Advances, challenges and a developing synthesis of ecological community assembly theory

Linking spatial self-organization to community assembly and biodiversity

Temporal shifts to drier climates impose environmental stresses on plant communities that may result in community reassembly and threatened ecosystem services, but also may trigger self-organization in spatial patterns of biota and resources, which act to relax these stresses. The complex relationships between these counteracting processes - community reassembly and spatial self-organization - have hardly been studied. Using a spatio-temporal model of dryland plant communities and a trait-based approach, we study the response of such communities to increasing water-deficit stress. We first show that spatial patterning acts to reverse shifts from fast-growing species to stress-tolerant species, as well as to reverse functional-diversity loss. We then show that spatial self-organization buffers the impact of further stress on community structure. Finally, we identify multistability ranges of uniform and patterned community states and use them to propose forms of non-uniform ecosystem management that integrate the need for provisioning ecosystem services with the need to preserve community structure.

Phenotypic plasticity can reverse the relative extent of intra- and interspecific variability across a thermal gradient

Intra- and interspecific variability can both ensure ecosystem functions. Generalizing the effects of individual and species assemblages requires understanding how much within and between species trait variation is genetically based or results from phenotypic plasticity. Phenotypic plasticity can indeed lead to rapid and important changes of trait distributions, and in turn community functionality, depending on environmental conditions, which raises a crucial question: could phenotypic plasticity modify the relative importance of intra- and interspecific variability along environmental gradients? We quantified the fundamental niche of five genotypes in monocultures for each of five ciliate species along a wide thermal gradient in standardized conditions to assess the importance of phenotypic plasticity for the level of intraspecific variability compared to differences between species. We showed that phenotypic plasticity strongly influences trait variability and reverses the relative extent of intra- and interspecific variability along the thermal gradient. Our results show that phenotypic plasticity may lead to either increase or decrease of functional trait variability along environmental gradients, making intra- and interspecific variability highly dynamic components of ecological systems.

Ecological causes of fluctuating natural selection on habitat choice in an amphibian

We estimated natural selection targeting three traits related to habitat choice in a frog (Pseudacris maculata) breeding in pools on the rocky shores of Isle Royale, Michigan, over 16 years. Our aim was to identify the form and ecological causes of annual variation in directional and correlational selection as expressed in the survival and growth of tadpoles. We found directional selection favoring early breeding, but pool choice was under weak stabilizing selection. However, the form of stabilizing selection and the position of the optimum trait value shifted among years with the severity of disturbance and the intensity of biotic interactions. In years when wave wash and pool desiccation were severe, selection shifted to favor tadpoles in habitats where these risks were less pronounced. If predatory dragonfly larvae were abundant, selection favored tadpoles in small pools where dragonflies did not occur. When intraspecific competition was strong, selection favored early broods within a broader range of pool types. The agents of selection in this study—biotic interactions and disturbance—are common to many ecological systems and frequently exhibit temporal variation; this suggests that fluctuating selection may be widespread in natural populations.

Unveiling ecological assembly rules from commonalities in trait distributions

Deciphering the effect of neutral and deterministic processes on community assembly is critical to understand and predict diversity patterns. The information held in community trait distributions is commonly assumed as a signature of these processes, but empirical and modelling attempts have most often failed to untangle their confounding, sometimes opposing, impacts. Here, we simulated the assembly of trait distributions through stochastic (dispersal limitation) and/or deterministic scenarios (environmental filtering and niche differentiation). We characterized the shape of trait distributions using the skewness-kurtosis relationship. We identified commonalities in the co-variation between the skewness and the kurtosis of trait distributions with a unique signature for each simulated assembly scenario. Our findings were robust to variation in the composition of regional species pools, dispersal limitation and environmental conditions. While ecological communities can exhibit a high degree of idiosyncrasy, identification of commonalities across multiple communities can help to unveil ecological assembly rules in real-world ecosystems.

Competition Restricts the Growth, Development, and Propagation of Carpinus tientaiensis: A Rare and Endangered Species in China

The protection and propagation of rare and endangered species are key to the preservation of their population development; however, due to the scarcity of individuals, the potential effects and status of rare and endangered species in the whole forest ecosystem are still poorly understood. Using data from a 60 × 140 m forest dynamic monitoring sample of the Carpinus tientaiensis (Betulaceae) species in Zhejiang of Southeast China. We assessed the population distribution and diameter at breast height (DBH) structure of the Carpinus tientaiensis species, which was a rare and endangered species, as well as intra- and interspecific correlation with other species. The results show that saplings (1 cm ≤ DBH < 5 cm) and juveniles (5 cm ≤ DBH < 10 cm) were more aggregated than larger individuals (DBH ≥ 20 cm) of Carpinus tientaiensis. The DBH size structure of all the trees shows an obvious inverted “J” distribution. With an increase in the DBH size category, the number of individuals gradually decreases. Due to the diffusion limitation, the spatial distribution patterns of all the tree individuals and roof geese in the sample land are increased at a small spatial scale, and as the spatial scale increases, the degree of aggregation decreases gradually. The relationship between different diameter stages of the population of Carpinus tientaiensis showed a consistent general trend. The spatial distribution of individuals with a large diameter on a small scale was significantly positively correlated (p < 0.001). With an increase in the scale, there was no significant correlation (p > 0.05) between individuals with a large diameter and individuals with a small diameter. There was no significant correlation (p > 0.05) between the population of Carpinus tientaiensis and other species in the sample, and the strong unidirectional competition of other species in the sample can be seen by the competition index. We found that interspecific competition restricts the growth and expansion of Carpinus tientaiensis, and it has adopted different ecological strategies to coexist with a population of common tree species occupying a similar living space.

The Small-Scale Variation of Herb-Layer Community Structure in a Riparian Mixed Forest

The role of spatial variables, soil properties and overstorey structure in spatial variation of the herb-layer community in a riparian mixed forest was shown. The research were conducted in the "Dnipro-Orils’kiy" Nature Reserve (Ukraine). The research polygon was laid in a forest in the floodplain of the River Protich, which is a left inflow of the River Dnipro. Plant abundance was quantified by measuring cover within experimental polygon. The experimental polygon consisted of 7 transects and each transect was made up of 15 test points. The distance between rows in the site was 3 m. At the site we established a plot of 45×21 m, with 105 subplots of 3×3 m organized in a regular grid. Vascular plant species lists were recorded for each 3×3 m subplot along with visual estimates of species cover projection. The plant community was represented by 43 species, of which 18.6% were phanerophytes, 39.5% were hemikriptophytes, 9.3% were therophytes, 7.0% were geophytes. An overall test of random labelling revealed the total nonrandom distribution of the tree stems within the site. Constrained correspondence analysis (CCA) was applied as ordination approach. The forward selection procedure allowed us to select 6 soil variables, which explain 28.3% of the herb-layer community variability. The list of the important soil variables includes soil mechanical impedance (at the depth 0–5, 30–35, 75–80, and 95–10 cm), soil moisture, and soil bulk density. The variation explained by pure spatial variables is equal to 11.0 %. The majority of the tree-distance structured variation in plant community composition was broad-scaled. The significant relationship was found between the pure spatial component of the community variation and a lot of phytoindicator estimations of which the variability of damping and humidity were of the greatest importance. Trees stand was demonstrated to be a considerable factor structuring both the herb-layer community and spatial variation of the physical properties of soil.

Intraspecific trait variability and community assembly in hawkmoths (Lepidoptera: Sphingidae) across an elevational gradient in the eastern Himalayas, India

We investigated some aspects of hawkmoth community assembly at 13 elevations along a 200- to 2770-m transect in the eastern Himalayas, a little studied biodiversity hot spot of global importance. We measured the morphological traits of body mass, wing loading, and wing aspect ratio of 3,301 free-ranging individuals of 76 species without having to collect or even constrain them. We used these trait measurements and T-statistic metrics to assess the strength of intracommunity ("internal") and extra-community ("external") filters which determine the composition of communities vis-a-vis the regional pool of species.The trait distribution of constituent species turned out to be nonrandom subsets of the community-trait distribution, providing strong evidence for internal filtering in all elevational communities. The external filter metric was more ambiguous. However, the elevational dependence of many metrics including that of the internal filter provided evidence for external (i.e., environmental) filtering. On average, a species occupied as much as 50%-75% of the total community-trait space, yet the T-statistic metric for internal filter was sufficiently sensitive to detect a strong nonrandom structure in the trait distribution.We suggest that the change in T-statistic metrics along the environmental gradient may provide more clues to the process of community assembly than previously envisaged. A large, smoothly varying and well-sampled environmental span would make it easier to discern them. Developing T-statistics for combined analysis of multiple traits will perhaps provide a more accurate picture of internal/filtering and niche complementarity. Moths are a hyperdiverse taxon and a very important component of many ecosystems. Our technique for accurately measuring body and wing dimensions of free-ranging moths can generate trait database for a large number of individuals in a time- and resource-efficient manner for a variety of community assembly studies using this important taxon.

Phenotypic Clumping Decreases With Flock Richness in Mixed-Species Bird Flocks

Animals that live in groups may experience positive interactions such as cooperative behavior or negative interactions such as competition from group members depending on group size and similarity between individuals. The effect of group size and phenotypic and ecological similarity on group assembly has not been well-studied. Mixed-species flocks are important subsets of bird communities worldwide. We examined associations within these in relation to flock size, to understand rules of flock assembly, in the Western Ghats of India. We examined the relationship between phenotypic clumping and flock richness using four variables—body size, foraging behavior, foraging height and taxonomic relatedness. Using a null model approach, we found that small flocks were more phenotypically clumped for body size than expected by chance; however, phenotypic clumping decreased as flocks increased in size and approached expected phenotypic variation in large flocks. This pattern was not as clear for foraging height and foraging behavior. We then examined a dataset of 55 flock matrices from 24 sites across the world. We found that sites with smaller flocks had higher values of phenotypic clumping for body size and sites with larger flocks were less phenotypically clumped. This relationship was weakly negative for foraging behavior and not statistically significant for taxonomic relatedness. Unlike most single-species groups, participants in mixed-species flocks appear to be able to separate on different axes of trait similarity. They can gain benefits from similarity on one axis while mitigating competition by dissimilarity on others. Consistent with our results, we speculate that flock assembly was deterministic up to a certain point with participants being similar in body size, but larger flocks tended to approach random phenotypic assemblages of species.

Characteristics and driving mechanisms of species beta diversity in desert plant communities

Species dissimilarity (beta diversity) primarily reflects the spatio–temporal changes in the species composition of a plant community. The correlations between β diversity and environmental factors and spatial distance can be used to explain the magnitudes of environmental filtering and dispersal. However, little is known about the relative roles and importance of neutral and niche-related factors in the assemblage of plant communities with different life forms in deserts. We found that in desert ecosystems, the β diversity of herbaceous plants was the highest, followed by that of shrubs and trees. The changes in the β diversity of herbs and shrubs had stronger correlations with the environment, indicating that community aggregation was strongly affected by niche processes. The soil water content and salt content were the key environmental factors affecting species distributions of the herb and shrub layers, respectively. Spatial distance explained a larger amount of the variation in tree composition, indicating that dispersal limitation was the main factor affecting the construction of the tree layer community. The results suggest that different life forms may determine the association between organisms and the environment. These findings suggest that the spatial patterns of plant community species in the Ebinur Lake desert ecosystem are the result of the combined effects of environmental filtering and dispersal limitation.

Disturbance reinforces community assembly processes differentially across spatial scales

BACKGROUND AND AIMS

There is a paucity of empirical research and a lack of predictive models concerning the interplay between spatial scale and disturbance as they affect the structure and assembly of plant communities. We proposed and tested a trait dispersion-based conceptual model hypothesizing that disturbance reinforces assembly processes differentially across spatial scales. Disturbance would reinforce functional divergence at the small scale (neighbourhood), would not affect functional dispersion at the intermediate scale (patch) and would reinforce functional convergence at the large scale (site). We also evaluated functional and species richness of native and exotic plants to infer underlying processes. Native and exotic species richness were expected to increase and decrease with disturbance, respectively, at the neighbourhood scale, and to show similar associations with disturbance at the patch (concave) and site (negative) scales.

METHODS

In an arid shrubland, we estimated species richness and functional dispersion and richness within 1 m2 quadrats (neighbourhood) nested within 100 m2 plots (patch) along a small-scale natural disturbance gradient caused by an endemic fossorial rodent. Data for the site scale (2500 m2 plots) were taken from a previous study. We also tested the conceptual model through a quantitative literature review and a meta-analysis.

KEY RESULTS

As spatial scale increased, disturbance sequentially promoted functional divergence, random trait dispersion and functional convergence. Functional richness was unaffected by disturbance across spatial scales. Disturbance favoured natives over exotics at the neighbourhood scale, while both decreased under high disturbance at the patch and site scales.

CONCLUSIONS

The results supported the hypothesis that disturbance reinforces assembly processes differentially across scales and hampers plant invasion. The quantitative literature review and the meta-analysis supported most of the model predictions.

Disentangling landscape and local drivers of ground-dwelling beetle community assembly in an urban ecosystem

Community assembly is the process by which local communities are organized and maintained from the regional species pool. Understanding processes of insect assembly are of interest in "shrinking" cities where vacant land has become abundant as a result of protracted economic decline and population loss. Vacant land represents a viable conservation space for insects such as beetles that contribute to ecosystem services including pest suppression, decomposition, and nutrient cycling. However, the inherent heterogeneity of cities may pose challenges for beetle dispersal from source populations, while quality of the urban environment may constrain establishment. The objective of this study was to investigate the constraints to ground-dwelling beetle community assembly in vacant lots and pocket prairies of Cleveland, Ohio using a functional trait-based approach. Functional traits with a strong predictive capacity for ecological functions were measured on beetle species collected via pitfall traps. Assembly of beetle communities was primarily constrained by dispersal limitations to colonization. Over 93% of species found within treatments were capable of flight, and functional diversity of beetle communities was higher across all treatments than expected by chance. Once beetles colonized, successful establishment was influenced by heavy metal contamination and mowing frequency, with these disturbances shaping communities based on body size, antennae length, and origin. Colonization of dispersal-limited species could be facilitated by increasing connectivity among greenspaces in cities, while establishment could be enhanced by managing local environmental conditions. Understanding how insect communities are structured in urban ecosystems provides context for observed patterns of biodiversity, advances conservation efforts, and fosters ecosystem services.

Demographic trends in community functional tolerance reflect tree responses to climate and altered fire regimes

Forests of the western United States are undergoing substantial stress from fire exclusion and increasing effects of climate change, altering ecosystem functions and processes. Changes in broad-scale drivers of forest community composition become apparent in their effect on survivorship and regeneration, driving demographic shifts. Here we take a community functional approach to forest demography, by investigating mean drought or shade functional tolerance in community assemblages. We created the Community Mean Tolerance Index (CMTI), a response metric utilizing drought/shade tolerance trade-offs to identify communities undergoing demographic change from a functional trait perspective. We applied the CMTI to Forest Inventory and Analysis data to investigate demographic trends in drought and shade tolerance across the southern Rocky Mountains. To find the major drivers of change in community tolerance within and across forest types, we compared index trends to climate and fire-exclusion-driven disturbance, and identified areas where demographic change was most pronounced. We predicted that greater shifts in drought tolerance would occur at lower forest type ecotones where climate stress is limiting and that shifts in shade tolerance would correspond to excursions from the historic fire regime leading to greater changes in forest types adapted to frequent, low-intensity fire. The CMTI was applied spatially to identify sites likely to transition to oak shrubfield, where disturbance history combined with a species-driven demographic shift toward drought tolerance. Within forest types, lower elevations are trending toward increased drought tolerance, while higher elevations are trending toward increased shade tolerance. Across forest types, CMTI difference peaked in mid-elevation ponderosa pine and mixed-conifer forests, where fire exclusion and autecology drive demographic changes. Peak CMTI difference was associated with fire exclusion in forest types adapted to frequent fire. At higher elevations, site-level stand dynamics appear to be influencing demographic tolerance trends more than broad climate drivers. Through a community demographic approach to functional traits, the CMTI highlights areas and forest types where ecosystem function is in the process of changing, before persistent vegetation type change occurs. Applied to regional plot networks, the CMTI provides an early warning of shifts in community functional processes as climate change pressures continue.

Long-term changes in macroalgae assemblages reveal a gradual biodiversity loss over the last 200 years in the hypereutrophic Guanabara Bay

Guanabara Bay, the second largest bay on the Brazilian coast, has tropical to subtropical, hypereutrophic water conditions. A survey of the macroalgae flora conducted over the past 200 years (1800-2013) yielded a list of 245 species, which represents 15.7% of the total macroalgae recorded in the Tropical Western Atlantic, while comprehends 29.9% and 50.2% of the marine flora in Brazil and Rio de Janeiro, respectively. When evaluating the macroalgal list throughout different periods, it is noticeable the gradual loss of biodiversity as anthropization increased over the last two centuries. The use of phytogeographic indexes and beta diversity confirmed the negative changes in macroalgae assemblages as a response to increasing environmental degradation. The use of floristic surveys to establish the reference conditions employing historical data and/or regionally referenced prove to be an efficient tool using macroalgae as an ecological indicator for water quality.

Global functional and phylogenetic structure of avian assemblages across elevation and latitude

Mountain systems are exceptionally species rich, yet the associated elevational gradients in functional and phylogenetic diversity and their consistency across latitude remain little understood. Here, we document how avian functional and phylogenetic diversity and structure vary along all major elevational gradients worldwide and uncover strong latitudinal differences. Assemblages in warm tropical lowlands and cold temperate highlands are marked by high functional overdispersion and distinctiveness, whereas tropical highlands and temperate lowlands appear strongly functionally clustered and redundant. We additionally find strong geographic variation in the interplay of phylogenetic and functional structure, with strongest deviations between the two in temperate highlands. This latitudinal and elevational variation in assemblage functional structure is underpinned by nuanced shifts in the position, shape and composition of multivariate trait space. We find that, independent of latitude, high-elevation assemblages emerge as exceptionally susceptible to functional change.

Materiality, Agency and Evolution of Lithic Technology: an Integrated Perspective for Palaeolithic Archaeology

Considerations of materiality and object-oriented approaches have greatly influenced the development of archaeological theory in recent years. Yet, Palaeolithic archaeology has been slow in incorporating this emerging body of scholarship and exploring its bearing on the human deep past. This paper probes into the potential of materiality theory to clarify the material dynamics of the Plio-Pleistocene and seeks to re-articulate the debate on the evolution of our species with materiality discourses in archaeology and the humanities more broadly. We argue that the signature temporalities and geospatial scales of observation provided by the Palaeolithic record offer unique opportunities to examine the active role of material things, objects, artefacts and technologies in the emergence, stabilisation and transformation of hominin lifeworlds and the accretion of long-term trajectories of material culture change. We map three axes of human-thing relations-ecological, technical and evolutionary-and deploy a range of case studies from the literature to show that a critical re-assessment of material agency not only discloses novel insights and questions, but can also refine what we already know about the human deep past. Our exploration underscores the benefits of de-centring human behaviour and intentionality and demonstrates that materiality lends itself as a productive nexus of exchange and mutual inspiration for diverging schools and research interests in Palaeolithic archaeology. An integrated object-oriented perspective calls attention to the human condition as a product of millennial-scale human-thing co-adaptation, in the course of which hominins, artefacts and technologies continuously influenced and co-created each other.

High environmental stress and productivity increase functional diversity along a deep-sea hydrothermal vent gradient

Productivity and environmental stress are major drivers of multiple biodiversity facets and faunal community structure. Little is known on their interacting effects on early community assembly processes in the deep sea (>200 m), the largest environment on Earth. However, at hydrothermal vents productivity correlates, at least partially, with environmental stress. Here, we studied the colonization of rock substrata deployed along a deep-sea hydrothermal vent gradient at four sites with and without direct influence of vent fluids at 1,700-m depth in the Lucky Strike vent field (Mid-Atlantic Ridge [MAR]). We examined in detail the composition of faunal communities (>20 μm) established after 2 yr and evaluated species and functional patterns. We expected the stressful hydrothermal activity to (1) limit functional diversity and (2) filter for traits clustering functionally similar species. However, our observations did not support our hypotheses. On the contrary, our results show that hydrothermal activity enhanced functional diversity. Moreover, despite high species diversity, environmental conditions at surrounding sites appear to filter for specific traits, thereby reducing functional richness. In fact, diversity in ecological functions may relax the effect of competition, allowing several species to coexist in high densities in the reduced space of the highly productive vent habitats under direct fluid emissions. We suggest that the high productivity at fluid-influenced sites supports higher functional diversity and traits that are more energetically expensive. The presence of exclusive species and functional entities led to a high turnover between surrounding sites. As a result, some of these sites contributed more than expected to the total species and functional β diversities. The observed faunal overlap and energy links (exported productivity) suggest that rather than operating as separate entities, habitats with and without influence of hydrothermal fluids may be considered as interconnected entities. Low functional richness and environmental filtering suggest that surrounding areas, with their very heterogeneous species and functional assemblages, may be especially vulnerable to environmental changes related to natural and anthropogenic impacts, including deep-sea mining.

Coexistence of plant species under harsh environmental conditions: an evaluation of niche differentiation and stochasticity along salt marsh creeks

Background

Ecologists have achieved much progress in the study of mechanisms that maintain species coexistence and diversity. In this paper, we reviewed a wide range of past research related to these topics, focusing on five theoretical bodies: (1) coexistence by niche differentiation, (2) coexistence without niche differentiation, (3) coexistence along environmental stress gradients, (4) coexistence under non-equilibrium versus equilibrium conditions, and (5) modern perspectives.

Results

From the review, we identified that there are few models that can be generally and confidently applicable to different ecological systems. This problem arises mainly because most theories have not been substantiated by enough empirical research based on field data to test various coexistence hypotheses at different spatial scales. We also found that little is still known about the mechanisms of species coexistence under harsh environmental conditions. This is because most previous models treat disturbance as a key factor shaping community structure, but they do not explicitly deal with stressful systems with non-lethal conditions. We evaluated the mainstream ideas of niche differentiation and stochasticity for the coexistence of plant species across salt marsh creeks in southwestern Denmark. The results showed that diversity indices, such as Shannon–Wiener diversity, richness, and evenness, decreased with increasing surface elevation and increased with increasing niche overlap and niche breadth. The two niche parameters linearly decreased with increasing elevation. These findings imply a substantial influence of an equalizing mechanism that reduces differences in relative fitness among species in the highly stressful environments of the marsh. We propose that species evenness increases under very harsh conditions if the associated stress is not lethal. Finally, we present a conceptual model of patterns related to the level of environmental stress and niche characteristics along a microhabitat gradient (i.e., surface elevation).

Conclusions

The ecology of stressful systems with non-lethal conditions will be increasingly important as ongoing global-scale climate change extends the period of chronic stresses that are not necessarily fatal to inhabiting plants. We recommend that more ecologists continue this line of research.

Trait-environment relationships differ between mixed-species flocking and nonflocking bird assemblages

Hypotheses about the mechanisms of community assembly suggest that biotic and abiotic filters constrain species establishment through selection on their functional traits. It is unclear how differences in traits influence the niche dimensions of closely related bird species when they coexist in spatiotemporally heterogeneous environments. Further, it is necessary to take into account their participation in mixed-species flocks, social systems that can include both competition and facilitation. For 6 yr, we conducted counts of forest bird species and took measurements of environmental variables along an elevational gradient in the Nanling Mountains, China. To disentangle different deterministic and historical/stochastic processes between flocking and nonflocking bird assemblages, we first compared phylogenetic and functional structure, and community-weighted mean trait values (CWM). We further assessed elevational variations in trait-environment relationships. We found that the flocking and nonflocking bird assemblages were structured by environmental gradients in contrasting ways. The nonflocking assemblage showed a strong change from over-dispersed to clustered community structure with increasing elevations, consistent with the strong selective pressures of a harsh environment (i.e., environmental filtering). The nonflocking assemblage also displayed significant trait-environment relationships in bivariate correlations and multivariate ordination space, including specific morphological and foraging traits that are linked to vegetation characteristics (e.g., short trees at high elevations). By contrast, flocking birds were more resilient to habitat change with elevation, with relatively consistent community membership, and showed fewer trait-environment associations. CWM of traits that are known to be associated with species' propensity to join mixed-species flocks, including small body size and broad habitat specificity, were linked to the flocking assemblage consistently across the elevational gradient. Collectively, our trait-based analyses provide strong evidence that trait-environment relationships differ between flocking and nonflocking bird assemblages. Besides serving as bellwethers of changing environments, emergent properties of flock systems may increase the resilience of animal communities undergoing environmental change. Mixed-species flocks present an ideal model with which to explore cooccurrence of closely related species, because habitat filtering may be buffered, and the patterns observed are therefore the outcomes of species interactions including both competition and facilitation.

Wave exposure reduces herbivory in post-disturbed reefs by filtering species composition, abundance and behaviour of key fish herbivores

Harsh environmental conditions limit how species use the landscape, strongly influencing the way assemblages are distributed. In the wake of repeated coral bleaching mortalities in Lakshadweep, we examined how wave exposure influences herbivory  in exposed and sheltered reefs. We used a combination of i. field observations of fish herbivore composition, abundance and activity across 6 exposed and 6 sheltered reefs; ii. experimental manipulations in a subset of these reefs (herbivore exclosures); and iii. opportunistic observations of fish recruitment, to determine how exposure influences herbivore biomass and herbivory. Species richness, biomass, abundance, total bite rates and species-specific per capita bite rates were lower in exposed compared to sheltered reefs, linked to strong environmental filtering of species composition, abundance and behaviour. For some critical species, this environmental filtering begins with differential recruitment and post-recruitment processes between exposures. Bite rates at sheltered sites were dominated by just a few species, most being laterally compressed surgeonfish that may find it difficult accessing or surviving in wave-battered shallow reefs. Exclosure experiments confirmed  that exposed reefs  were less controlled by herbivores than sheltered reefs. In post-disturbed reefs like Lakshadweep, environmental gradients appear to be key mediators of critical functions like herbivory by determining species composition, abundance and behaviour.

The effect of lateral connectedness on the taxonomic and functional structure of fish communities in a lowland river floodplain

In river floodplains many conservation programs focus on the main river channel as the richest in species. Lateral floodplain waterbodies, which contribute largely to functional processes in river systems, often remain overlooked and exposed to anthropogenic pressures. Although the role of hydrological connectedness between lateral waterbodies and the main river on taxonomic composition of fish communities is well understood, effects on functional community composition is much less studied. Abundance data of fish communities were gathered from 152 electrofishing sites in the main channel and lateral floodplain waterbodies of the river Lippe (Germany), over 18 years. These data were used to compare taxonomic, functional, conservation and recreational fishing aspects along the floodplain lateral connectedness gradient. Fish species richness decreased along the lateral continuum from the main river channel to isolated floodplain waterbodies. In contrast, the relative abundance of endangered and also of non-native species increased along this gradient, highlighting the ecological and conservational importance of floodplain waterbodies. Species composition in floodplain waterbodies differed across the connectedness gradient showing distinct assemblages which were not merely subsets of the main channel. The variability of life-history and feeding strategists among classes of lateral connectedness confirmed the importance of each connectivity class in contributing to the overall floodplain functional diversity. This study highlights the need of preserving fish taxonomic and functional biodiversity across the floodplain as one integrated hydrosystem. Conservation and restoration measures should therefore extend to include the whole floodplain area and the complete spectrum of differently connected floodplain waterbodies in addition to the main channel of the river.

Effect of aridity on species assembly in gypsum drylands: a response mediated by the soil affinity of species

Previous studies found that plant communities on infertile soils are relatively resistant to climatic variation due to stress tolerance adaptations. However, the species assemblies in gypsum soil habitats require further investigation. Thus, we considered the following questions. (1) Do harsher arid conditions determine the characteristics of the species that form plant assemblages? (2) Is the selection of the species that assemble in arid conditions mediated by their ability to grow on gypsum soils? (3) Is the selection of species that assemble in harsher conditions related to phylogenetically conserved functional traits? Perennial plant communities were analysed in 89 gypsum-soil sites along a 400 km climate gradient from the central to southeastern Iberian Peninsula. Each local assemblage was analysed in 30 × 30 m plots and described based on taxonomic, functional (soil plant affinity) and phylogenetic parameters. The mean maximum temperatures in the hottest month, mean annual precipitation and their interaction terms were used as surrogates for the aridity conditions in generalized linear models. In the hottest locations, the gypsophily range narrowed and the mean gypsophily increased at the community level, thereby suggesting the filtering of species and the dominance of soil specialists in the actual plant assemblies. Drier sites had higher taxonomic diversity. The species that formed the perennial communities were close in evolutionary terms at the two ends of the aridity gradient. The mean maximum temperatures in the hottest month had the main abiotic filtering effect on perennial plant communities, which was mediated by the ability of species to grow on gypsum soils, and thus gypsum specialists dominated the species assemblies in the hottest locations. In contrast, the perennial communities on gypsum soils were relatively resistant to changes in precipitation. Our findings suggest that the warmer environmental conditions predicted by global change models will favour gypsum specialists over generalists.

Social and Ecological Dimensions of Urban Conservation Grasslands and Their Management through Prescribed Burning and Woody Vegetation Removal

Natural grasslands are threatened globally. In south-eastern Australia, remnants of critically endangered natural grasslands are increasingly being isolated in urban areas. Urbanisation has led to reduced fire frequency and woody plant encroachment in some patches. Grasslands are currently being managed under the assumption that desirable management actions to address these threats (prescribed burning and removing woody vegetation) (1) lead to improved conservation outcomes and (2) are restricted by negative public attitudes. In this study, we tested these two assumptions in the context of native grassland conservation reserves in Melbourne, Australia. Firstly, we investigated differences in species and functional trait composition between patches that had been recently burnt, patches that were unburnt and patches subject to woody vegetation encroachment. We found that the functional traits of species converged in areas subject to woody plant encroachment and areas frequently disturbed by fire. Burning promoted native species, and patches of woody plants supressed the dominant grass, providing a wider range of habitat conditions. Secondly, we surveyed 477 residents living adjacent to these grassland conservation reserves to measure values, beliefs and attitudes and the acceptance of prescribed burning and removing woody vegetation. We found conflict in people’s attitudes to grasslands, with both strongly positive and strongly negative attitudes expressed. The majority of residents found prescribed burning an acceptable management practice (contrary to expectations) and removing trees and shrubs from grasslands to be unacceptable. Both cognitive factors (values and beliefs) and landscape features were important in influencing these opinions. This research provides some guidance for managing urban grassland reserves as a social–ecological system, showing that ecological management, community education and engagement and landscape design features can be integrated to influence social and ecological outcomes.

Consistent metacommunity structure despite inconsistent drivers of assembly at the continental scale

A fundamental goal of community ecology is to understand the drivers of community assembly and diversity. Local factors acting on community assembly are typically related to environmental conditions while regional factors are typically related to dispersal. Previous research has not consistently demonstrated the importance of local or regional factors, but this is likely because these factors act in concert and not in isolation. Studies that simultaneously integrate local and regional factors into analyses of community assembly can be a useful avenue to further our understanding of this core concept in community ecology. Here, we aimed to identify metacommunity structure and diversity and the local and regional drivers of community assembly at the continental scale. We evaluated metacommunity structure and drivers of assembly of macroinvertebrate communities in 941 rivers and streams nested within nine ecoregions distributed across the conterminous United States. Pattern-based metacommunity analyses and boosted regression tree techniques were used to (a) assign metacommunity structures and (b) identify the environmental, landscape and network drivers of assembly. We also evaluated how biodiversity scaled across hierarchical levels and varied among ecoregions. Metacommunity structures were consistent for the conterminous United States and each of the nine ecoregion subsets, with each ecoregional metacommunity displaying a Clementsian structure. Environmental variables were the predominant drivers of assembly, suggesting the importance of species sorting and environmental filtering on community structure; however, the identity of the most influential environmental variables differed among ecoregions and suggested hierarchical filtering on assembly. Partitioned diversity was found to be lower at the local and ecoregional levels, but turnover in diversity among ecoregions was higher than expected. Our results demonstrate contingencies in community assembly, notwithstanding consistency in metacommunity structure and support the importance of environmental control over community assembly and biodiversity. Moreover, biodiversity at the continental scale is likely maintained through this inherent variation in the drivers of assembly and concomitant changes in community composition among ecoregions. We suggest that further work should evaluate the assembly of other facets of community structure and the underlying mechanisms of the contingency in assembly drivers.

Lasting signature of planting year weather on restored grasslands

Ecological restoration - the rebuilding of damaged or destroyed ecosystems - is a critical component of conservation efforts, but is hindered by inconsistent, unpredictable outcomes. We investigated a source of this variation that is anecdotally suggested by practitioners, but for which empirical evidence is rare: the weather conditions during the first growing season after planting. The idea of whether natural communities face long-term consequences from conditions even many years in the past, called historical contingency, is a debated idea in ecological research. Using a large dataset (83 sites) across a wide geographic distribution (three states), we find evidence that precipitation and temperatures in the planting year (2-19 years before present) affected the relative dominance of the sown (native target species) and non-sown (mostly non-native) species. We find strong support for lasting planting year weather effects in restored tallgrass prairies, thereby supporting the historically contingent model of community assembly in a real-world setting.

Patterns of zeta diversity in ectoparasite communities harboured by small mammals at three hierarchical scales: taxon-invariance and scale-dependence

We studied compositional turnover in communities of fleas and mites harboured by small mammals using zeta diversity metric (similarity between multiple communities) and asked whether the patterns of zeta diversity decline with an increase in the number of communities differ between taxa and hierarchical scales [infracommunities (parasite assemblages on individual hosts), component communities (parasite assemblages harboured by host populations), and compound communities (all parasite species in a locality)]. The average number of shared species declined with an increasing number of communities (zeta order). It attained zero at higher orders in infracommunities of both taxa with the shape of the zeta decline being best fitted by the negative exponential function, and the retention rate curves being modal. In contrast, zeta diversity values for compound communities of mites and fleas did not attain zero at higher zeta orders, and the form of the zeta decline was best fitted by the power-law function, whereas the retention rate curves were asymptotic. In component communities, the form of zeta decline was best fitted by either exponential or power-law function in dependence of whether communities were considered within a host across localities or across hosts within a locality and whether ubiquitous species were taken into account. Our main conclusions are that (a) the rules governing compositional turnover in parasite communities for the lowest and the highest hierarchical scales are taxon-invariant but scale-dependent and (b) species composition of infracommunities is mainly driven by stochastic assembly processed, whereas that of compound communities is mainly driven by niche-based processes.

Soil fungal assemblage complexity is dependent on soil fertility and dominated by deterministic processes

In the processes controlling ecosystem fertility, fungi are increasingly acknowledged as key drivers. However, our understanding of the rules behind fungal community assembly regarding the effect of soil fertility level remains limited. Using soil samples from typical tea plantations spanning c. 2167 km north-east to south-west across China, we investigated the assemblage complexity and assembly processes of 140 fungal communities along a soil fertility gradient. The community dissimilarities of total fungi and fungal functional guilds increased with increasing soil fertility index dissimilarity. The symbiotrophs were more sensitive to variations in soil fertility compared with pathotrophs and saprotrophs. Fungal networks were larger and showed higher connectivity as well as greater potential for inter-module connection in more fertile soils. Environmental factors had a slightly greater influence on fungal community composition than spatial factors. Species abundance fitted the Zipf-Mandelbrot distribution (niche-based mechanisms), which provided evidence for deterministic-based processes. Overall, the soil fungal communities in tea plantations responded in a deterministic manner to soil fertility, with high fertility correlated with complex fungal community assemblages. This study provides new insights that might contribute to predictions of fungal community complexity.

Trait Gradient Analysis for Evergreen and Deciduous Species in a Subtropical Forest

Leaf habits (e.g., evergreen or deciduous) can reflect strategies of species adapting to varied environmental conditions. However, how species with different leaf habits coexist within a plant community is still poorly understood. Trait gradient analysis is a new approach to partition plant functional trait variations into alpha (within-community) and beta (among-community) components to quantify the effects of environmental filtering and biotic competition on community assembly. Here, on the basis of establishing forty-eight forest dynamic plots in a subtropical evergreen and deciduous broadleaved mixed forest in central China and measuring of seven functional traits, we compared the trait variation patterns and influencing factors of evergreen and deciduous species by using the trait gradient analysis method. The results showed that there were significant differences between functional traits for evergreen and deciduous species. Alpha trait components consistently varied more widely than beta components. The correlation between species trait mean and the alpha of each trait was highly significant, but there was no significant correlation between beta and alpha trait values. There were relatively weak or nonexistent significant correlations among species mean trait values and alpha trait values of different functional traits. However, the beta trait value showed high and significant correlations in both evergreen and deciduous species. Our results indicated that evergreen and deciduous species adopt similar adaptation strategies (beta component) in the context of environmental change in the community. However, the species initially came to coexist via the ecological positioning of traits (alpha component), which helped reduce competition so individuals could obtain more resources.

Effect of urbanization on the communities of Tardigrades in Argentina

Urban tardigrades are studied little worldwide. In Salta (Argentina), two studies have evaluated their diversity in an urban–native gradient. Vehicular traffic in Salta city has increased over the last decade, increasing environmental pollution. Our aim is to determine whether biotic homogenization occurs in this city. Sampling sites were selected considering different vehicular traffic intensities (high, medium and low). Tardigrades were sampled from bryophytes growing on tree trunks. Specimens were treated according to the usual study methodology. Different biotic and environmental abiotic variables of the microhabitat were considered. A total of 3049 specimens of 16 species of heterotardigrades and eutardigrades were reported. The low vehicular transit habitat was more diverse than the others. Ordination analyses evidenced a nested pattern in the studied urban habitats. Milnesium sp. nov. 3 and Minibiotus sp. nov. 1 were reported as detector species for habitats with medium vehicular traffic. The partition of the β diversity showed that the species turnover was higher among low and high vehicular traffic communities than medium, whereas a loss of species was evidenced between high and medium habitats. The high diversity of tardigrades in Salta city and their assemblages are partly dependent on several factors, including vehicular traffic.

Patterns of Understory Community Assembly and Plant Trait-Environment Relationships in Temperate SE European Forests

We analyzed variation in the functional composition and diversity of understory plant communities across different forest vegetation types in Slovenia. The study area comprises 10 representative forest sites covering broad gradients of environmental conditions (altitude, geology, light availability, soil type and reaction, nutrient availability, soil moisture), stand structural features and community attributes. The mean and variation of the trait values were quantified by community-weighted means and functional dispersion for four key plant functional traits: plant height, seed mass, specific leaf area and leaf dry matter content. At each study site, forest vegetation was surveyed at two different spatial scales (4 and 100 m2) in order to infer scale-dependent assembly rules. Patterns of community assembly were tested with a null model approach. We found that both trait means and diversity values responded to conspicuous gradients in environmental conditions and species composition across the studied forests. Our results mainly support the idea of abiotic filtering: more stressful environmental conditions (e.g., high altitude, low soil pH and low nutrient content) were occupied by communities of low functional diversity (trait convergence), which suggests a selective effect for species with traits adapted to such harsh conditions. However, trait convergence was also detected in some more resource-rich forest sites (e.g., low altitude, high soil productivity), most likely due to the presence of competitive understory species with high abundance domination. This could, at least to some extent, indicate the filtering effect of competitive interactions. Overall, we observed weak and inconsistent patterns regarding the impact of spatial scale, suggesting that similar assembly mechanisms are operating at both investigated spatial scales. Our findings contribute to the baseline understanding of the role of both abiotic and biotic constraints in forest community assembly, as evidenced by the non-random patterns in the functional structure of distinct temperate forest understories.

Combined effects of land-use intensification and plant invasion on native communities

Land-use intensification (LUI) and biological invasions are two of the most important global change pressures driving biodiversity loss. However, their combined impacts on biological communities have been seldom explored, which may result in misleading ecological assessments or mitigation actions. Based on an extensive field survey of 445 paired invaded and control plots of coastal vegetation in SW Spain, we explored the joint effects of LUI (agricultural and urban intensification) and invasion on the taxonomic and functional richness, mean plant height and leaf area of native plants. Our survey covered five invasive species with contrasting functional similarity and competitive ability in relation to the native community. We modeled the response of native communities for the overall and invader-specific datasets, and determined if invader-native functional differences could influence the combined impacts of LUI and invasion. Overall, we found that urban intensification reduced taxonomic richness more strongly at invaded plots (synergistic interactive effects). In contrast, functional richness loss caused by urban intensification was less pronounced at invaded plots (antagonistic interactive effects). Overall models showed also that urban intensification led to reduced mean leaf area, while agriculture was linked to higher mean plant height. When exploring invader-specific models, we observed that the combined effects of agricultural and urban intensification with invasion were heterogeneous. At invaded plots, invader-native functional differences accounted for part of this variability. Our findings demonstrate the importance of considering the interactive effects of global change pressures for a better assessment and management of ecosystems.

Assessment of congruence between co-occurrence and functional networks: A new framework for revealing community assembly rules

Describing how communities change over space and time is crucial to better understand and predict the functioning of ecosystems. We propose a new methodological framework, based on network theory and modularity concept, to determine which type of mechanisms (i.e. deterministic versus stochastic processes) has the strongest influence on structuring communities. This framework is based on the computation and comparison of two networks: the co-occurrence (based on species abundances) and the functional networks (based on the species traits values). In this way we can assess whether the species belonging to a given functional group also belong to the same co-occurrence group. We adapted the Dg index of Gauzens et al. (2015) to analyze congruence between both networks. This offers the opportunity to identify which assembly rule(s) play(s) the major role in structuring the community. We illustrate our framework with two datasets corresponding to different faunal groups and ecosystems, and characterized by different scales (spatial and temporal scales). By considering both species abundance and multiple functional traits, our framework improves significantly the ability to discriminate the main assembly rules structuring the communities. This point is critical not only to understand community structuring but also its response to global changes and other disturbances.

Soil characteristics and land-use drive bacterial community assembly patterns

Land-use and soil characteristics drive variation in soil community composition, but the influences of these factors on dispersal and community assembly at regional scale remain poorly characterized. Land-use remains a consistent driver of soil community composition even when exhibiting patchy spatial distribution at regional scale. In addition, disturbed and early successional soils often exhibit stochastic community assembly patterns. These observations suggest local community composition is influenced by dispersal and assembly from regional species pools. We examined bacterial community assembly within agricultural cropland, old-field, and forested sites across 10 landscapes in the region around Ithaca, New York (USA). We found that the Sloan neutral model explained assembly well at regional scale (R2 = 0.763), but that both soil pH and land-use imposed selection that shaped community composition. We show that homogeneous selection was a dominant assembly process with respect to both soil pH and land-use regime, but that these two factors interacted in their effects on bacterial community assembly. We conclude that bacterial community assembly at a regional scale is driven by dispersal from regional species pools and local selection on the basis of soil pH and other soil characteristics that vary with land-use.

Intercontinental trends in functional and phylogenetic structure of stream fish assemblages

Understanding of community assembly has been improved by phylogenetic and trait-based approaches, yet there is little consensus regarding the relative importance of alternative mechanisms and few studies have been done at large geographic and phylogenetic scales. Here, we use phylogenetic and trait dispersion approaches to determine the relative contribution of limiting similarity and environmental filtering to community assembly of stream fishes at an intercontinental scale. We sampled stream fishes from five zoogeographic regions. Analysis of traits associated with habitat use, feeding, or both resulted in more occurrences of trait underdispersion than overdispersion regardless of spatial scale or species pool. Our results suggest that environmental filtering and, to a lesser extent, species interactions were important mechanisms of community assembly for fishes inhabiting small, low-gradient streams in all five regions. However, a large proportion of the trait dispersion values were no different from random. This suggests that stochastic factors or opposing assembly mechanisms also influenced stream fish assemblages and their trait dispersion patterns. Local assemblages tended to have lower functional diversity in microhabitats with high water velocity, shallow water depth, and homogeneous substrates lacking structural complexity, lending support for the stress-dominance hypothesis. A high prevalence of functional underdispersion coupled with phylogenetic underdispersion could reflect phylogenetic niche conservatism and/or stabilizing selection. These findings imply that environmental filtering of stream fish assemblages is not only deterministic, but also influences assemblage structure in a fairly consistent manner worldwide.

Identifying models of trait-mediated community assembly using random forests and approximate Bayesian computation

Ecologists often use dispersion metrics and statistical hypothesis testing to infer processes of community formation such as environmental filtering, competitive exclusion, and neutral species assembly. These metrics have limited power in inferring assembly models because they rely on often-violated assumptions. Here, we adapt a model of phenotypic similarity and repulsion to simulate the process of community assembly via environmental filtering and competitive exclusion, all while parameterizing the strength of the respective ecological processes. We then use random forests and approximate Bayesian computation to distinguish between these models given the simulated data. We find that our approach is more accurate than using dispersion metrics and accounts for uncertainty in model selection. We also demonstrate that the parameter determining the strength of the assembly processes can be accurately estimated. This approach is available in the R package CAMI; Community Assembly Model Inference. We demonstrate the effectiveness of CAMI using an example of plant communities living on lava flow islands.