Grazing exclusion-induced shifts,the relative importance of environmental filtering,biotic interactions and dispersal limitation in shaping desert steppe communities,northern China

Grazing exclusion is one of the most efficient approaches to restore degraded grassland but may negatively affects the recovery of species diversity. Changes in plant species diversity should be a consequence of the ecological assembly process. Local community assembly is influenced by environmental filtering, biotic interactions, and dispersal. However, how these factors potentially contribute to changes to species diversity is poorly understood, especially in harsh environments. In this study, two management sites within a Stipa breviflora desert steppe community(typical natural steppe) were selected in northern China. In one of the two management sites, grazing has been excluded since 2010 and in the other with open grazing by sheep. In August 2016, three plots were established and 100 sampling units were created within each plot in a 5 m×5 m area at the two management sites. To assess the effects of grazing exclusion on S. breviflora steppe, we analyzed the vegetation biomass, species diversity,soil organic carbon, and soil particle size distribution using paired T-tests. In addition, variation partitioning was applied to determine the relative importance of environmental filtering and dispersal limitation. Null mode analysis was used to quantify the influence of biotic interactions in conjunction with Eco Sim niche overlap and co-occurrence values. Our results demonstrated that(1) species diversity significantly decreased and the main improvements in soil quality occurred in the topsoil 0–10 cm after the grazing exclusion;(2) environmental filtering was important for community assembly between grazed and fenced grassland and this appears particularly true for soil particle size distribution, which may be well correlated with soil hydrological processes; and(3) however, competitive exclusion may play a significant role within the exclusion. The multiple pathways of assembly may collectively determine negative effects on the restoration of species diversity. Therefore, designers should be aware of the risk of reducing grazing exclusion-induced species diversity and account for manipulating processes. This in turn will reduce dominant species and promote environmental heterogeneity to maximize species diversity in semi-arid regions.     

Spatial organization of multiple plant species in arid ecosystems: linking patterns and processes

Spatial organization of multiple plant species that appears as a non-random distribution of vegetative patches is one of the mostly observed spatial patterns in arid ecosystems. Yet understanding of ecological processes allowing this spatial pattern to emerge through interspecific interactions is still lacking. With a proposed conceptual model involving interspecific trade-offs between species competitive ability and colonization ability, we have argued that within patch abundance dynamics regulated by the mechanisms of competition are strongly influenced by the between patches colonization dynamics that are maintained via this trade-offs and it holds a positive, intraspecific occupancy-abundance relationship, in which increased patch occupancy increases species density within inhabiting patches. In a constant environment, while local abundance dynamics approach toward a stable equilibrium point, a fixed spatial arrangement of species can be retained through this coupled dynamics. However, in fluctuating environments where existence of such stable equilibriums is highly uncertain, it may involve continuous transitions from one community state to another as species re-organized themselves over space through the rapid changes in local species abundances. While some of the inhabiting patches are destroyed exogenously or endogenously, or species responses to increasing environmental fluctuations vary increasingly with time, discontinuous transitions into an abrupt, irreversible state of the community dynamics may occur, as with this effect the inherent positive relationship between occupancy and abundance of species is no longer maintained.     

Spatial heterogeneity of plant species on the windward slope of active sand dunes in a semi-arid region of China

Species richness and abundance are two important species diversity variables that have attracted particular attention because of their significance in determining present and future species composition conditions.This paper aims to explain the qualitative and quantitative relationships between species diversity pattern and grain size(i.e.size of the sampling unit),and species diversity pattern and sampling area,and to analyze species diversity variability on active sand dunes in the Horqin Sandy Land,northeastern Inner Mongolia,China.A 50 m×50 m sampling plot was selected on the windward slope,where the dominant species was annual herb Agriophyllum squarrosum.Species composition and abundance at five grain sizes were recorded,and the species-area curves were produced for thirteen grain sizes.The range of values for species abundance tended to increase with increasing grain size in the study area,whereas,generally,species richness did not follow this rule because of poor species richness on the windward slope of active sand dunes.However,the homogeneity of species richness increased significantly.With the increase in sampling area,species abundance increased linearly,but richness increased logarithmically.Furthermore,variograms showed that species diversity on the windward slope of active sand dunes was weakly anisotropic and the distribution pattern was random,according to the Moran Coefficient.The results also showed that species richness was low,with a random distribution pattern.This conflicts with the results of previous studies that showed spatial aggregation in lower richness in a sampling area within a community and inferred that the physical processes play a more important role in species diversity than distribution pattern on active sand dunes.Further research into different diversity patterns and mechanisms between active sand dunes and interdune lowlands should be conducted to better understand biodiversity conservation in sand dune fields.     

Profiling soil free-living nematodes in the Namib Desert,Namibia

Functional structure and diversity of soil free-living nematodes in a desert environment depend on plant gender and sampling site.The objective of this study was to compare the composition,abundance and tropic group of soil free-living nematodes in the upper 0–10 cm soil layer under the male and female Acanthosicyos horridus Welw.ex Hook.f.plants and in the inter-shrub open areas(control)in the Namib Desert,Namibia in April 2015.Soil moisture,organic matter(OM)and pH was also analyzed.Free-living nematodes were extracted from 100 g soil using the Baermann funnel procedure,and total number of nematodes was counted under a microscope.Community composition and diversity of soil free-living nematodes were analyzed using 18S rDNA sequences.Results indicated that a total of 67 groups,including 64 species,2 genera and 1 family were identified.Feeding behavior of 58 species were identified as follows:15 bacteria-feeding species,12 fungi-feeding species,10 plant-parasite species,5 omnivorous-predator species,8 animal-parasite species,5 invertebrate-parasite species and 3 non-free-living nematodes,known as marine species.Moreover,soil free-living nematodes were found to be affected by sampling locations and plant gender,and community composition and density of these nematodes were strongly influenced by soil OM content.Result confirmed that spatial location and plant cover were main factors influencing the diversity of soil free-living nematodes.Moreover,molecular tools were found to be very useful in defining the richness of soil non-free-living nematodes.In conclusion,the results elucidated the importance of biotic variables in determining the composition and abundance of soil free-living nematodes in the Namib Desert,Namibia.     

Influence of groundwater depth on species composition and community structure in the transition zone of Cele oasis

The paper analyzes the hypothesis that the distribution of dominant plant species and characteristics of plant communities are related to groundwater depth. The results showed that variations of groundwater depth impacted distributions and characteristics of dominant plant communities. However, besides groundwater depth, the community composition and species diversity were also influenced by physiognomy of the habitat. Based on the similarity coefficient, the differences between dominant plant communities were significant at different groundwater depths. Compared with other results relating to desert vegetation and groundwater depth, variations of community distribution were similar at the large spatial scale. However, in this extremely arid region, there were significant differences in community type and community succession when compared with other arid regions, especially in relationship to deep groundwater depth. With groundwater depth from deep to shallow, communities transformed with the sequence of Alhagi communities, Tamarix spp. communities, Populus communities, Phragmites communities, and Sophora communities. At groundwater depth of less than 6.0 m, the community type and composition changed, and the species diversity increased. Among these dominant species, Tamarix exhibited the biggest efficiency in resource utilization according to niche breadth, which means it possessed the best adaptability to environmental conditions at the oasis margins.     

Plant assemblage and diversity variation with human disturbances in coastal habitats of the western Arabian Gulf

Knowledge about plant diversity along disturbance gradients is essential for conservation and management of fragmented coastal habitats.This study examined the effects of human disturbance intensity in coastal habitats of Kuwait on diversity,composition,identity and assemblage of vascular plant species.Plant survey data from 113 plots (5m×5m each) were randomly selected in 51 sites at coastal fragmented habitats at three levels of disturbance intensities (high,moderate and low) and were statistically analyzed.The results revealed that about 76% of the recorded species are considered threatened species in Kuwait,most of which are being lost in high disturbed habitats.Disturbance led to the dominance of Zygophyllum qatarense,Cornulaca aucheri and Salsola imbricata,which are species of disturbance indicators.Richness,total plant cover and species diversity were higher in moderate and low disturbed habitats than in high disturbed habitats.Beta diversity between high and low disturbed habitats was higher than either between high and moderate,or between moderate and low disturbed habitats.Cluster analyses showed statistically significant differences in composition of plant assemblages,which indicate high beta diversity between the habitat types.Intensive urbanization and industrialization are among the most serious threats that contribute to declines in biological diversity and rapid fragmentation of coastal habitats in Kuwait.Establishing protective enclosures in the disturbed habitats,planting endangered and vulnerable species,and establishing a natural reserve at Nuwaiseeb are recommended conservation actions to avoid loss of the fragmented coastal habitats and to facilitate restoration of native plants.     

Potential habitat modeling for reintroduction of three native plant species in central Iran

Potential habitat modeling of endemic species is an appropriate method to maintain biodiversity,ecosystem function and rehabilitation of rangeland ecosystems.Astragalus caragana,A.cyclophyllon and A.podolobus are endemic in Iran's rangelands and some neighboring countries.The three native species could endure environmental stresses due to their distinctive ecophysiological characteristics.They play important roles in sustainable pastures production,recreation and improvement.They suffer severe threat from many factors including;grazing,agriculture and invasive exotic species.We analyzed the potential habitat of three native plant species in central Iran basing on the grid map with the resolution of 1-km.We used inventory records from field surveys,herbarium collections and 22 environmental factors to explore the environmental influences on given species distribution by Maximum entropy(Maxent) model.Maxent is a species distribution model that uses species occurrence and environmental data for predicting potential species.The results of our study indicated species occurrence has strong correlation with environmental factors such as mean temperature of wettest season,elevation and precipitation of coldest season.We evaluated the model accuracy by AUC(area under the receiver operating characteristic curve) based on an independent test data set.AUC values indicated the high power of Maxent to create potential habitat map(AUCA.caragana=0.988,AUCA.cyclophyllon=0.927,AUCA.podolobus=0.923).It is important to consider that AUC values tend to be lower for species that have broad distribution scope,such as A.podolobus distribution.Most suitable potential habitat distributions of the three species were predicted in the western and southwestern parts of rangelands in Isfahan province.Visual comparisons of the actual distribution map of the three species with produced Maxent maps represent a good agreement.In general,the model demonstrated that the occurrence of the given species is highly probable when the elevation is between 2,200 and 3,000 m and mean temperature of wettest season less than 3°C.This model,therefore,can be applied to recognize potential sites for rangeland reclamation projects.     

Phenology of desert plants from an arid gravel plain in eastern United Arab Emirates

Phenological events for desert plants were recorded and rainfall and temperature data were gathered over a three-year time scale at a gravel plain in the eastern region of the United Arab Emirates. Variations of phenological periods were analyzed and correlations between phenological periods and climate factors were discussed. The study showed that the growth and flowering of therophytes were significantly correlated with air temperature. The timing and abundance of rainfall came to be another factor significantly correlated with the onset and duration of chamaephyte flowering as well as the duration of therophyte growth and flowering. The variation in rainfall did not affect the onset of flowering in phanerophytes. Peak growing seasons were from November to March and flowering from November to December(also February to March depending on the availability of rainfall). Flowering in phanerophytes and chamaephytes occurs 4–6 weeks and in therophytes 4–8 weeks after rain. The results showed that variations in growth and phenology between species were correlated with environmental factors, such as temperature and rainfall or, maybe, differences in life form and photosynthetic pathways, each being adapted for utilizing a particular phase of the seasonally and yearly variable rainfall. The sequence of flowering for the species under study was more or less constant despite the difference in the amount of rainfall. The fruiting patterns of most of the species were also found parallel to their flowering patterns. Our results emphasized the need to study multiple species at many sites for the understanding and forecast of regional changes in phenology.     

Predicting the dynamics of local adaptation in invasive species

An invasive plant species may restrict its spread to only one type of habitat,or,after some time,may continue to spread into a different,often stressful,secondary,habitat.The question of whether evolution is required for an invasive species to spread from one habitat to another is currently hotly debated.In order for local adaptation to occur,genetic variation must be present within invasive populations.In this paper,I focus on the effect of habitat on the maintenance of genetic variation during the lag phase,the phase of population stability prior to expansion.Genetic diversity in invasive plant populations accumulates through multiple introductions,gene flow,mutation,and hybridization,but diversity is maintained by population level processes influencing effective population size(Ne).I show that when the plastic response to the environment results in little variation in reproductive output among individuals,Ne is maximized and genetic variation is maintained.Established models of plant competition show that below-ground competition reduces the variation in reproductive output,whereas competition for light increases variation in reproductive output.The same environments that maintain high Ne also reduce the opportunity for selection and minimize the response to selection,and thus the effects of the environment are synchronized to prevent genetic purges.When the primary invasion habitat supports high Ne,conditions are ripe for local adaptation to a secondary habitat,particularly if the secondary habitat has high opportunity for selection.When the primary invasion habitat supports low Ne,genetic diversity is less likely to be sufficient for local adaptation to secondary habitat to occur.     

Contribution of biodiversity to ecosystem functioning:a non-equilibrium thermodynamic perspective

Ecosystem stays far from thermodynamic equilibrium. Through the interactions among biotic and abiotic components,and encompassing physical environments,ecosystem forms a dissipative structure that allows it to dissipate energy continuously and thereby remains functional over time. Biotic regulation of energy and material fluxes in and out of the ecosystem allows it to maintain a homeostatic state which corresponds to a self-organized state emerged in a non-equilibrium thermodynamic system. While the associated self-organizational processes approach to homeostatic state,entropy(a measure of irreversibility) degrades and dissipation of energy increases. We propose here that at a homeostatic state of ecosystem,biodiversity which includes both phenotypic and functional diversity,attains optimal values. As long as biodiversity remains within its optimal range,the corresponding homeostatic state is maintained. However,while embedded environmental conditions fluctuate along the gradient of accelerating changes,phenotypic diversity and functional diversity contribute inversely to the associated self-organizing processes. Furthermore,an increase or decrease in biodiversity outside of its optimal range makes the ecosystem vulnerable to transition into a different state.     

The relative generality of plant invasion mechanisms and predicting future invasive plants

This study presents an updated analysis of the relative generality of invasion mechanisms in invasive plants. We categorised eight invasion mechanisms into three classes, according to the ecological processes behind the invasions: physical environment mechanisms (phenotypic plasticity in environmental tolerance and evolutionary adaptation to physical environment), resource use mechanisms (resource competition, resource utilisation and allelopathy) and enemy release mechanisms (high growth/reproduction rate, evolutionarily increased competitive ability and phenotypic plasticity in resource allocation). An analysis of 133 invasive plant species in the literature showed the enemy release mechanisms are equally general as resource use mechanisms, while the physical environment mechanisms are less general. Among the eight specific invasion mechanisms, phenotypic plasticity (either in resource allocation or in environmental tolerance), allelopathy, evolutionarily increased competitive ability and high resource-use efficiency are fairly common. Furthermore, chemical defence behaviour is very common in plant invaders. Species invading through enemy release were more likely to originate from Europe and those invading through resource competition/utilisation were often native to tropical and North America and Asia. Invaders with allelopathy were more likely to be from tropical and North America and those showing evolutionarily increased competitive ability were often native to temperate regions such as Europe and North America. Collectively, phenotypic plasticity, defence strategy and native status can be used not only to predict a plant's invasiveness, but also for identifying the potential invasion mechanisms.     

新疆乌鲁木齐南部山区地面生地衣生态位研究

为了确定乌鲁木齐南部山区森林生态系统地面生地衣对资源的利用特征,采用Levins的生态位宽度计算公式和王刚提出的生态位重叠值公式,对该地区地面生地衣生态位宽度和生态位重叠进行了研究。同时以地面生地衣生态位重叠值为指标应用主分量分析法,对地衣群落进行了分类。研究结果显示:该地区发现地面生地衣共17种,大部分的地面生地衣生态位宽度比较窄。它们组成四种主要的地面生地衣类群。地衣植物对环境的变化特别敏感。应该重视地面生地衣植物在多资源维上的生态位重叠值和生态位宽度,以便更好的保护地衣物种多样性和揭示它们的生态适应特点。     

宁夏麻黄山草原蝗虫及天敌对土壤含水量及坡度的响应特征

为明确宁夏麻黄山蝗虫种群、天敌种群与土壤含水量及坡度之间的关系,于2012年在麻黄山蝗区设置3个样点进行2次调查,使用土壤水分温度速测仪测量不同深度土壤含水量,通过数字高程模型获取坡度数据,以网捕目测法对蝗虫及天敌进行采集,并利用典型性相关分析法分析不同深度土层土壤含水量、坡度与蝗虫物种多样性及种群密度的关系。结果显示,在能够影响蝗虫种群的20~40 cm土层上,首次调查中土壤含水量都体现出坡顶最高、坡底最低的相似趋势;第2次调查中则在不同地形上呈现出多样化。蝗虫物种多样性与20、30 cm土层含水量关系不明显,相关系数仅为0.1306和-0.0157;蝗虫种群密度与20、30 cm土壤含水量关系也不明显,相关系数为0.0077和-0.0118。随着坡度的增加,蝗虫物种多样性与种群密度都有所下降,相关系数分别为0.7223和0.4315;坡度在30°以内的蝗虫物种多样性与种群密度都比较高,超过30°后二者急剧下降。表明在西部干旱、半干旱草原地区,土壤含水量对蝗虫种群的空间分布没有决定性影响,而坡度是影响蝗虫分布的重要因素。     

春甘蓝田小菜蛾及主要捕食性天敌时空生态位

在田间系统调查的基础上,研究了春甘蓝田小菜蛾及主要捕食性天敌的时空生态位。结果表明,小菜蛾的时间生态位宽度较小而空间生态位宽度相对较大,说明小菜蛾种群的发生有高峰期明显和全株为害的特点。小菜蛾的时间动态、空间生态位宽度与种群密度无显著相关性,说明小菜蛾对甘蓝的为害情况比较复杂。田间调查共记录捕食性天敌13种,主要优势种依次为草间钻头蛛、七星瓢虫、血色高亮腹蛛、八斑鞘腹蛛、四点高亮腹蛛和三突花蛛。主要捕食性天敌中,七星瓢虫和三突花蛛与小菜蛾的时空二维生态位重叠度最大,其次为四点高亮腹蛛和血色高亮腹蛛,八斑鞘腹蛛和草间钻头蛛最小。这表明七星瓢虫和三突花蛛对小菜蛾有较好的跟随和潜在控制作用。     

异质生境对生防昆虫莲草直胸跳甲数量分布的影响

昆虫的种群动态和数量分布与环境因素密切相关。本研究通过在中国南方空心莲子草大规模入侵的区域分别设置水、陆样地,采用曲线回归和冗余分析等方法探讨了异质生境中空心莲子草形态特征、入侵群落植物多样性及无机环境对生防昆虫莲草直胸跳甲数量分布的影响。结果显示:水、陆两境中莲草直胸跳甲数量均随着空心莲子草株高增加而上升;莲草直胸跳甲数量随水生型空心莲子草分枝数及入侵盖度的增加而上升。莲草直胸跳甲数量与陆生群落植物物种多样性无显著相关性,但在水生环境中,莲草直胸跳甲数量随群落的Simpson、Shannon和Pielou指数上升而显著下降。水、陆两境中决定莲草直胸跳甲数量的主导性环境因子均为年均气温、纬度和海拔。研究结果可为入侵植物的生物防治及引进天敌提供参考。     

黄秋葵田主要害虫、天敌种群动态及时间生态位分析

为了解黄秋葵田害虫和天敌的种类及种群动态变化情况,分析主要害虫和天敌间的生态调控机制,为黄秋葵田害虫的生物防控提供科学依据。通过目测法和5点取样法调查记录黄秋葵田害虫、天敌种类和数量,并对害虫和天敌间的生态位进行分析。结果表明,黄秋葵田内共发现6目13科21种昆虫,害虫优势种分别为棉蚜和棉大卷叶螟,两者在田间均呈聚集分布。天敌优势种分别为龟纹瓢虫、异色瓢虫、三突花蛛、草间小黑蛛、卷叶螟绒茧蜂、卷叶螟姬小蜂。其中蚜虫生态位宽度值最小,田间发生时间比较集中,主要在6月。棉大卷叶螟生态位宽度值最大,田间发生时间比较长,主要在7—9月。蚜虫与瓢虫的时间生态位重叠值为0.8593,瓢虫对蚜虫有明显的跟随效应,是蚜虫的优势性天敌。棉大卷叶螟与蜘蛛、卷叶螟绒茧蜂、卷叶螟姬小蜂的时间生态位重叠值分别为0.7220、0.7175、0.6797,这三类天敌对棉大卷叶螟有明显的跟随效应,对其种群具有协同控制效果,是控制棉大卷叶螟的三种优势天敌。以上结果表明,天敌对害虫在时间生态位上有一定的同步性,在田间管理时应加强对这些天敌的保护和利用,以增强天敌的自然控害功能。     

民勤荒漠区霸王群落植物生态位研究

运用Simpson生态位宽度指数及Pianka生态位重叠指数,对民勤荒漠区霸王群落两大分布区的植物生态位宽度及生态位重叠度进行计算,从生态位角度分析霸王群落内各物种对资源的利用状况,揭示霸王群落的结构及分布格局。结果表明:两大分布区建群种均为霸王,霸王的生态位宽度较其他物种都要宽;建群种霸王与猫头刺、沙生针茅相互之间的重叠度指数相对较高,猫头刺、沙生针茅与霸王相互间对资源利用竞争激烈,对群落的结构共同起着重要的作用。生态位宽度大的物种不一定和其他物种有大的重叠指数,较高的生态位宽度和较高的生态位重叠度之间并不存在直接的线性关系,说明植被在演替进化的过程中对资源利用分化严重,环境资源存在着高度的空间异质性。     

What are the best predictors for invasive potential of weeds? Transferability evaluations of model predictions based on diverse environmental data sets for Flaveria bidentis

Ecological niche models are widely used in the study of weed invasions, yet best approaches for selecting ecologically relevant environmental predictors for weeds remain unresolved. Here, we evaluate niche model transferability based on diverse environmental data sets for an invasive herb, Flaveria bidentis. This species is native to South America, but has established populations in China that pose a threat to agriculture and animal husbandry. Relevant environmental data sets were selected via five statistical approaches: permutation importance (PI) and jackknife test (JK) in Maxent, variable importance identified by boosting regression trees (BRT), ecological niche factor analysis (ENFA) and a newly released algorithm based on a fluctuation index (FI). Climate spaces occupied by native South American and introduced Chinese populations were compared based on these environmental data sets. Native niche model predictions in China were compared across environmental data sets and model settings (i.e. default versus fine‐tuned Maxent settings). Results suggest that native and introduced populations occupy two distinct climate spaces, but that this divergence likely results from background effects. Niche models based on fine‐tuned Maxent settings generally showed better discrimination ability than those based on default settings. The best model discrimination in China was attained in the FI model using fine‐tuned settings, followed by the BRT model on default settings. The best models suggest that highly suitable areas at risk of invasion in China are to the west and north‐east of present distributional areas. Results presented here provide predictions for F. bidentis in particular, but also shed light on procedures for selecting ecologically relevant predictors for invasive species distributional predictions more generally.     

Disturbance of plateau zokor-made mound stimulates plant community regeneration in the Qinghai-Tibetan Plateau,China

Mounds constructed by plateau zokors, which is widely distributed in alpine meadows significantly modified plant community structure. However, the variations of plant community structure under the disturbance of plateau zokor-made mound are less concerned. Therefore, we investigated the responses of plant community on zokor-made mound of different years (1 a and 3-4 a), and compared with undisturbed sites (no mound) in an alpine meadow in the eastern Qinghai-Tibetan Plateau (QTP), China. Species richness, coverage and Simpson diversity index were all significantly reduced by the presence of zokor-made mound, but plant heights were significantly increased, particularly in grasses and sedges. Several perennial forage species showed an increased importance value and niche breadth, including Koeleria macrantha, Elymus nutans and Poa pratensis. The effect of zokor-made mound on niche overlap showed that more intense interspecific competition produced a greater utilization of environmental resources. And this interspecific niche overlap was strengthened as succession progressed. The bare mound created by zokor burrowing activities provided a colonizing opportunity for non-dominant forage species, resulting in abundant plant species and plant diversity during the succession period. We concluded that presence of zokor-made mound was conducive to regeneration and vitality of plant community in alpine meadows, thus improving their resilience to anthropogenic stress.