Relative Contributions of Spatial and Environmental Processes and Biotic Interactions in a Soil Collembolan Community

Understanding the underlying processes of how communities are structured remains a central question in community ecology. However, the mechanisms of the soil animal community are still unclear, especially for communities on a small scale. To evaluate the relative roles of biotic interactions and environmental and spatial processes in a soil collembolan community, a field experiment was carried out on a small scale(50 m) in the farmland ecosystem of the Sanjiang Plain, Northeast China. In August and October, 2011, we took 100 samples each month in a 50 m × 50 m plot using a spatially delimited sampling design. Variation partitioning was used to quantify the relative contributions of the spatial and environmental variables. A null model was selected to test for the non-randomness pattern of species co-occurrence and body size in assemblages of collembolans and to test whether the pattern observed was the result of environmental or biotic processes that structured the community on a small scale. The results showed that large variance was accounted for by spatial variables(18.99% in August and 21.83% in October, both were significant). There were relatively lower effects of environmental variation(3.56% in August and 1.45% in October, neither was significant), while the soil water content, soil p H and soybean height explained a significant portion of the variance that was observed in the spatial pattern of the collembolan community. Furthermore, the null model revealed more co-occurrence than expected by chance, suggesting that collembolan communities had a non-random co-occurrence pattern in both August and October. Additionally, environmental niche overlap and the body size ratio of co-occurrence showed that interspecific competition was not influential in collembolan community structuring. Considering all of the results together, the contributions of spatial and environmental processes were stronger than biotic interactions in the small-scale structuring of a soil collembolan community.

Responses of biotic interactions of dominant and subordinate species to decadal warming and simulated rotational grazing in Tibetan alpine meadow

Warming increases competition among plant species in alpine communities by ameliorating harsh environmental conditions,such as low temperatures. Grazing, as the main human activity, may mitigate the effect of warming, as previously reported.However, it is critical to refine the effects of warming on biotic interactions among species, for example, by taking the competitive ability of species into consideration. Based on a 10-year warming and grazing experiment in a Tibetan alpine meadow, we evaluated interspecific biotic interactions of dominant and subordinate species, using the approach of interspecific spatial associations. Warming significantly increased competition between subordinate and dominant species as well as among subordinate species, but not among dominant species. Moreover, facilitation of dominant-subordinate species also increased under warming. Simulated rotational grazing had similar effects to warming, with increasing interspecific competition. Our results show that, when studying the effects of warming on biotic interactions among species, it is necessary to characterize different species pairs relative to their competitive ability, and that simulated rotational grazing does not mitigate the effects of warming in the long term. Our results also provide evidence that the spatial pattern of species is a critical mechanism in species coexistence.

Distribution and influencing factors of microeukaryote in different water layers of the southwestern Indian Ocean Ridge

Microeukaryotes play a vital role in shaping marine ecosystems,especially in marine productivity,the microbial food web,and carbon cycle.The Indian Ocean is one of the largest oligotrophic areas in the world,but little is known about the biodiversity of microeukaryotes in the area.The community composition and geographical distribution of microeukaryotes collected from the surface(SUR)and deep chlorophyll maximum(DCM)layers in the southwestern Indian Ocean were studied using high-throughput sequencing of the 18S rRNA gene.The metagenomic data helped quantify the impact of environmental factors on microeukaryotic communities.The relative abundance of different taxa groups exhibited distinct patterns between SUR and DCM layers,except for the most dominant Dinoflagellata that accounted for more than 40.6%abundance in each sample.Radiolaria was much more abundant in the nutrient-rich DCM layer than the SUR layer.The community similarity of microeukaryotes decreased with increasing of geographic distance,whereas the temperature and inorganic nitrogen were the most important environmental parameters to community structure.Abundant communities were more influenced by dispersal limitations and rare communities were more responsive to environmental factors.Correlation network analyses revealed strong biotic interactions indicative of parasitism,predation and competition,and their contribution to microeukaryotic population in diverse environments.Overall,this study provided insights into the biodiversity of microeukaryotes by characterizing the differences between water layers and identifying the driving factors in the ocean.

Protist Interactions and Seasonal Dynamics in the Coast of Yantai, Northern Yellow Sea of China as Revealed by Metabarcoding

Facilitated by the high-throughput sequencing(HTS)technique,the importance of protists to aquatic systems has been widely acknowledged in the last decade.However,information of protistan biotic interactions and seasonal dynamics is much less known in the coast ecosystem with intensive anthropic disturbance.In this study,year-round changes of protist community composition and diversity in the coastal water of Yantai,a city along the northern Yellow Sea in China,were investigated using HTS for the V4 region of 18S rDNA.The interactions among protist groups were also analyzed using the co-occurrence network.Data analyses showed that Alveolata,Chlorophyta,and Stramenopiles are the most dominant phytoplanktonic protists in the investigated coastal area.The community composition displayed strong seasonal variation.The abundant families Dino-Group-I-Clade-1 and Ulotrichales_X had higher proportions in spring and summer,while Bathycoccaceae exhibited higher ratios in autumn and winter.Alpha diversities(Shannon and Simpson)were the highest in autumn and the lowest in spring(ANOVA test,P<0.05).Nutrients(SiO42−,PO43−),total organic carbon(TOC),and pH seemed to drive the variation of alpha diversity,while temperature,PO43−and TON were the most significant factors influencing the whole protist community.Co-variance network analyses reveal frequent co-occurrence events among ciliates,chlorophytes and dinoflagellate,suggesting biotic interactions have been induced by predation,parasitism and mixotrophy.

Trends in Global and Mexico Research in Wildfires: A Bibliometric Perspective

In the last two decades, unprecedented changes have taken place in the frequency and severity of wildfires;in different regions of the world, some fires were even classified as megafires. Although there are studies about the diverse effects of fire, which have made significant theoretical contributions, a comprehensive review of the changes in fire research is required to understand worldwide patterns, particularly in those countries where fire activity is on the rise, such is the case of Mexico. The objective of this study was to analyze the trends in the research on wildfires published in Mexico and worldwide over a 40-year timescale. For this purpose, the Web of Science database, bibliometric tools, and the keywords TI = Forest fire* OR TI = Wildfire* were used to extract as many articles as possible related to fires from 1980 to 2020, without being restricted to those studies whose title included any of the variants of the keywords. There were 8458 publications about fires in the vegetation cover, with a notable increase in the frequency of studies in the previous decade;52% of the studies were concentrated in five countries and 20% of the articles focused on the study of different aspects of the soil. Mexico ranks thirteenth in volume of scientific production and studies in the country have focused mainly on the description of the quantitative relationship between the size of the affected area and the number of occurrences in the landscape, meanwhile, studies on fires and the consequences on the biotic interactions have been little explored.

Inference of Fish Community Assembly in Intermittent Rivers Using Joint Species Distribution Models and Trophic Guilds

Mechanisms that drive species co-occurrence are poorly documented for intermittent rivers of semiarid regions. Here, we investigated fish community assembly in response to habitat types and physicochemical conditions in intermittent rivers of the lower Okavango Delta, Botswana. Using Joint Species Distribution Models, we inferred relative influences of environmental filtering and species interactions on patterns of species co-occurrence. Fishes were surveyed from multiple locations during drought and flood phases of the annual hydrological cycle. Species were classified into trophic guilds to facilitate inference about the types of species interactions that influence community structure. Water physicochemistry (depth, temperature, and dissolved oxygen concentration) was significantly associated with species distributions, whereas habitat type (ephemeral vs. permanent) was not significantly associated with species distribution. Controlling for the influence of environmental covariates resulted in three negative residual correlations, of which two involved non-predatory fishes from different trophic guilds, suggesting a behavioral selection of, or enhanced survival within, habitats with different environmental conditions. There was a negative residual correlation between a predator and a potential prey species, suggesting a role for predation mortality or threat in species segregation. Our results demonstrate that using trophic guilds in conjunction with JSDMs can enhance inferences about mechanisms of community assembly.

Heavy collecting induces smaller and deeper Fritillariae Cirrhosae Bulbus in the wild

Fritillariae Cirrhosae Bulbus is a well-known traditional Chine medicinal herb. However, the demand for this herb is leading to over-collection and its decline in the wild. This study aims to investigate the ecological conditions of Fritillaria cirrhosa under the influence of human disturbance, biotic species interactions and climatic conditions. We established a total of 78 plots at 14 sites of F. cirrhosa in its distribution center, the Hengduan Mountains area. At each site, we estimated the abundance of F. cirrhosa at different distances from roads. The diameter and height of F. cirrhosa fruit and bulbs were measured and compared to underground bulb depth. We then analyzed the effects of environmental conditions and human disturbance on the abundance of F. cirrhosa using variance partitioning. We found that(1)abundance of F. cirrhosa and their underground bulb depth showed a significant linear increase with the distance from the main road;(2) the diameter/height of fruits and the diameter/height of fruits/bulbs showed significantly different responses to the human disturbance;(3) the community associates,climate and spatial conditions can explain 58%, 22% and 27%, respectively, of the variance in the F. cirrhosa abundance. These results highlight the fact that human disturbance and biotic factors have a great influence on the survival of F. cirrhosa, even more than climate conditions.

Effects of soil organism interactions and temperature on carbon use efficiency in three different forest soils

Microbial carbon use efficiency(CUE)affects the soil C cycle to a great extent,but how soil organisms and the abiotic environment combine to influence CUE at a regional scale remains poorly understood.In the current study,microcosms were used to investigate how microbial respiration,biomass,and CUE responded to biotic and abiotic factors in natural tropical,subtropical,and temperate forests.Soil samples from the forests were collected,sterilized,and populated with one or a combination of three types of soil organisms(the fungus Botrytis cinerea,the bacterium Escherichia coli,and the nematode Caenorhabditis elegans).The microcosms were then kept at the mean soil temperatures of the corresponding forests.Microbial respiration,biomass,and CUE were measured over one-month incubation period.The results showed that microbial biomass and CUE were significantly higher,but microbial respiration lower in the subtropical and temperate forest soils than in tropical forest soil.Biotic factors mainly affected CUE by their effect on microbial biomass,while temperature affected CUE by altering respiration.Our results indicate that temperature regulates the interactive effects of soil organisms on microbial biomass,respiration,and CUE,which would provide a basis for understanding the soil C cycle in forest ecosystems.

Diverse outcomes of species interactions in an invaded annual plant community

Aims Despite acknowledgement that interactions among native and exotic species are important for determining the structure and diversity of novel communities,directed experiments using mul-tiple exotics from the same system are rare.Recent observational studies have highlighted distinct ways that exotic species interface with resident natives across invaded communities.The correlative nature of these studies,however,has provided few details about the mechanisms driving distinct interaction outcomes within the same communities.Our aim was to determine how three exotic annual plant species with distinct relationships with local plant diversity impact the performance of a co-occurring native annual in the York gum-jam woodlands of Western Australia.Methods We grew species in experimental communities in growth chambers at varying total planting densities to assess how interaction out-comes varied among natives and exotics across a gradient of com-petition intensity.We measured a variety of performance responses,including survival,biomass and population-level and individual-level reproductive investment.Important Findings Overall,the effects of interspecific versus intraspecific competi-tion on performance varied with the identity and density of exotic competitors.The exploitative exotic grass Bromus madritensis was dominant in polyculture,whereas the diminutive grass Pentameris airoides conferred weak intraspecific competition and interspe-cific facilitation on native Waitzia nitida.The exotic broadleaf forb,Hypochaeris glabra,suppressed growth and survival of W.nitida,while W.nitida had weakly negative,neutral or positive effects on all exotics.These outcomes highlight the complexity of interac-tions impacting the diversity,stability and structure of novel plant communities.As few of these communities contain a single exotic species,understanding the competitive dynamics occurring in diverse novel communities is critical for their conservation and restoration.

The grassland carbon cycle:Mechanisms,responses to global changes,and potential contribution to carbon neutrality

Grassland is one of the largest terrestrial biomes,providing critical ecosystem services such as food production,biodiversity conservation,and climate change mitigation.Global climate change and land-use intensification have been causing grassland degradation and desertification worldwide.As one of the primary medium for ecosystem energy flow and biogeochemical cycling,grassland carbon(C)cycling is the most fundamental process for maintaining ecosystem services.In this review,we first summarize recent advances in our understanding of the mechanisms underpinning spatial and temporal patterns of the grassland C cycle,discuss the importance of grasslands in regulating inter-and intra-annual variations in global C fluxes,and explore the previously unappreciated complexity in abiotic processes controlling the grassland C balance,including soil inorganic C accumulation,photochemical and thermal degradation,and wind erosion.We also discuss how climate and land-use changes could alter the grassland C balance by modifying the water budget,nutrient cycling and additional plant and soil processes.Further,we examine why and how increasing aridity and improper land use may induce significant losses in grassland C stocks.Finally,we identify several priorities for future grassland C research,including improving understanding of abiotic processes in the grassland C cycle,strengthening monitoring of grassland C dynamics by integrating ground inventory,flux monitoring,and modern remote sensing techniques,and selecting appropriate plant species combinations with suitable traits and strong resistance to climate fluctuations,which would help design sustainable grassland restoration strategies in a changing climate.

Quantifying the relative importance of direct and indirect effects influencing bird nestling growth

Nestling growth parameters are integral components of avian life-history strategies as they are crucial determinants of individual survival.Although many factors impact on nestling growth,the relative contribution of each one is still debated in the literature.Most studies rely on the assumption that each factor directly affects nestling growth,but indirect effects mediated by other factors are usually the rule in nature.In this study,we present a comprehensive view of both direct and indirect factors affecting nestling growth using the Red-crested Cardinal(Paroaria coronata)as model system.We evaluated the relative importance of different habitat(forest structure),biotic interactions(botfly larvae ectoparasitism,number of siblings,hatching order),and temporal factors(time of breeding)on nestling growth parameters in 278 nestlings of 128 nests by using piecewise structural equation models.We found that botfly ectoparasitism had the strongest direct effect on nestling growth and,in turn,forest structure increased the probability of botfly occurrence.Besides,the interaction between the number of siblings and hatching order influenced nestling growth,indicating that the first and second nestlings had disproportionately higher growth rates in large than in small clutches.Time of breeding also showed a strong positive indirect effect on botfly occurrence,as well as a weak direct positive effect on nestling growth.Our results demonstrate that,under natural conditions,nestling growth is driven by different factors acting not only directly,but also indirectly on this essential life history trait,and that these factors weave a complex web of interrelated variables.

Effects of drought and slope aspect on canopy facilitation in a mountainous rangeland

Aims Species composition and diversity of the mountainous rangelands are results of interactions between environmental severities,het-erogeneous topography and facilitative effects by nurse plants.This research was aimed to compare relative effects of these three envi-ronmental variables on the natural vegetation of a mountainous rangeland.For a more detailed understanding,effects of four dif-ferent nurse species were separately compared on the various plant growth forms and on two community plant responses(diversity and abundance).Methods A mountainous semiarid rangeland was selected in Baharkish,Quchan,Northeast of Iran.Density and canopy cover of all plant species were recorded under the canopy of four different shrubs and in open areas,in north and south-facing aspects,and in a normal and a drought year.Shannon diversity,total abundance(%cover)and the abundance of different growth forms were used as crite-ria for assessing effects of the environmental variables.Data were arranged in a factorial combination and analyzed by three-way analysis of variance using a GLM analysis.Important Findings(i)Drought,aspect and canopy created niche differentiation:annual forbs and shrubs were more affected by drought,whereas geophytes and grasses were more responsive to slope aspects.Effects of drought and slope aspect were more profound on species diversity,whereas that of canopy facilitation was stronger on plant abundance.(ii)Canopy facili-tation was dependent on severity of the abiotic factors and life history of interacting species.Canopy facilitation allowed for the persistence of only annual forbs,but it was disadvantaged during the drought year.Plant community responses to abiotic factors(slope and drought)were more dependent on the plant growth form,while responses to canopy facilitation were more dependent on the morphology and/or ecology of nurse shrubs.(iii)Effect of shrubs was dependent on their morphology and ecology:shrubs with larger canopy area and nitrogen fixation capa-bility increased,but those with allelopathic effects or a dense canopy structure decreased the diversity of the understory species.