The structure of plant spatial association networks is linked to plant diversity in global drylands

Taxonomic inventory and distributions of Chenopodiaceae (Amaranthaceae s.l.) in Orenburg Region, Russia

Background

Orenburg Region is located in the South Urals, mostly in the steppe zone and is characterised by various landscapes suitable for many Chenopodiaceae. The species of Chenopodiaceae are present in all major plant communities (saline vegetation, steppes, on limestone, chalk and sand, and as degraded or ruderal communities). In the steppe zone, many native subshrubby species (Atriplexcana, Caroxylonlaricinum, Suaedaphysophora) playing a crucial role in semi-deserts (known as southern steppes in the recent Russian literature) located southwards of Orenburg Region are locally found, and several annuals (Salicorniaperennans, Suaeda spp.) are most common dominants in plant communities. Some typical semi-desert species (Kalidiumfoliatum, Bassiahyssopifolia, Sodafoliosa, Spirobassiahirsuta) are found in the easternmost part of the region.

New information

We compiled a checklist of Chenopodiaceae in Orenburg Region, with two new records (Chenopodiumvirgatum, Corispermumlaxiflorum), based on our critical revision, comprehensive inventory of herbarium specimens and documented observations and field research. In total, we report 76 species in the Region, which is the third-highest number of the Chenopodiaceae species compared with other administrative territories of European Russia, North Caucasus and West Siberia. Alien and native taxa are distinguished. Zonal patterns of species distributions are confirmed. A preliminary conservation status is proposed for each native species. Three species are recommended for exclusion from the Red Data Book of Orenburg Region: Petrosimoniatriandra (because of its extensive distribution), Kalidiumfoliatum and Anabasissalsa (because of the lack of actual threat to their populations). Arthrophytumlehmannianum and Salsolarosacea are considered threatened (Vulnerable) because of their restricted occurrence and population size and because their localities are under anthropogenic pressure. Atriplexhortensis, Atriplexrosea, Chenopodiumacuminatum, C.karoi, C.praetericola, C.vulvaria, Climacopteraaffinis, C.crassa, Halimocnemiskarelinii, Salsolapaulsenii and Xylosalsolaarbuscula are excluded from the checklist, based on various reasons as discussed in the paper. Point distribution maps are provided for each species. Agriophyllumpungens (Vahl) Link is accepted as the correct authorship instead of "M.Bieb. ex C.A.Mey."

Positive associations fuel soil biodiversity and ecological networks worldwide

Microbial interactions are key to maintaining soil biodiversity. However, whether negative or positive associations govern the soil microbial system at a global scale remains virtually unknown, limiting our understanding of how microbes interact to support soil biodiversity and functions. Here, we explored ecological networks among multitrophic soil organisms involving bacteria, protists, fungi, and invertebrates in a global soil survey across 20 regions of the planet and found that positive associations among both pairs and triads of soil taxa governed global soil microbial networks. We further revealed that soil networks with greater levels of positive associations supported larger soil biodiversity and resulted in lower network fragility to withstand potential perturbations of species losses. Our study provides unique evidence of the widespread positive associations between soil organisms and their crucial role in maintaining the multitrophic structure of soil biodiversity worldwide.

Contextualizing the ecology of plant-plant interactions and constructive networks

Botanical concepts have traditionally viewed the environment as a static box containing plants. In this box, plants compete with one another and act as passive resource consumers subjected to the environment in a top-down manner. This entails that plants have only negative effects on other plants and have no influence on the environment. By contrast, there is increasing evidence that plants have positive, bottom-up engineering effects and diversity effects on other plants and on the environment. Here, to overcome the limitations of top-down environmental control, antagonistic-only and pairwise interactions, I propose the concept of constructive networks. Constructive networks unify niche construction and network theory recognizing that (i) plants have manifold ecological functions and impacts on their neighbours, and (ii) the environment shapes and is shaped by diverse organisms, primarily plants. Constructive networks integrate both plant-environment and plant-plant interactions in a relational context. They address how plants influence the environment and support or inhibit other plant species by physically, biochemically and ecologically shaping environmental conditions. Constructive networks acknowledge the fact that diverse plants change and create novel environmental conditions and co-produce, share and transform resources, thereby influencing biological communities and the environment in constructive ways. Different interaction types are considered simultaneously in constructive networks. Yet, the main limitation to understanding constructive networks is the identification of plant links. This barrier may be overcome by applying complexity theory and statistical mechanics to comparative data and experimental field botany. Considering multiple interaction types and feedback between plants and the environment may improve our understanding of mechanisms responsible for biodiversity maintenance and help us to better anticipate the response of plant systems to global change.

Genomic epidemiology of Escherichia coli: antimicrobial resistance through a One Health lens in sympatric humans, livestock and peri-domestic wildlife in Nairobi, Kenya

BACKGROUND

Livestock systems have been proposed as a reservoir for antimicrobial-resistant (AMR) bacteria and AMR genetic determinants that may infect or colonise humans, yet quantitative evidence regarding their epidemiological role remains lacking. Here, we used a combination of genomics, epidemiology and ecology to investigate patterns of AMR gene carriage in Escherichia coli, regarded as a sentinel organism.

METHODS

We conducted a structured epidemiological survey of 99 households across Nairobi, Kenya, and whole genome sequenced E. coli isolates from 311 human, 606 livestock and 399 wildlife faecal samples. We used statistical models to investigate the prevalence of AMR carriage and characterise AMR gene diversity and structure of AMR genes in different host populations across the city. We also investigated household-level risk factors for the exchange of AMR genes between sympatric humans and livestock.

RESULTS

We detected 56 unique acquired genes along with 13 point mutations present in variable proportions in human and animal isolates, known to confer resistance to nine antibiotic classes. We find that AMR gene community composition is not associated with host species, but AMR genes were frequently co-located, potentially enabling the acquisition and dispersal of multi-drug resistance in a single step. We find that whilst keeping livestock had no influence on human AMR gene carriage, the potential for AMR transmission across human-livestock interfaces is greatest when manure is poorly disposed of and in larger households.

CONCLUSIONS

Findings of widespread carriage of AMR bacteria in human and animal populations, including in long-distance wildlife species, in community settings highlight the value of evidence-based surveillance to address antimicrobial resistance on a global scale. Our genomic analysis provided an in-depth understanding of AMR determinants at the interfaces of One Health sectors that will inform AMR prevention and control.

Correlation between Vegetation Structure and Species Diversity in Traditional Villages in Karst Topographic Regions of the Zunyi City, China

Studying the relationship between vegetation structure and diversity is important in an area having karst topography and unique traditional customs. We selected a total of six traditional villages in Zunyi City, China, to collect vegetation data. Additionally, using one-way ANOVA and the Pearson correlation coefficient analytic method to analyze, the results showed that, overall, plant communities were mostly regularly distributed. The overall differentiation degree was low aggregation, intensity, and the extreme intensity mixed state. Overall, competitive pressure, growth vigor and stability were better than the natural forest. The community stability index at lower altitude was significantly higher than that at higher altitude. The recorded plant communities in the living space were typically aggregated, the plant communities were randomly distributed in the production space, and the plant communities were uniformly distributed in the ecological space. In general, the diversity indexes, except the Jh index, were the highest in the herb layer; the second was in the shrub layer and the lowest was in the tree layer. Species diversity at the middle altitude was higher than that at low and high altitudes (except for the shrub at a high altitude of 1100-1160 m). The overall plant species diversity was highest in the living space, second highest in the ecological space and lowest in the production space. On the whole, there was a significant correlation between the spatial structure of plant communities and the species diversity of plant communities at different altitudes, and in PLE spaces. The main objective of this study was to reveal the plant community structure, species diversity, and their relationship under the dual effects of national traditional culture and karst landform. Additionally, we sought to provide theoretical guidance for the construction of plant community protection and biodiversity conservation in traditional villages in karst areas.

Spatial associations between plants and vegetation community characteristics provide insights into the processes influencing plant rarity

Determining the drivers of plant rarity is a major challenge in ecology. Analysing spatial associations between different plant species can provide an exploratory avenue for understanding the ecological drivers of plant rarity. Here, we examined the different types of spatial associations between rare and common plants to determine if they influence the occurrence patterns of rare species. We completed vegetation surveys at 86 sites in woodland, forest, and heath communities in south-east Australia. We also examined two different rarity measures to quantify how categorisation criteria affected our results. Rare species were more likely to have positive associations with both rare and common species across all three vegetation communities. However, common species had positive or negative associations with rare and other common species, depending on the vegetation community in which they occurred. Rare species were positively associated with species diversity in forest communities. In woodland communities, rare species were associated negatively with species diversity but positively associated with species evenness. Rare species with high habitat specificity were more clustered spatially than expected by chance. Efforts to understand the drivers of plant rarity should use rarity definitions that consider habitat specificity. Our findings suggest that examining spatial associations between plants can help understand the drivers of plant rarity.

Network motifs involving both competition and facilitation predict biodiversity in alpine plant communities

Biological diversity depends on multiple, cooccurring ecological interactions. However, most studies focus on one interaction type at a time, leaving community ecologists unsure of how positive and negative associations among species combine to influence biodiversity patterns. Using surveys of plant populations in alpine communities worldwide, we explore patterns of positive and negative associations among triads of species (modules) and their relationship to local biodiversity. Three modules, each incorporating both positive and negative associations, were overrepresented, thus acting as "network motifs." Furthermore, the overrepresentation of these network motifs is positively linked to species diversity globally. A theoretical model illustrates that these network motifs, based on competition between facilitated species or facilitation between inferior competitors, increase local persistence. Our findings suggest that the interplay of competition and facilitation is crucial for maintaining biodiversity.

Plant-plant co-occurrences under a complex land-use gradient in a temperate forest

Land-use generates multiple stress factors, and we need to understand their effects on plant-plant interactions to predict the consequences of land-use intensification. The stress-gradient hypothesis predicts that the relative strength of positive and negative interactions changes inversely under increasing environmental stress. However, the outcome of interactions also depends on stress factor's complexity, the scale of analysis, and the role of functional traits in structuring the community. We evaluated plant-plant co-occurrences in a temperate forest, aiming to identify changes in pairwise and network metrics under increasing silvopastoral use intensity. Proportionally, positive co-occurrences were more frequent under high than low use, while negative co-occurrences were more frequent under low than high. Networks of negative co-occurrences showed higher centralization under low use, while networks of positive co-occurrences showed lower modularity and higher centralization under high use. We found a partial relationship between co-occurrences and key functional traits expected to mediate facilitation and competition processes. Our results shows that the stress-gradient hypothesis predicts changes in spatial co-occurrences even when two stress factors interact in a complex way. Networks of negative co-occurrences showed a hierarchical effect of dominant species under low use intensity. But positive co-occurrence network structure partially presented the characteristics expected if the facilitation was an important mechanism characterizing the community under high disturbance intensity. The partial relationship between functional traits and co-occurrences may indicate that other factors besides biotic interactions may be structuring the observed negative spatial associations in temperate Patagonian forests.

Integration of remote sensing in spatial ecology: assessing the interspecific interactions of two plant species in a semi-arid woodland using unmanned aerial vehicle (UAV) photogrammetric data

The spatial structure of plant communities in semi-arid regions is mostly derived by plant-plant interactions and environmental heterogeneity. In this study, we investigated the intra- and interspecific interactions and their contribution to growth inhibition in the patches of Pistacia trees and Amygdalus shrubs in semi-arid woodland communities through the implementation of photogrammetric data provided by unmanned aerial vehicle (UAV). This study was conducted in a part of Wild Pistachio Natural Reserve covered by Pistacia-Amygdalus stands in Zagros Mountains, western Iran. We used univariate and bivariate forms of pair- and mark correlation functions and Analytical Global Envelopes under inhomogeneous Poisson process which allow detection of the interactions of the species within the 45-ha study area. Our results indicated that the UAV-derived photogrammetric data proved to be efficient in identification of the plant individuals (F-score ≈ 0.92 for both species). Additionally, strong coefficients of determination (R² = 0.98 and 0.94 for Pistacia and Amygdalus, respectively) supported prediction of crown area. We observed the aggregation of the species individuals in clusters of conspecifics and heterospecifics at small spatial scales, most likely as a result of aggregation in favourable parts of the study area. The aggregation of the species within patches had a marked effect on their size (i.e., crown area, height) inferred as growth inhibition, probably due to intra- and interspecific competition. Our findings demonstrated that promising UAV photogrammetric data can be effectively utilized by ecologists for investigation of plant associations, hence increasing the potentiality of remote sensing in spatial ecology of vegetation patches in semi-arid environments.

Effects of Water and Energy on Plant Diversity along the Aridity Gradient across Dryland in China

Plants need water and energy for their growth and reproduction. However, how water and energy availability influence dryland plant diversity along the aridity gradient in water-limited regions is still lacking. Hence, quantitative analyses were conducted to evaluate the relative importance of water and energy to dryland plant diversity based on 1039 quadrats across 184 sites in China's dryland. The results indicated that water availability and the water-energy interaction were pivotal to plant diversity in the entire dryland and consistent with the predictions of the water-energy dynamic hypothesis. The predominance of water limitation on dryland plant diversity showed a weak trend with decreasing aridity, while the effects of energy on plants were found to be significant in mesic regions. Moreover, the responses of different plant lifeforms to water and energy were found to vary along the aridity gradient. In conclusion, the study will enrich the limited knowledge about the effects of water and energy on plant diversity (overall plants and different lifeforms) in the dryland of China along the aridity gradient.

Global ecosystem thresholds driven by aridity

Aridity, which is increasing worldwide because of climate change, affects the structure and functioning of dryland ecosystems. Whether aridification leads to gradual (versus abrupt) and systemic (versus specific) ecosystem changes is largely unknown. We investigated how 20 structural and functional ecosystem attributes respond to aridity in global drylands. Aridification led to systemic and abrupt changes in multiple ecosystem attributes. These changes occurred sequentially in three phases characterized by abrupt decays in plant productivity, soil fertility, and plant cover and richness at aridity values of 0.54, 0.7, and 0.8, respectively. More than 20% of the terrestrial surface will cross one or several of these thresholds by 2100, which calls for immediate actions to minimize the negative impacts of aridification on essential ecosystem services for the more than 2 billion people living in drylands.

Superhost Plants Alter the Structure of Plant-Galling Insect Networks in Neotropical Savannas

Host plants may harbor a variable number of galling insect species, with some species being able to harbor a high diversity of these insects, being therefore called superhost plants. In the present study, we tested the hypothesis that the occurrence of superhost plant species of genus Qualea (Vochysiaceae) affects the structure of plant–galling insect ecological networks in Brazilian Cerrado. We sampled a total of 1882 plants grouped in 131 species and 43 families, of which 64 species and 31 families of host plants hosted 112 galling insect species. Our results showed that occurrence of superhosts of genus Qualea increased the linkage density of plant species, number of observed interactions, and the size of plant–galling insect networks and negatively affected the network connectance (but had no effect on the residual connectance). Although the occurrence of Qualea species did not affect the plant species richness, these superhosts increased the species richness and the number of interactions of galling insects. Our study represents a step forward in relation to previous studies that investigated the effects of plant diversity on the plant–insect networks, showing that few superhost plant species alter the structure of plant–herbivore networks, even without having a significant effect on plant diversity.

Plant species-area relationships are determined by evenness, cover and aggregation in drylands worldwide

AIM

Species-area relationships (also known as 'species-area curves' and 'species accumulation curves') represent the relationship between species richness and the area sampled in a given community. These relationships can be used to describe diversity patterns while accounting for the well-known scale-dependence of species richness. Despite their value, their functional form and parameters, as well as their determinants, have barely been investigated in drylands.

LOCATION

171 drylands from all continents except Antarctica.

TIME PERIOD

2006-2013.

MAJOR TAXA STUDIED

Perennial plants.

METHODS

We characterized species-area relationships of plant communities by building accumulation curves describing the expected number of species as a function of the number of sampling units, and later compared the fit of three functions (power-law, logarithmic and Michaelis-Menten). We tested the prediction that the effects of aridity, soil pH on SAR are mediated by vegetation attributes such as evenness, cover, and spatial aggregation.

RESULTS

We found that the logarithmic relationship was the most common functional form (c.50%), followed by Michaelis-Menten (c.33%) and power-law (c.17%). Functional form was mainly determined by evenness. Power-law relationships were found mostly under low evenness, logarithmic relationships peaked under intermediate evenness and the Michalis-Menten function increased in frequency with increasing evenness. The SAR parameters approximated by the logarithmic model ('small-scale richness' (b₀ ) and 'accumulation coefficient' (b₁ )) were determined by vegetation attributes. Increasing spatial aggregation had a negative effect on the small-scale richness and a positive effect on the accumulation coefficient, while evenness had an opposite effect. In addition, accumulation coefficient was positively affected by cover. Interestingly, aridity decreased small scale richness but did not affect the accumulation coefficient.

MAIN CONCLUSIONS

Our findings highlight the role of evenness, spatial aggregation and cover as main drivers of species area relationships in drylands, the Earth's largest biome.

Intransitivity increases plant functional diversity by limiting dominance in drylands worldwide

1. Biotic interactions are key determinants of plant community structure. Indirect interactions such as intransitivity (i.e. in the absence of competitive hierarchies among species) have been hypothesized to benefit diversity within plant communities. However, their effect on functional diversity remains scarcely explored in real communities. Here we develop a novel approach to infer intransitivity from plant spatial patterns and functional traits (height and specific leaf area), and quantify its effect on different components of plant diversity along environmental gradients in 100 drylands from all continents except Antarctica. 2. We first calculated the spatial association pattern for all perennials to infer competition between species. Trait values were used as a proxy of competitive hierarchies to infer the direction of these interactions. We used multiple regression models to evaluate how intransitivity responds to environmental variables (mean annual temperature and precipitation, precipitation seasonality, soil pH, sand content and woody cover). We also used confirmatory path analysis to evaluate the effects of intransitivity on species richness and evenness, trait dispersion and functional diversity. 3. Intransitivity mostly responded to climatic variables, and significantly increased with precipitation scarcity and seasonality. We found that intransitivity had significant effects on functional diversity, mostly by promoting plant community evenness. However, the dominance of woody vegetation (steppes vs. shrublands) modulated this effect.

SYNTHESIS

Intransitivity increased the functional diversity of drylands, particularly under high rainfall seasonality, by limiting functionally dominant species. Our findings specify how intransitivity structures the functional diversity of dryland vegetation worldwide. Intransitivity may be particularly important in ecosystems where the availability of abiotic resources changes over time, thereby breaking down inherent competitive hierarchies between plant species. Neglecting intransitivity will bias our estimation of the impacts of biotic interactions on plant communities, a fundamental issue to fully understand how plant communities will respond to ongoing environmental changes.