤细菌的种群分类单元、Ace指数、Chao1指数和Shannon指数均随着演替进行呈减少趋势,演替早期阶段土壤的细菌总数、菌群丰富度及复杂程度最高。不同演替阶段细菌群落结构存在显著差异,其中,针阔混交林的差异最大,变形菌门和酸杆菌门为各演替序列共有的优势类群,放线菌门、绿弯菌门和Patescibacteria是演替早期的优势类群,且随着演替进行呈现减少趋势;变形菌门和WPS-2相对多度随演替进行呈增加趋势。土壤pH和乔木层物种丰富度是驱动次生演替过程中土壤细菌群落组成变化的关键因子。随着演替的进行,土壤细菌多样性减少,群落组成差异加大。     

典型农田退耕后土壤真菌与细菌群落的演替

土壤真菌和细菌作为地下生态系统的重要组成部分,其群落的恢复状况是评价农田退耕还林生态效益的重要指标。以云南省维西县典型退耕还林农田为对象,利用高通量测序等方法比较了不同退耕年限的农田土壤中真菌和细菌群落随植被演替的变化特征。结果发现,农田撂荒后土壤细菌多样性先显著降低后缓慢上升,真菌多样性变化不明显;地上部植被由草本经灌丛再向林地演替的过程中,土壤真菌的群落组成随植被变化呈现明显的改变,主要体现在粪壳菌纲(Sordariomycetes)所占比例的减少(由30%减至10%左右)和伞菌纲(Agaricomycetes)所占比例的增加(由5%以下增至20%以上);而细菌的群落组成无明显变化。聚类分析的结果显示,真菌的群落组成变化与植物群落的演替规律更为同步。不同演替阶段的退耕农田土壤真菌和细菌群落均明显区别于未经扰动的天然林,表明人为扰动对土壤微生物群落的影响可能在较长时间内持续存在。研究揭示了云南典型农田退耕后地下土壤真菌和细菌群落随植被演替的变化特征,为全面评价该地区退耕还林的生态效益提供了数据支撑。     

森林次生演替和土壤层次对微生物群落结构的影响

森林次生演替与生态系统结构和功能的动态变化密切相关。大多数研究主要关注植物群落以及土壤有机碳(SOC)的变化,然而土壤微生物群落如何响应森林次生演替还需要进一步探究。本研究以长白山森林次生演替序列(20、80、120、200和≥300年)和两个土壤层次为对象,采用磷脂脂肪酸微生物标志物,探究温带森林次生演替过程中地下微生物群落结构变化。森林次生演替改变了土壤微生物群落结构,主要归因于某些特定微生物类群的变化,演替前期革兰氏阴性菌和腐生真菌占主导,而在演替后期革兰氏阳性菌和丛枝菌根真菌占主导。另外,土壤有机质数量和质量差异是影响微生物群落结构和生物量的主要环境因素。森林演替前期和中期增加的SOC含量促进了微生物生物量,而演替后期增加的难分解芳香族有机组分抑制了微生物生物量合成。土壤层次间理化性质的差异导致微生物群落变化,有机质层高的SOC以及氮含量导致更多微生物生物量的合成。微生物群落在时间和空间尺度的变化及其驱动因素反映了生态系统结构和功能对环境变化的响应。     

Effects of microbial mats on germination and seedling survival of typical dune slack species in the Netherlands

The role of microbial mats in wet dune slack succession is often discussed. We tested if presence of microbial mats may retard dune slack succession by lowering the germination and seeding survival of successor species. This hypothesis was tested on a set of typical dune slack species of the Frisian Islands in two climate chamber experiments. The species were separated into early-, intermediate- and late successional species. There were large differences in germination rates between species (2% – >200% compared to the reference), but within a species the high germination rates were mostly found on sand without a microbial mat. Only the germination of Agrostis stolonifera appeared to be stimulated by the presence of a well-developed microbial mat, they were even higher than in the reference. Seedling survival also did not show different responses between successional stages. Seedlings placed on top of a microbial mat showed for most species lower survival rates compared to seedlings that were planted or placed on top of the sand. Growth was the only measured variable that differed between successional groups. Species of the early- and intermediate successional stages grew significantly better if a microbial mat was present whereas late successional species were not stimulated. Early and intermediate successional species seem to be favored by the presence of a microbial mat. An explanation for this may be that they can profit from the enhanced nitrogen availability caused by N₂-fixation by cyanobacteria in the microbial mat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Primary succession of Bistorta vivipara (L.) Delabre (Polygonaceae) root‐associated fungi mirrors plant succession in two glacial chronosequences

Glacier chronosequences are important sites for primary succession studies and have yielded well‐defined primary succession models for plants that identify environmental resistance as an important determinant of the successional trajectory. Whether plant‐associated fungal communities follow those same successional trajectories and also respond to environmental resistance is an open question. In this study, 454 amplicon pyrosequencing was used to compare the root‐associated fungal communities of the ectomycorrhizal (ECM) herb Bistorta vivipara along two primary succession gradients with different environmental resistance (alpine versus arctic) and different successional trajectories in the vascular plant communities (directional replacement versus directional non‐replacement). At both sites, the root‐associated fungal communities were dominated by ECM basidiomycetes and community composition shifted with increasing time since deglaciation. However, the fungal community's successional trajectory mirrored the pattern observed in the surrounding plant community at both sites: the alpine site displayed a directional‐replacement successional trajectory, and the arctic site displayed a directional‐non‐replacement successional trajectory. This suggests that, like in plant communities, environmental resistance is key in determining succession patterns in root‐associated fungi. The need for further replicated study, including in other host species, is emphasized.     

Changes in diversity and importance of clonal plants during sand dune succession in northeastern China

Many major biomes throughout the world are dominated by plants with clonal growth forms. While many recent studies have examined the effects of clonality on the growth of individual plants, relatively few studies have tested the community level effects of clonality as a function of environmental characteristics. By investigating six sand dune sites that have undergone different numbers of years of natural restoration constituting a succession sere, we quantified if the abundance and importance of clonal plants was related to successional age in the stressful environment of a semi-arid sand dune region in northeastern China. We expected that clonal plants would dominate at every stage of the succession sere. We also predicted that species diversity would decrease in later stages of the succession sere due to the extremely high proportion of clonal plants in the community. Our results showed that, through 45 years of succession, the total plant species richness and Shannon–Wiener diversity index continually increased. While the species number of clonal plants was consistently low during the succession, the importance of clonal plants increased gradually from none at 3 years to 49 % of the total, approximately equal to that of aclonal plants, at the 45-year site. Clonal plants with phalanx strategies were more important than guerillas at all ages in sand dune succession. At the beginning and early stages of sand dune succession, aclonal plants were more important than clonal plants, perhaps due to greater seed propagation. The distribution or arrangement of aclonal and clonal plants in the whole process of sand dune complemented each other. The results presented give new perceptions on the function of biodiversity in maintaining ecosystems.     

Fungal importance extends beyond litter decomposition in experimental early‐successional streams

Fungi are important decomposers of leaf litter in streams and may have knock‐on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand‐bottom streams in an early‐successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early‐successional catchment circulate through the channels for 5 weeks. Whole‐stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well‐established direct involvement in leaf litter decomposition.     

Above and belowground community development in a marine sand dune ecosystem

Transects across undisturbed marine sand dunes sequentially traversed the following plant communities: littoral, foredunes, intradune complex of ridges and hollows, deflation plain, myrtle forest, and mature conifer forest. Organic carbon levels were low in the littoral zone and increased across the dune ecosystem landward to the forest communities. The highest percentage of nutrients was isolated from the heavy fraction of soil residues. Soil microfloral populations responded to vegetation, physical dune characteristics, and seasonal moisture patterns. Populations of bacteria and actinomycetes were higher in winter than during summer sampling periods in all communities. Populations of microscopic fungi were higher in winter in all the communities except the hollows. The distribution of vesicular-arbuscular mycorrhizal fungi responded to vegetation and sand dune succession but did not display seasonality. Species of Gigaspora and Acaulospora were the most commonly isolated VAM fungi.Sand aggregation increased along the dune transect and was correlated to plant community succession: the most highly aggregated soil was found in the two forest communities. With scanning electron microscopy, sand grains and organic residues were observed entangled by strands of filamentous mciroorganisms. Many of the filaments were of the dimensions of VAM fungi, which may be important for the release of nutrients associated with the cementing agents of sand aggregates and for the survival of early pioneer plants of sand dunes.     

A comparison of seed banks across a sand dune successional gradient at Lake Michigan dunes (Indiana,USA)

In habitats where disturbance is frequent, seed banks are important for the regeneration of vegetation. Sand dune systems are dynamic habitats in which sand movement provides intermittent disturbance. As succession proceeds from bare sand to forest, the disturbance decreases. At Indiana Dunes National Lakeshore, we examined the seed banks of three habitat types across a successional gradient: foredunes, secondary dunes, and oak savanna. There were differences among the types of species that germinated from each of the habitats. The mean seed bank density increased across the successional gradient by habitat, from 376 to 433 to 968 seeds m⁻², but with foredune and secondary dune seed bank densities being significantly lower than the savanna seed bank density. The number of seeds germinated was significantly correlated with soil organic carbon, demonstrating for this primary successional sequence that seed density increases with stage and age. The seed bank had much lower species richness than that of the aboveground vegetation across all habitats. Among sites within a habitat type, the similarity of species germinated from the seed banks was very low, illustrating the variability of the seed bank even in similar habitat types. These results suggest that restoration of these habitats cannot rely on seed banks alone.     

镜泊湖岩溶台地不同植被类型土壤微生物群落特征

为了探讨不同演替阶段植被类型土壤微生物群落特征,分别选取镜泊湖岩溶台地草本、矮灌木、高灌木、小乔木与灌木混生(简称混生)群落、落叶阔叶林及针阔混交林6种典型植被类型,进行植物群落调查和对土壤微生物生物量、群落结构和多样性指标、土壤物理化学性质的测定。结果表明：从土壤微生物量、土壤微生物群落组成、土壤微生物代谢动力学过程和代谢功能多样性的角度来看,各种植被类型土壤微生物群落具有明显的差异。演替前期的草本群落土壤微生物量碳氮、细菌生物量、真菌生物量,代谢活性及丰富度指数均最低,但Shannon-Wiener多样性指数和均匀度指数显著(P<0.05)高于其他植被类型。矮灌木土壤微生物群落组成显著受植被类型的影响。高灌木群落和混生(小乔木与灌木混生)群落具有极强的相似性, 但在碳源利用类型上两者表现出一定的差异。落叶阔叶林代谢活性最高,碳源利用能力最强,能利用BIOLOG微孔板中的所有31种碳源,这与其具有较高的微生物量碳氮和细菌生物量一致,其代谢功能丰富度最高。演替后期的针阔混交林下的土壤pH最低,真菌比例升高,在碳源丰富的条件下具有极强的竞争优势(仅次于落叶阔叶林),但在碳源贫瘠的条件下其利用碳源能力较弱(仅高于草本)。植被可能主要通过土壤全磷和有机质影响土壤微生物代谢功能多样性。     

Resilience to fire of phylogenetic diversity across biological domains

Fire alters the structure and composition of above‐ and belowground communities with concurrent shifts in phylogenetic diversity. The inspection of postfire trends in the diversity of ecological communities incorporating phylogenetic information allows to better understand the mechanisms driving fire resilience. While fire reduces plant phylogenetic diversity based on the recruitment of evolutionarily related species with postfire seed persistence, it increases that of soil microbes by limiting soil resources and changing the dominance of competing microbes. Thus, during postfire community reassembly, plant and soil microbes might experience opposing temporal trends in their phylogenetic diversity that are linked through changes in the soil conditions. We tested this hypothesis by investigating the postfire evolution of plant and soil microbial (fungi, bacteria and archaea) communities across three 20‐year chronosequences. Plant phylogenetic diversity increased with time since fire as pioneer seeders facilitate the establishment of distantly related late‐successional shrubs. The postfire increase in plant phylogenetic diversity fostered plant productivity, eventually recovering soil organic matter. These shifts over time in the soil conditions explained the postfire restoration of fungal and bacterial phylogenetic diversity, which decreased to prefire levels, suggesting that evolutionarily related taxa with high relative fitness recover their competitive superiority during community reassembly. The resilience to fire of phylogenetic diversity across biological domains helps preserve the evolutionary history stored in our ecosystems.     

Dynamics of vesicular-arbuscular mycorrhizae during old field succession

Summary The species composition of vesicular-arbuscular mycorrhizal (VAM) fungal communities changed during secondary succession of abandoned fields based on a field to forest chronosequence. Twenty-five VAM fungal species were identified. Seven species were clearly early successional and five species were clearly late successional. The total number of VAM fungal species did not increase with successional time, but diversity as measured by the Shannon-Wiener index tended to increase, primarily because the community became more even as a single species, Glomus aggregatum, became less dominant in the older sites. Diversity of the VAM fungal community was positively correlated with soil C and N. The density of VAM fungi, as measured by infectivity and total spore count, first increased with time since abandonment and then decreased in the late successional forest sites. Within 12 abandoned fields, VAM fungal density increased with increasing soil pH, H₂O soluble soil C, and root biomass, but was inversely related to extractable soil P and percent cover of non-host plant species. The lower abundance of VAM fungi in the forest sites compared with the field sites agrees with the findings of other workers and corresponds with a shift in the dominant vegetation from herbaceous VAM hosts to woody ectomycorrhizal hosts.     

Plant functional diversity enhances associations of soil fungal diversity with vegetation and soil in the restoration of semiarid sandy grassland

The trait‐based approach shows that plant functional diversity strongly affects ecosystem properties. However, few empirical studies show the relationship between soil fungal diversity and plant functional diversity in natural ecosystems. We investigated soil fungal diversity along a restoration gradient of sandy grassland (mobile dune, semifixed dune, fixed dune, and grassland) in Horqin Sand Land, northern China, using the denaturing gradient gel electrophoresis of 18S rRNA and gene sequencing. We also examined associations of soil fungal diversity with plant functional diversity reflected by the dominant species' traits in community (community‐weighted mean, CWM) and the dispersion of functional trait values (FD_is). We further used the structure equation model (SEM) to evaluate how plant richness, biomass, functional diversity, and soil properties affect soil fungal diversity in sandy grassland restoration. Soil fungal richness in mobile dune and semifixed dune was markedly lower than those of fixed dune and grassland (P < 0.05). Soil fungal richness was positively associated with plant richness, biomass, CWM plant height, and soil gradient aggregated from the principal component analysis, but SEM results showed that plant richness and CWM plant height determined by soil properties were the main factors exerting direct effects. Soil gradient increased fungal richness through indirect effect on vegetation rather than direct effect. The negative indirect effect of FDis on soil fungal richness was through its effect on plant biomass. Our final SEM model based on plant functional diversity explained nearly 70% variances of soil fungal richness. Strong association of soil fungal richness with the dominant species in the community supported the mass ratio hypothesis. Our results clearly highlight the role of plant functional diversity in enhancing associations of soil fungal diversity with community structure and soil properties in sandy grassland ecosystems.     

Richness, diversity, and rate of primary succession over 20?year in tropical coastal dunes

The tropical coastal dunes in central Gulf of Mexico have been stabilizing over the last decades resulting in reduced substrate mobility, and promoting primary succession. We describe changes in species richness and diversity in dune vegetation during 20?years. Our questions: (a) Do species richness and diversity increase over time as predicted by models of ecological succession or do they show a hump-backed manner similar to the observations in temperate coastal dunes?, (b) What is the interaction between vegetation cover and diversity and species richness?, (c) Is there a relationship between species diversity and succession rate and does succession rate change over time?, and (d) How do plant functional types change during succession? In order to answer these questions, we set 140 4?×?4?m permanent plots in a mobile dune area and monitored vegetation cover and species richness from 1991 to 2011. In time, diversity increased in a logistic manner toward an asymptotic value once vegetation cover surpassed 60?%. Species richness increased in a humped-back shape, also reaching a maximum peak at 60?% vegetation cover. The succession rate of diversity was measured by the Euclidean distance, and showed a significant humped-back relation, meaning that it was slower in early and late successional stages. The study supports the intermediate disturbance theory. The conservation of coastal dunes vegetation should focus on all, species-poor and species-rich habitats that help to maintain the ecological integrity of these ecosystems. The understanding of community dynamics and diversity patterns becomes an essential component of coastal dune management and conservation.     

Soil physicochemical properties drive the variation in soil microbial communities along a forest successional series in a degraded wetland in northeastern China

The Sanjiang Plain is the biggest freshwater wetland locating in northeastern China. Due to climate change and human activities, that wetland has degraded to a successional gradient from the original flooded wetland to dry shrub vegetation and a forest area with lower ground water level, which may result in changes in soil microbiologic structure and functions. The present study investigated the microbial diversity and community structure in relation to soil properties along that successional gradient. The soil physico‐chemical properties changed significantly with degradation stage. The Shannon diversity index of both soil bacteria (5.90–6.42) and fungi (1.7–4.19) varied significantly with successional stage (both p < .05). The community structures of soil bacteria and fungi in the early successional stages (i.e., the wetland) were significantly determined by water content, total nitrogen, and available nitrogen concentrations in soils, while those in the later successional stages (i.e., forests) were significantly structured by soil organic carbon, soil pH, and available phosphorus concentrations. These results suggest that the soil microbial structure is mainly determined by soil properties rather than by plant community such as plant species composition along successional stages.     

Specificity in Arabidopsis thaliana recruitment of root fungal communities from soil and rhizosphere

Biotic and abiotic conditions in soil pose major constraints on growth and reproductive success of plants. Fungi are important agents in plant soil interactions but the belowground mycobiota associated with plants remains poorly understood. We grew one genotype each from Sweden and Italy of the widely-studied plant model Arabidopsis thaliana. Plants were grown under controlled conditions in organic topsoil local to the Swedish genotype, and harvested after ten weeks. Total DNA was extracted from three belowground compartments: endosphere (sonicated roots), rhizosphere and bulk soil, and fungal communities were characterized from each by amplification and sequencing of the fungal barcode region ITS2. Fungal species diversity was found to decrease from bulk soil to rhizosphere to endosphere. A significant effect of plant genotype on fungal community composition was detected only in the endosphere compartment. Despite A. thaliana being a non-mycorrhizal plant, it hosts a number of known mycorrhiza fungi in its endosphere compartment, which is also colonized by endophytic, pathogenic and saprotrophic fungi. Species in the Archaeorhizomycetes were most abundant in rhizosphere samples suggesting an adaptation to environments with high nutrient turnover for some of these species. We conclude that A. thaliana endosphere fungal communities represent a selected subset of fungi recruited from soil and that plant genotype has small but significant quantitative and qualitative effects on these communities.