The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change,alien insect invasions and plant viruses

The suite of environments and anthropogenic modifications of sub-Antarctic islands provide key opportunities to improve our understanding of the potential consequences of climate change and biological species invasions on terrestrial ecosystems. The profound impact of human introduced invasive species on indigenous biota, and the facilitation of establishment as a result of changing thermal conditions, has been well documented on the French sub-Antarctic Kerguelen Islands (South Indian Ocean). The present study provides an overview of the vulnerability of sub-Antarctic terrestrial communities with respect to two interacting factors, namely climate change and alien insects. We present datasets assimilated by our teams on the Kerguelen Islands since 1974, coupled with a review of the literature, to evaluate the mechanism and impact of biological invasions in this region. First, we consider recent climatic trends of the Antarctic region, and its potential influence on the establishment, distribution and abundance of alien insects, using as examples one fly and one beetle species. Second, we consider to what extent limited gene pools may restrict alien species’ colonisations. Finally, we consider the vulnerability of native communities to aliens using the examples of one beetle, one fly, and five aphid species taking into consideration their additional impact as plant virus vectors. We conclude that the evidence assimilated from the sub-Antarctic islands can be applied to more complex temperate continental systems as well as further developing international guidelines to minimise the impact of alien species.     

Drought consistently alters the composition of soil fungal and bacterial communities in grasslands from two continents

The effects of short‐term drought on soil microbial communities remain largely unexplored, particularly at large scales and under field conditions. We used seven experimental sites from two continents (North America and Australia) to evaluate the impacts of imposed extreme drought on the abundance, community composition, richness, and function of soil bacterial and fungal communities. The sites encompassed different grassland ecosystems spanning a wide range of climatic and soil properties. Drought significantly altered the community composition of soil bacteria and, to a lesser extent, fungi in grasslands from two continents. The magnitude of the fungal community change was directly proportional to the precipitation gradient. This greater fungal sensitivity to drought at more mesic sites contrasts with the generally observed pattern of greater drought sensitivity of plant communities in more arid grasslands, suggesting that plant and microbial communities may respond differently along precipitation gradients. Actinobateria, and Chloroflexi, bacterial phyla typically dominant in dry environments, increased their relative abundance in response to drought, whereas Glomeromycetes, a fungal class regarded as widely symbiotic, decreased in relative abundance. The response of Chlamydiae and Tenericutes, two phyla of mostly pathogenic species, decreased and increased along the precipitation gradient, respectively. Soil enzyme activity consistently increased under drought, a response that was attributed to drought‐induced changes in microbial community structure rather than to changes in abundance and diversity. Our results provide evidence that drought has a widespread effect on the assembly of microbial communities, one of the major drivers of soil function in terrestrial ecosystems. Such responses may have important implications for the provision of key ecosystem services, including nutrient cycling, and may result in the weakening of plant–microbial interactions and a greater incidence of certain soil‐borne diseases.     

Plant diversity predicts beta but not alpha diversity of soil microbes across grasslands worldwide

Aboveground–belowground interactions exert critical controls on the composition and function of terrestrial ecosystems, yet the fundamental relationships between plant diversity and soil microbial diversity remain elusive. Theory predicts predominantly positive associations but tests within single sites have shown variable relationships, and associations between plant and microbial diversity across broad spatial scales remain largely unexplored. We compared the diversity of plant, bacterial, archaeal and fungal communities in one hundred and forty‐five 1 m² plots across 25 temperate grassland sites from four continents. Across sites, the plant alpha diversity patterns were poorly related to those observed for any soil microbial group. However, plant beta diversity (compositional dissimilarity between sites) was significantly correlated with the beta diversity of bacterial and fungal communities, even after controlling for environmental factors. Thus, across a global range of temperate grasslands, plant diversity can predict patterns in the composition of soil microbial communities, but not patterns in alpha diversity.     

Herbivore impact on grassland plant diversity depends on habitat productivity and herbivore size

Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.     

Plant community response to loss of large herbivores: comparing consequences in a South African and a North American grassland

Loss of biodiversity poses one of the greatest threats to natural ecosystems throughout the world. However, a comprehensive understanding of the impacts of species losses from upper trophic levels is still emerging. Here we compare the impacts of large mammalian herbivore species loss on grassland plant community structure and composition in a South African and North American grassland. Herbaceous plant communities were surveyed at sites without large mammalian herbivores present and at sites with a single species of herbivore present in both locations, and additionally at one site in South Africa with multiple herbivore species. At both the North American and South African locations, plant communities on sites with a single herbivore species were more diverse and species rich than on sites with no herbivores. At the multi-herbivore site in South Africa, plant diversity and richness were comparable to that of the single herbivore site early in the growing season and to the no herbivore site late in the growing season. Analyses of plant community composition, however, indicated strong differences between the multi-herbivore site and the single and no herbivore sites, which were more similar to each other. In moderate to high-productivity ecosystems with one or a few species of large herbivores, loss of herbivores can cause a significant decrease in plant diversity and richness, and can have pronounced impacts on grassland plant community composition. In ecosystems with higher herbivore richness, species loss may also significantly alter plant community structure and composition, although standard metrics of community structure may obscure these differences.     

Effects of an introduced mustard,Thlaspi arvense,on soil fungal communities in subalpine meadows

The subalpine meadows of the Rocky Mountains, USA, are at the advancing front of global change; however, little is known about the sensitivities of high-elevation soil fungal communities to ongoing ecological changes. Soil fungi are sensitive to abiotic and biotic environmental stressors, including climate change, soil disturbance, and the presence of introduced, non-native plants. Invasive plants in the Brassicaceae (mustard family) are known to alter fungal community structure, suppress arbuscular mycorrhizal fungi, and change their relationship with native plant hosts in forest ecosystems, but these phenomena have not been studied in the subalpine zone where non-native mustard plants are becoming established. Here, we investigated whether the presence of the introduced mustard plant, Thlaspi arvense, is associated with distinct properties of the whole fungal and arbuscular mycorrhizal fungal communities in subalpine meadow ecosystems. We observed clear differences in the composition, relative abundance of core taxa, and mean taxon relatedness of soil fungal communities in plots with T. arvense relative to those with only native vegetation. A suite of novel fungi were associated with T. arvense, and overall patterns of AMF phylogenetic diversity were drastically reduced in association with its presence. Our results suggest that T. arvense introduction impacts the soil fungal community, with potential implications for native plant communities and soil nutrient cycling in high elevation meadows of the Rocky Mountains.     

Fungal community composition in neotropical rain forests: the influence of tree diversity and precipitation

Plant diversity is considered one factor structuring soil fungal communities because the diversity of compounds in leaf litter might determine the extent of resource heterogeneity for decomposer communities. Lowland tropical rain forests have the highest plant diversity per area of any biome. Since fungi are responsible for much of the decomposition occurring in forest soils, understanding the factors that structure fungi in tropical forests may provide valuable insight for predicting changes in global carbon and nitrogen fluxes. To test the role of plant diversity in shaping fungal community structure and function, soil (0-20?cm) and leaf litter (O horizons) were collected from six established 1-ha forest census plots across a natural plant diversity gradient on the Isthmus of Panama. We used 454 pyrosequencing and phospholipid fatty acid analysis to evaluate correlations between microbial community composition, precipitation, soil nutrients, and plant richness. In soil, the number of fungal taxa increased significantly with increasing mean annual precipitation, but not with plant richness. There were no correlations between fungal communities in leaf litter and plant diversity or precipitation, and fungal communities were found to be compositionally distinct between soil and leaf litter. To directly test for effects of plant species richness on fungal diversity and function, we experimentally re-created litter diversity gradients in litter bags with 1, 25, and 50 species of litter. After 6?months, we found a significant effect of litter diversity on decomposition rate between one and 25 species of leaf litter. However, fungal richness did not track plant species richness. Although studies in a broader range of sites is required, these results suggest that precipitation may be a more important factor than plant diversity or soil nutrient status in structuring tropical forest soil fungal communities.     

Anthropogenic disturbance equalizes diversity levels in arbuscular mycorrhizal fungal communities

The arbuscular mycorrhizal (AM) symbiosis is a key plant–microbe interaction in sustainable functioning ecosystems. Increasing anthropogenic disturbance poses a threat to AM fungal communities worldwide, but there is little empirical evidence about its potential negative consequences. In this global study, we sequenced AM fungal DNA in soil samples collected from pairs of natural (undisturbed) and anthropogenic (disturbed) plots in two ecosystem types (10 naturally wooded and six naturally unwooded ecosystems). We found that ecosystem type had stronger directional effects than anthropogenic disturbance on AM fungal alpha and beta diversity. However, disturbance increased alpha and beta diversity at sites where natural diversity was low and decreased diversity at sites where natural diversity was high. Cultured AM fungal taxa were more prevalent in anthropogenic than natural plots, probably due to their efficient colonization strategies and ability to recover from disturbance. We conclude that anthropogenic disturbance does not have a consistent directional effect on AM fungal diversity; rather, disturbance equalizes levels of diversity at large scales and causes changes in community functional structure.     

The interactions between plant life form and fungal traits of arbuscular mycorrhizal fungi determine the symbiotic community

Arbuscular mycorrhizal (AM) fungi have traditionally been considered generalist symbionts. However, an increasing number of studies are pointing out the selectivity potential of plant hosts. Plant life form, determined by plant life history traits, seems to drive the AM fungal community composition. The AM fungi also exhibit a wide diversity of functional traits known to be responsible for their distribution in natural ecosystems. However, little is known about the role of plant and fungal traits driving the resultant symbiotic assemblages. With the aim of testing the feedback relationship between plant and fungal traits on the resulting AM fungal community, we inoculated three different plant life forms, i.e. annual herbs, perennial herbs and perennial semi-woody plants, with AM fungal communities sampled in different seasons. We hypothesized that the annual climate variation will induce changes in the mean traits of the AM fungal communities present in the soil throughout the year. Furthermore, the association of plants with different life forms with AM fungi with contrasting life history traits will show certain preferences according to reciprocal traits of the plants and fungi. We found changes in the AM fungal community throughout the year, which were differentially disrupted by disturbance and altered by plant growth form and plant biomass. Both plant and fungal traits clearly contributed to the resultant AM fungal communities. The revealed process can have implications for the functioning of ecosystems since changes in dominant plant life forms or climatic variables could influence the traits of AM fungal communities in soil and hence ecosystem processes.     

Responses of belowground communities to large aboveground herbivores: Meta‐analysis reveals biome‐dependent patterns and critical research gaps

The importance of herbivore–plant and soil biota–plant interactions in terrestrial ecosystems is amply recognized, but the effects of aboveground herbivores on soil biota remain challenging to predict. To find global patterns in belowground responses to vertebrate herbivores, we performed a meta‐analysis of studies that had measured abundance or activity of soil organisms inside and outside field exclosures (areas that excluded herbivores). Responses were often controlled by climate, ecosystem type, and dominant herbivore identity. Soil microfauna and especially root‐feeding nematodes were negatively affected by herbivores in subarctic sites. In arid ecosystems, herbivore presence tended to reduce microbial biomass and nitrogen mineralization. Herbivores decreased soil respiration in subarctic ecosystems and increased it in temperate ecosystems, but had no net effect on microbial biomass or nitrogen mineralization in those ecosystems. Responses of soil fauna, microbial biomass, and nitrogen mineralization shifted from neutral to negative with increasing herbivore body size. Responses of animal decomposers tended to switch from negative to positive with increasing precipitation, but also differed among taxa, for instance Oribatida responded negatively to herbivores, whereas Collembola did not. Our findings imply that losses and gains of aboveground herbivores will interact with climate and land use changes, inducing functional shifts in soil communities. To conceptualize the mechanisms behind our findings and link them with previous theoretical frameworks, we propose two complementary approaches to predict soil biological responses to vertebrate herbivores, one focused on an herbivore body size gradient, and the other on a climate severity gradient. Major research gaps were revealed, with tropical biomes, protists, and soil macrofauna being especially overlooked.     

Plant traits determine the phylogenetic structure of arbuscular mycorrhizal fungal communities

Functional diversity in ecosystems has traditionally been studied using aboveground plant traits. Despite the known effect of plant traits on the microbial community composition, their effects on the microbial functional diversity are only starting to be assessed. In this study, the phylogenetic structure of arbuscular mycorrhizal (AM) fungal communities associated with plant species differing in life cycle and growth form, that is, plant life forms, was determined to unravel the effect of plant traits on the functional diversity of this fungal group. The results of the 454 pyrosequencing showed that the AM fungal community composition differed across plant life forms and this effect was dependent on the soil collection date. Plants with ruderal characteristics tended to associate with phylogenetically clustered AM fungal communities. By contrast, plants with resource‐conservative traits associated with phylogenetically overdispersed AM fungal communities. Additionally, the soil collected in different seasons yielded AM fungal communities with different phylogenetic dispersion. In summary, we found that the phylogenetic structure, and hence the functional diversity, of AM fungal communities is dependent on plant traits. This finding adds value to the use of plant traits for the evaluation of belowground ecosystem diversity, functions and processes.     

Ecological consequences of Late Quaternary extinctions of megafauna

Large herbivorous vertebrates have strong interactions with vegetation, affecting the structure, composition and dynamics of plant communities in many ways. Living large herbivores are a small remnant of the assemblages of giants that existed in most terrestrial ecosystems 50 000 years ago. The extinction of so many large herbivores may well have triggered large changes in plant communities. In several parts of the world, palaeoecological studies suggest that extinct megafauna once maintained vegetation openness, and in wooded landscapes created mosaics of different structural types of vegetation with high habitat and species diversity. Following megafaunal extinction, these habitats reverted to more dense and uniform formations. Megafaunal extinction also led to changes in fire regimes and increased fire frequency due to accumulation of uncropped plant material, but there is a great deal of variation in post-extinction changes in fire. Plant communities that once interacted with extinct large herbivores still contain many species with obsolete defences against browsing and non-functional adaptations for seed dispersal. Such plants may be in decline, and, as a result, many plant communities may be in various stages of a process of relaxation from megafauna-conditioned to megafauna-naive states. Understanding the past role of giant herbivores provides fundamental insight into the history, dynamics and conservation of contemporary plant communities.     

Fungal diversity increases soil fungistasis and resistance to microbial invasion by a non resident species

Biodiversity decline is a major concern for ecosystem functioning. Recent research efforts have been mostly focused on terrestrial plants, while, despite their importance in both natural and artificial ecosystems, little is known about soil microbial communities. This work aims at investigating the effects of fungal species richness on soil invasion by non resident microbes. Synthetic fungal communities with a species diversity ranging from 1 to 8 were assembled in laboratory microcosms and used in three factorial experiments to assess the effect of diversity on soil fungistasis, microbial invasion of soil amended with plant litter and of plant rhizosphere. The capability of different microbes to colonize environments characterized by different resident microbial communities was measured. The number of microbial species in the microcosms positively affected soil fungistasis that was also induced more rapidly in presence of synthetic communities with more species. Moreover, the increase of resident fungal diversity dramatically reduced the invasibility of both soil and plant rhizosphere. We found lower variability of soil fungistasis and invasibility in microcosms with higher species richness of microbial communities. Our study pointed out the existence of negative relationships between fungal diversity and soil invasibility by non resident microbes. Therefore, the loss of microbial species may adversely affect ecosystem functionality under specific environmental conditions.     

Drought‐resistant fungi control soil organic matter decomposition and its response to temperature

Microbial‐mediated decomposition of soil organic matter (SOM) ultimately makes a considerable contribution to soil respiration, which is typically the main source of CO₂ arising from terrestrial ecosystems. Despite this central role in the decomposition of SOM, few studies have been conducted on how climate change may affect the soil microbial community and, furthermore, on how possible climate‐change induced alterations in the ecology of microbial communities may affect soil CO₂ emissions. Here we present the results of a seasonal study on soil microbial community structure, SOM decomposition and its temperature sensitivity in two representative Mediterranean ecosystems where precipitation/throughfall exclusion has taken place during the last 10 years. Bacterial and fungal diversity was estimated using the terminal restriction fragment length polymorphism technique. Our results show that fungal diversity was less sensitive to seasonal changes in moisture, temperature and plant activity than bacterial diversity. On the other hand, fungal communities showed the ability to dynamically adapt throughout the seasons. Fungi also coped better with the 10 years of precipitation/throughfall exclusion compared with bacteria. The high resistance of fungal diversity to changes with respect to bacteria may open the controversy as to whether future ‘drier conditions’ for Mediterranean regions might favor fungal dominated microbial communities. Finally, our results indicate that the fungal community exerted a strong influence over the temporal and spatial variability of SOM decomposition and its sensitivity to temperature. The results, therefore, highlight the important role of fungi in the decomposition of terrestrial SOM, especially under the harsh environmental conditions of Mediterranean ecosystems, for which models predict even drier conditions in the future.     

Small mammal herbivores mediate the effects of soil nitrogen and invertebrate herbivores on grassland diversity

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Eradication of feral goats,not population control,as a strategy to conserve plant communities on Mediterranean islets

The introduction of mammalian herbivores negatively affects insular vegetation, especially plant species with narrow distributions that are vulnerable to herbivory or that evolved in the absence of native mammal herbivores. Eradication programs have been performed on many islands worldwide, though assessment of the responses of vegetation is crucial to guarantee the success of these programs. In this study, we aimed to evaluate vegetation recovery after a reduction in the feral goat population on Es Vedrà, an islet in the Balearic Islands (Western Mediterranean Basin). We monitored nine permanent plots in three different habitats (rocky areas, grasslands, shrublands) to evaluate the variation in plant coverage, functional traits and diversity indexes over time. We obtained data for each plot by annual field sampling in mid-May of 2016–2019. We observed a significant recovery of the predominant species in each plant community and an increase in many functional traits. We found significant variation in taxonomic diversity in rocky areas and grasslands, but not in shrublands, and functional diversity only varied in rocky areas. Therefore, plant diversity benefited from the reduction in the goat population and functional redundancy increased in rocky areas, improving the capacity to respond against disturbances. However, the reduction in the goat population was not sufficient to preserve plant communities and negative effects reappeared in 2019, coinciding with the increase in the goat population. Therefore, absolute eradication of introduced herbivores represents a unique, efficient strategy to guarantee the ecological restoration of affected habitats in microinsular ecosystems.     

Spatial factors and plant attributes influence soil fungal community distribution patterns in the lower reaches of the Heihe River Basin,Northwest China

Inland river basins include critical habitats and provide various ecosystem services in extremely arid lands. However, we know little about the distribution patterns of soil fungal communities in these river basins. We investigated the distribution patterns of soil fungal communities from the riparian oasis zone (ROZ) to the circumjacent desert zone (CDZ) at the lower reaches of the Heihe River. The results indicated that soil fungal communities were mainly dominated by the phyla Ascomycota and Basidiomycota across all samples. The dominant soil fungi taxa were significantly different between ROZ and CDZ habitats at both the phylum and genus levels. Fungal alpha diversity was mainly affected by spatial factors and plant functional traits, and Pearson correlation analysis revealed that fungal alpha diversity was more closely related to plant functional traits than soil properties. Furthermore, fungal community structure was best explained by spatial factors and plant attributes (including plant diversity and plant functional traits). Together, our findings provide new insights into the significance of spatial factors and plant attributes for predicting distributions of fungal communities in arid inland river basins, which will help us better understand the functions and services of these ecosystems.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Influences of Plant Species Composition,Fertilisation and <Emphasis Type="Italic">Lolium perenne</Emphasis> Ingression on Soil Microbial Community Structure in Three Irish Grasslands

Semi-natural grassland soils are frequently fertilised for agricultural improvement. This practice often comes at a loss of the indigenous flora while fast-growing nitrogen-responsive species, such as Lolium perenne, take over. Since soil microbial communities depend on plant root exudates for carbon and nitrogen sources, this shift in vegetation is thought to influence soil microbial community structure. In this study, we investigated the influence of different plant species, fertilisation and L. perenne ingression on microbial communities in soils from three semi-natural Irish grasslands. Bacterial and fungal community compositions were determined by automated ribosomal intergenic spacer analysis, and community changes were linked to environmental factors by multivariate statistical analysis. Soil type had a strong effect on bacterial and fungal communities, mainly correlated to soil pH, as well as soil carbon and nitrogen status. Within each soil type, plant species composition was the main influencing factor followed by nitrogen fertilisation and finally Lolium ingression in the acidic upland and mesotrophic grassland. In the alkaline grassland, however, Lolium ingression had a stronger effect than fertilisation. Our results suggest that a change in plant species diversity strongly influences the microbial community structure, which may subsequently lead to significant changes in ecosystem functioning.     

Belowground biodiversity effects of plant symbionts support aboveground productivity

Soil microbes play key roles in ecosystems, yet the impact of their diversity on plant communities is still poorly understood. Here we demonstrate that the diversity of belowground plant-associated soil fungi promotes plant productivity and plant coexistence. Using additive partitioning of biodiversity effects developed in plant biodiversity studies, we demonstrate that this positive relationship can be driven by complementarity effects among soil fungi in one soil type and by a selection effect resulting from the fungal species that stimulated plant productivity the most in another soil type. Selection and complementarity effects among fungal species contributed to improving plant productivity up to 82% and 85%, respectively, above the average of the respective fungal species monocultures depending on the soil in which they were grown. These results also indicate that belowground diversity may act as insurance for maintaining plant productivity under differing environmental conditions.