Species and functional group composition of ant communities across an elevational gradient in the Eastern Himalaya

Elevational gradients in mountains show rapid changes in environmental conditions across a small geographic extent. This results in habitat specialization in animal communities which results in changes in species composition across space. We explore changes in species and functional group composition of ants using the first ever data on the distribution of ants across an elevational gradient in the Eastern Himalaya. Ants were sampled from 600 to 2400 m elevations at 200 m intervals using Winklers and pitfall traps. The sampling yielded 166 species of ants from 10,560 individuals, which were then classified into functional groups. We used redundancy analysis to test the effects of environmental factors (temperature, leaflitter volume, understory vegetation) and spatial predictors on species as well as functional group composition of communities at different elevations. Our results show that species diversity within all functional groups decreases towards higher elevations. The functional group composition of ant communities shows a gradient from high evenness at low elevations to being dominated by opportunist species at higher elevations. Redundancy analyses shows that most of the variation in species as well as functional group composition is driven by spatially structured environmental variation. This is most likely due to the high correlation between temperature and elevation. In summary, the changes in species as well as functional group composition are likely driven by a gradient in climate across the elevation gradient.     

Environmental determinants of leaf litter ant community composition along an elevational gradient

Ant communities are extremely diverse and provide a wide variety of ecological functions in tropical forests. Here, we investigated the abiotic factors driving ant composition turnover across an elevational gradient at Mont Itoupé, French Guiana. Mont Itoupé is an isolated mountain whose top is covered by cloud forests, a biogeographical rarity that is likely to be threatened according to climate change scenarios in the region. We examined the influence of six soil, climatic, and LiDAR‐derived vegetation structural variables on leaf litter ant assembly (267 species) across nine 0.12‐ha plots disposed at three elevations (ca. 400, 600, and 800m asl). We tested (a) whether species cooccurring within a same plot or a same elevation were more similar in terms of taxonomic, functional, and phylogenetic composition, than species from different plots/elevations, and (b) which environmental variables significantly explained compositional turnover among plots. We found that the distribution of species and traits of ant communities along the elevational gradient was significantly explained by a turnover of environmental conditions, particularly in soil phosphorus and sand content, canopy height, and mean annual relative humidity of soil. Our results shed light on the role exerted by environmental filtering in shaping ant community assembly in tropical forests. Identifying the environmental determinants of ant species distribution along tropical elevational gradients could help predicting the future impacts of global warming on biodiversity organization in vulnerable environments such as cloud forests.     

Phylogenetic beta diversity in bacterial assemblages across ecosystems: deterministic versus stochastic processes

Increasing evidence has emerged for non-random spatial distributions of microbes, but knowledge of the processes that cause variation in microbial assemblage among ecosystems is lacking. For instance, some studies showed that deterministic processes such as habitat specialization are important, while other studies hold that bacterial communities are assembled by stochastic forces. Here we examine the relative influence of deterministic and stochastic processes for bacterial communities from subsurface environments, stream biofilm, lake water, lake sediment and soil using pyrosequencing of the 16S ribosomal RNA gene. We show that there is a general pattern in phylogenetic signal in species ecological niches across recent evolutionary time for all studied habitats, enabling us to infer the influences of community assembly processes from patterns of phylogenetic turnover in community composition. The phylogenetic dissimilarities among-habitat types were significantly higher than within them, and the communities were clustered according to their original habitat types. For communities within-habitat types, the highest phylogenetic turnover rate through space was observed in subsurface environments, followed by stream biofilm on mountainsides, whereas the sediment assemblages across regional scales showed the lowest turnover rate. Quantifying phylogenetic turnover as the deviation from a null expectation suggested that measured environmental variables imposed strong selection on bacterial communities for nearly all sample groups. For three sample groups, spatial distance reflected unmeasured environmental variables that impose selection, as opposed to spatial isolation. Such characterization of spatial and environmental variables proved essential for proper interpretation of partial Mantel results based on observed beta diversity metrics. In summary, our results clearly indicate a dominant role of deterministic processes on bacterial assemblages and highlight that bacteria show strong habitat associations that have likely emerged through evolutionary adaptation.     

Environmental changes affect the assembly of soil bacterial community primarily by mediating stochastic processes

Both ‘species fitness difference’‐based deterministic processes, such as competitive exclusion and environmental filtering, and ‘species fitness difference’‐independent stochastic processes, such as birth/death and dispersal/colonization, can influence the assembly of soil microbial communities. However, how both types of processes are mediated by anthropogenic environmental changes has rarely been explored. Here we report a novel and general pattern that almost all anthropogenic environmental changes that took place in a grassland ecosystem affected soil bacterial community assembly primarily through promoting or restraining stochastic processes. We performed four experiments mimicking 16 types of environmental changes and separated the compositional variation of soil bacterial communities caused by each environmental change into deterministic and stochastic components, with a recently developed method. Briefly, because the difference between control and treatment communities is primarily caused by deterministic processes, the deterministic change was quantified as (mean compositional variation between treatment and control) – (mean compositional variation within control). The difference among replicate treatment communities is primarily caused by stochastic processes, so the stochastic change was estimated as (mean compositional variation within treatment) – (mean compositional variation within control). The absolute of the stochastic change was greater than that of the deterministic change across almost all environmental changes, which was robust for both taxonomic and functional‐based criterion. Although the deterministic change may become more important as environmental changes last longer, our findings showed that changes usually occurred through mediating stochastic processes over 5 years, challenging the traditional determinism‐dominated view.     

Using functional trait diversity to evaluate the contribution of multiple ecological processes to community assembly during succession

Successional chronosequences provide a unique opportunity to study the effects of multiple ecological processes on plant community assembly. Using a series of 0.5 × 0.5 m² plots (n = 30) from five successional sub‐alpine meadow plant communities (ages 3, 5, 9, 12, and undisturbed) in the Qinghai‐Tibetan Plateau, we investigated whether community assembly is stochastic or deterministic for species and functional traits. We tested directional change in species composition, functional trait composition, and then functional trait diversity measured by Rao's quadratic entropy for four traits – plant height, leaf dry matter content, specific leaf area, and seed mass – along two comparable successional chronosequences. We then evaluated the importance of species interactions, habitat filtering and stochasticity by comparing with random communities and partitioning the environmental and spatial components of Rao's quadratic entropy. We found no directional change in species composition, but clear directionality in functional trait composition. None of the abiotic environmental variables (except P) showed linear change with successional age, but soil moisture and nitrogen were positively related to functional diversity within meadows. Functional trait diversity increased significantly with the increase in successional age. Comparison with random communities showed a significant shift from trait divergence in early stages of succession (3‐ and 5‐yr) to convergence in the later stages of succession 9‐, 12‐yr and undisturbed). The relative importance of abiotic variables and spatial structure for functional trait diversity changed in a predictable manner with successional age. Stochasticity at the species level may indicate dispersal limitation, but deterministic effects on functional trait distributions show the role of both habitat effects and biotic interactions.     

Contrasting community assembly processes structure lotic bacteria metacommunities along the river continuum

The heterogeneous nature of lotic habitats plays an important role in the complex ecological and evolutionary processes that structure the microbial communities within them. Due to such complexity, our understanding of lotic microbial ecology still lacks conceptual frameworks for the ecological processes that shape these communities. We explored how bacterial community composition and underlying ecological assembly processes differ between lotic habitats by examining community composition and inferring community assembly processes across four major habitat types (free-living, particle-associated, biofilm on benthic stones and rocks, and sediment). This was conducted at 12 river sites from headwater streams to the main river in the River Thames, UK. Our results indicate that there are distinct differences in the bacterial communities between four major habitat types, with contrasting ecological processes shaping their community assembly processes. While the mobile free-living and particle-associated communities were consistently less diverse than the fixed sediment and biofilm communities, the latter two communities displayed higher homogeneity across the sampling sites. This indicates that the relative influence of deterministic environmental filtering is elevated in sediment and biofilm communities compared with free-living and particle-associated communities, where stochastic processes play a larger role.     

What is the effect of soil use on ant communities?

Studies on ant communities in agroecosystems have contributed to the knowledge of the effect of agricultural activities on biological communities. The aim of this study is to explain the effect of soil use on ant communities. We tested the hypothesis that there was a decrease in ant species richness and a change in the species composition at habitats with more intense soil use. We collected ants using sardine baits, subterranean traps and direct sampling at four habitats with different soil use (secundary forest, Acacia forestry, initial stage of succession and mixed crops). The ant species richness did not decrease with intensity of soil use. In successional habitat the species numbers collected using sardine baits and subterranean traps were significantly different. Species composition of communities had a pronounced variation, with the epigaeic and hypogaeic ant faunas of the habitat with high intense soil use (mixed crops) had low similarity with ant communities of the three other habitats. The predator species were restricted to habitats with low intensity of soil use. Then, species composition could better reflect the functional changes on ant communities than species richness. Our data can help to choose the component of ant community that better reflect the response of biodiversity to agricultural impacts.     

Assembly rules of ectoparasite communities across scales: combining patterns of abiotic factors,host composition,geographic space,phylogeny and traits

We investigated the role of environmental filtering as an underlying mechanism of assembly of compound communities of fleas parasitic on Palearctic small mammals at two spatial scales; a continental scale (encompassing regions across the entire Palearctic) and a regional scale (across sampling localities within Slovakia). We used the three‐table ordination (the RLQ analysis) and its extended version that links species occurrences with geographic space, environmental variables, and species traits and phylogeny (the ESLTP analysis). We asked whether environmental filtering acts as an assembly rule of compound communities of fleas and, if yes, a) whether the effect of environment on species composition of compound communities of fleas differs between spatial scales and b) what are the relative importance of the abiotic and host environments. We found that compound communities of fleas are, to a great extent, assembled via environmental filters that represent interplay between filtering via abiotic environment and filtering via host composition. The relative importance of these two components of environmental filtering differed between spatial scales. Host composition had a stronger effect on flea assembly than abiotic environment on the continental scale, while the opposite was true for the regional scale. The likely reason behind this scale‐dependence is that communities on the regional scale are mainly governed by ecological and epidemiological processes, while communities on the continental scale are mainly affected by evolutionary, biogeographic and historical forces.     

Underdispersion and overdispersion of traits in terrestrial snail communities on islands

Understanding and disentangling different processes underlying the assembly and diversity of communities remains a key challenge in ecology. Species can assemble into communities either randomly or due to deterministic processes. Deterministic assembly leads to species being more similar (underdispersed) or more different (overdispersed) in certain traits than would be expected by chance. However, the relative importance of those processes is not well understood for many organisms, including terrestrial invertebrates. Based on knowledge of a broad range of species traits, we tested for the presence of trait underdispersion (indicating dispersal or environmental filtering) and trait overdispersion (indicating niche partitioning) and their relative importance in explaining land snail community composition on lake islands. The analysis of community assembly was performed using a functional diversity index (Rao's quadratic entropy) in combination with a null model approach. Regression analysis with the effect sizes of the assembly tests and environmental variables gave information on the strength of under‐ and overdispersion along environmental gradients. Additionally, we examined the link between community weighted mean trait values and environmental variables using a CWM‐RDA. We found both trait underdispersion and trait overdispersion, but underdispersion (eight traits) was more frequently detected than overdispersion (two traits). Underdispersion was related to four environmental variables (tree cover, habitat diversity, productivity of ground vegetation, and location on an esker ridge). Our results show clear evidence for underdispersion in traits driven by environmental filtering, but no clear evidence for dispersal filtering. We did not find evidence for overdispersion of traits due to diet or body size, but overdispersion in shell shape may indicate niche differentiation between snail species driven by small‐scale habitat heterogeneity. The use of species traits enabled us to identify key traits involved in snail community assembly and to detect the simultaneous occurrence of trait underdispersion and overdispersion.     

Do Species Sorting and Mass Effects Drive Assembly in Tropical Agroecological Landscape Mosaics?

Recent assessments of biodiversity in tropical agroecosystems have revealed surprisingly high functional and taxonomic diversity in systems with low management intensity. This biodiversity is the product of community assembly. Because agroecosystems are novel ecosystems and occur in landscape mosaics, the assembly processes generating communities in agroecosystems are poorly resolved. Broadly, two models have been proposed to explain landscape assembly: trade‐offs in species performance across habitats (species sorting) and source‐sink dynamics between habitats of differential quality (mass effects). These models are largely untested in tropical agroecosystems. We utilize an extensive data set on a tropical twig‐nesting ant community from five microhabitat types in a shaded coffee agroecosystem to test for species sorting, mass effects, or a mixed model. To test among these models, we used community similarity and a variance decomposition on a focal microhabitat (a moderate‐shade coffee farm) to partition community variance into spatial and environmental components. To identify the source habitat for mass effects and assess their strength, we measured dispersing alates (winged reproductives), artificial nests, and colony and nest size in shade trees and coffee. We found significant environmental and spatial signal and evidence for both species sorting and mass effects. We find sorting occurs among common species, but that mass effects are prevalent among rare species and likely originate in the shade trees. Our results indicate that both metacommunity models occur in tropical landscape mosaics, but they may not apply equally to all species in communities, habitat gradients, or timescales.     

Environmental predictability of taxonomic and functional community composition in high‐latitude streams

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Partitioning the impact of abiotic factors and spatial patterns on species richness and community structure of ground ant assemblages in four Bornean rainforests

Contrasting theories have been proposed to explain the structure of ecological communities. Here, we studied the impact of environmental factors and spatial patterns on ground‐foraging ant communities in four different forest types of Gunung Mulu National Park in Sarawak, Borneo, Malaysia. Forest types differed in their environmental parameters and were inhabited by distinct ant communities, with various indicator species characteristic for single forest types. Three environmental parameters, soil volume, number of trees and amount of leaf litter, had the most influence on ant communities. Spatial patterns were correlated with environmental parameters and also influenced ant communities. Environmental parameters influenced community composition only moderately (r²=0.14), but had a high impact on species richness (r²=0.44). Spatial patterns explained only a small fraction of the total variance in species patterns, while much of the residual space in the ordination space of ant community patterns remained unexplained. We conclude that environmental parameters shape the number of niches within a tropical soil habitat, but identities of species that occupy those niches are accounted for by other factors like competition, traits and neutral processes that may further reduce unexplained variance in species ordination.     

Floristic patterns and plant traits of Mediterranean communities in fragmented habitats

Aim To contrast floristic spatial patterns and the importance of habitat fragmentation in two plant communities (grassland and scrubland) in the context of ecological succession. We ask whether plant assemblages are affected by habitat fragmentation and, if so, at what spatial scale? Does the relative importance of the niche differentiation and dispersal‐limitation mechanisms change throughout secondary succession? Is the dispersal‐limitation mechanism related to plant functional traits? Location A Mediterranean region, the massif of Albera (Spain). Methods Using a SPOT satellite image to describe the landscape, we tested the effect of habitat fragmentation on species composition, determining the spatial scale of the assemblage response. We then assessed the relative importance of dispersal‐related factors (habitat fragmentation and geographical distance) and environmental constraints (climate‐related variables) influencing species similarity. We tested the association between dispersal‐related factors and plant traits (dispersal mode and life form). Results In both community types, plant composition was partially affected by the surrounding vegetation. In scrublands, animal‐dispersed and woody plants were abundant in landscapes dominated by closed forests, whereas wind‐dispersed annual herbs were poorly represented in those landscapes. Scrubby assemblages were more dependent on geographical distance, habitat fragmentation and climate conditions (temperature, rainfall and solar radiation); grasslands were described only by habitat fragmentation and rainfall. Plant traits did not explain variation in spatial structuring of assemblages. Main conclusions Plant establishment in early Mediterranean communities may be driven primarily by migration from neighbouring established communities, whereas the importance of habitat specialization and community drift increases over time. Plant life forms and dispersal modes did not explain the spatial variation of species distribution, but species richness within the community with differing plant traits was affected by habitat patchiness.     

A functional trait-based approach to understand community assembly and diversity–productivity relationships over 7 years in experimental grasslands

Several multi-year biodiversity experiments have shown positive species richness–productivity relationships which strengthen over time, but the mechanisms which control productivity are not well understood. We used experimental grasslands (Jena Experiment) with mixtures containing different numbers of species (4, 8, 16 and 60) and plant functional groups (1–4; grasses, legumes, small herbs, tall herbs) to explore patterns of variation in functional trait composition as well as climatic variables as predictors for community biomass production across several years (from 2003 to 2009). Over this time span, high community mean trait values shifted from the dominance of trait values associated with fast growth to trait values suggesting a conservation of growth-related resources and successful reproduction. Increasing between-community convergence in means of several productivity-related traits indicated that environmental filtering and exclusion of competitively weaker species played a role during community assembly. A general trend for increasing functional trait diversity within and convergence among communities suggested niche differentiation through limiting similarity in the longer term and that similar mechanisms operated in communities sown with different diversity. Community biomass production was primarily explained by a few key mean traits (tall growth, large seed mass and leaf nitrogen concentration) and to a smaller extent by functional diversity in nitrogen acquisition strategies, functional richness in multiple traits and functional evenness in light-acquisition traits. Increasing species richness, presence of an exceptionally productive legume species (Onobrychis viciifolia) and climatic variables explained an additional proportion of variation in community biomass. In general, community biomass production decreased through time, but communities with higher functional richness in multiple traits had high productivities over several years. Our results suggest that assembly processes within communities with an artificially maintained species composition maximize functional diversity through niche differentiation and exclusion of weaker competitors, thereby maintaining their potential for high productivity.     

黄土高原油松和辽东栎林下植物β多样性差异及影响因素

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Ant community structure during forest succession in a subtropical forest in South-East China

Understanding how communities respond to environmental gradients is critical to predict responses of species to changing habitat conditions such as in regenerating secondary habitats after human land use. In this study, ground-living ants were sampled with pitfall traps in 27 plots in a heterogeneous and diverse subtropical forest to test if and how a broad set of environmental variables including elevation, successional age, and tree species richness influence ant diversity and community composition. In total, 13,441 ant individuals belonging to 71 species were found. Ant abundance was unrelated to all environmental variables. Rarefied ant species richness was negatively related to elevation, and Shannon diversity decreased with shrub cover. There was considerable variation in ant species amongst plots, associated with elevation, successional age, and variables related to succession such as shrub cover. It is shown that younger secondary forests may support a species-rich and diverse community of ants in subtropical forests even though the species composition between younger and older forests is markedly different. These findings confirm the conservation value of secondary subtropical forests, which is critical because subtropical forests have been heavily exploited by human activities globally. However, the findings also confirm that old-growth forest should have priority in conservation as it supports a distinct ant community. Our study identifies a set of ant species which are associated with successional age and may thus potentially assist local conservation planning.     

Restoring Lepidopteran Communities to Oak Savannas: Contrasting Influences of Habitat Quantity and Quality

Ecological restoration is deemed important for the long‐term conservation of biodiversity, but ecologists still lack an understanding of how habitat availability and habitat quality in a restored system interact to determine species diversity. This problem seems particularly apparent in Tallgrass Prairie and savanna ecoregions, where restored management units represent the majority of extant habitat. In this study, we tested three principal hypotheses, each stating that the diversity of Lepidoptera would be greater in (1) patches of savanna habitat that were larger; (2) patches that were of higher habitat quality; and (3) patches that had greater connectivity to management units of similar physiognomy. Lepidoptera were sampled in 2003 from 13 unmanaged woodland remnants within Neal Smith National Wildlife Refuge, a 2,292‐ha prairie and savanna reconstruction project. We also measured 11 environmental variables within each site to assess variation in habitat quantity and quality. Principal components analysis (PCA) was used to identify major gradients of environmental variation among the 13 sites. Our PCA differentiated among woodlands along three environmental gradients, defined by (1) stand size, shape, topography, and oak dominance; (2) degree of disturbance; and (3) isolation. Total lepidopteran species richness, however, was only predicted by variation in the first principal component. Species richness of Lepidoptera known to be oak specialists was significantly affected by variation along all three PCA gradients. Surprisingly, more isolated woodland remnants contained a greater richness of oak feeders. Our results suggest that approaches to restoring oak savannas should emphasize aspects of both habitat quantity and quality. Beyond making individual management units larger, priority sites for restoration should possess a low importance of trees that are indicative of past habitat disturbance (e.g., Honey locust, White mulberry) even if canopy closure is substantial. Connectivity among restored habitats may benefit savanna moth communities only when habitat linkages contain a flora similar in composition to focal patches.     

The spatial variation in ant species composition and functional groups across the Subantarctic-Patagonian transition zone

The role of ecotones in the maintenance of species diversity is rather controversial; they may represent either biodiversity hotspots with unique and rare forms, or be transitional areas that hold marginal populations of species. We analyse the taxonomic and functional composition of ant species assemblages across the Subantarctic-Patagonian transition to evaluate the role that transitional shrublands may play in the maintenance of the taxonomic and functional differentiation. We collected ants using 450 pitfall traps within a ~150 × 150 km area. Species were classified into functional groups in relation to stress and disturbance, and in foraging groups according to their foraging behavior. An indicator value for each species in each habitat was calculated. The steppes and the forests strongly differed in ant species and functional composition. Climatic effects combined with structural components of plant environment explained about 23–27 % of the variation in ant composition. The shrublands did not show a distinctive fauna, and show greater similarity in ant species composition and in the proportional occupancy of functional groups to the steppes than to the forests. They harbor neither rare nor indicator species, except for Lasiophanes valdiviensis, and thus this reinforces the idea that they are not a habitat source of species, but an area of encounter between two distinct forest- and steppe- ant faunas, where a high number of local distributional limits of ant species overlap.     

Diversity of Eastern North American Ant Communities along Environmental Gradients

Studies of species diversity patterns across regional environmental gradients seldom consider the impact of habitat type on within-site (alpha) and between-site (beta) diversity. This study is designed to identify the influence of habitat type across geographic and environmental space, on local patterns of species richness and regional turnover patterns of ant diversity in the northeastern United States. Specifically, I aim to 1) compare local species richness in paired open and forested transects and identify the environmental variables that best correlate with richness; and 2) document patterns of beta diversity throughout the region in both open and forested habitat. I systematically sampled ants at 67 sites from May to August 2010, spanning 10 degrees of latitude, and 1000 meters of elevation. Patterns of alpha and beta diversity across the region and along environmental gradients differed between forested and open habitats. Local species richness was higher in the low elevation and warmest sites and was always higher in open habitat than in forest habitat transects. Richness decreased as temperature decreased or elevation increased. Forested transects show strong patterns of decreasing dissimilarity in species composition between sites along the temperature gradient but open habitat transects did not. Maximum temperature of the warmest month better predicted species richness than either latitude or elevation. I find that using environmental variables as key predictors of richness yields more biologically relevant results, and produces simpler macroecological models than commonly used models which use only latitude and elevation as predictors of richness and diversity patterns. This study contributes to the understanding of mechanisms that structure the communities of important terrestrial arthropods which are likely to be influenced by climatic change.