Niche Assembly Research Articles

Simulating the role of connectivity in shaping stream insect metacommunities under colonization cycle dynamics

We simulated metacommunities in a hypothetical riverine network to investigate whether and how connectivity affects the species richness of groups with different dispersal abilities under different niche- and dispersal-assembly rules. The simulated network was represented by a graph in which the nodes represented habitat patches, each with a particular environmental condition. We built our model based on Müller's “colonization cycle” hypothesis, in which adult aquatic insects compensate for the gradual downstream movement of larvae by flying preferentially upstream to oviposition. Species occurrence in each patch was probabilistically determined, taking into account environmental filtering and/or dispersal ability in different assembly scenarios. Variation in local species richness was explained by connectivity in both assembly scenarios, especially when dispersal ability was weak. In niche assembly metacommunities, the effect of connectivity was also dependent on the location of preferable patches. In general, local species richness was higher when dispersal ability was strong. Our findings suggest that the role of connectivity in shaping riverine metacommunities is context dependent and that the spatial structure of riverine networks should be considered in monitoring freshwater biota.

Phylogenetic properties of Tertiary relict flora in the east Asian continental islands: imprint of climatic niche conservatism and in situ diversification

Understanding biodiversity patterns on islands has long been a central aim in ecology and conservation biology. Island‐specific biogeographical processes play substantial roles in the formation of endemic biota. Here, we examined how climate niche conservatism and geohistorical factors are interactively associated with in situ diversification of Tertiary relict flora in the east Asian continental islands. We generated two novel datasets for species distribution and phylogeny that included all of the known vascular plant species in Japan (5575). Then we tested phylogenetic signal of climatic tolerance, in terms of absolute minimum temperature and water balance, and explored environmental predictors of phylogenetic structure (evolutionary derivedness and clustering) of species assemblages. Although phylogenetic signal of climatic tolerance was significant across the phylogeny of most species, the strength of climatic niche conservatism differed among ferns, gymnosperms, angiosperm trees, and angiosperm herbs. For angiosperm trees, cold temperatures acted as environmental filters that generated phylogenetic derivedness/clustering of species assemblages. For fern and angiosperm herb species, however, phylogenetic properties were not associated with climatic harshness. These contrasting patterns among groups reflected climate niche evolution in vascular plants with different growth forms and traits; for example, diversification of angiosperm trees (but not fern and herb) occurred in response to historical climatic cooling. More importantly, geographical constraints contributed to evolutionary radiation that resulted from isolation by distance from the continent or by elevation. Quaternary climate change was also associated with clade‐specific radiation in refugial habitats. The degree to which geographical, geological, and palaeoclimatic variables explain the phylogenetic structure underscores the importance of isolation‐ and habitat‐stability‐related geohistorical processes in driving in situ diversification despite climatic niche conservatism. We propose that the highly endemic flora of the east Asian islands resulted from the interplay of idiosyncratic regional factors, and ecological and evolutionary processes, such as climate niche assembly and adaptive/nonadaptive radiation.

Detection of fine scale niche assembly in a tropical forest through analysis of indirect environmental variables

Topographic or forest structural variables have been called indirect environmental variables in forest ecology studies. We tested the hypothesis that relative elevation and tallest tree height in the sampling unit, two of these indirect environmental variables, play a role in fine scale community organization in an Atlantic forest in Southeastern Brazil. One hundred 10m×10m sampling units were used to sample the tree community in a 6.5ha stand. We modeled space through Moran’s eigenvectors maps and selected spatial eigenfuctions through an individual species based approach to reach an effective control of type I error. Redundancy analysis and variation partitioning were applied to correlate community composition and environmental variables while maintaining space constant. A significant global p-value for the Redundancy analysis was found. The pure environmental fraction of variation in community composition was also significant, increasing from 1% (adjusted R2) for the simple environmental data to 3.3% for the polynomial (third degree) environmental data. The pure and spatially structured environmental fractions summed up 4% and 6% for the simple and polynomial environmental data, respectively. The influence of the environmental variables varied between species functional groups: non-pioneer species were predominantly affected by relative elevation while pioneers were almost strictly associated with tallest tree height. Our findings amplify the importance of niche assembly processes in fine scale community organization because of the choice of a grain size (10m×10m) and an extent (6.5ha) smaller than those considered elsewhere, along with rigorous control of type I error rate.

Recruitment patterns and potential mechanisms of community assembly in an Andean cloud forest

AbstractQuestionWhat are the conditions necessary for the establishment and development of seedlings and an early stage sapling community in an old‐growth cloud forest? Cloud forests are one of the most threatened ecosystems in the world; however, recruitment patterns in these forests remain largely unknown.LocationCloud Forest, Northern Peruvian Andes.MethodsWe constructed a sapling community model through the unconstrained technique of non‐metric multidimensional scaling. We related the distribution of saplings of each species to the distance from conspecific adult (potential parent) trees through a point pattern analysis. We also used zero‐inflated Poisson models to investigate the relationship between sapling distributions and environmental conditions and forest structure.ResultsWe found that recruitment in woody plant species tends to be widely spread throughout the forest. The distribution of some sapling species was either positively or negatively related to the position of adult conspecific trees. Several species tended to occur within particular microhabitat conditions, with some differentiation between canopy and understorey species.ConclusionsCloud forest species recruitment may require the cover provided by the forest canopy. Under closed canopy conditions, both dispersal assembly and niche assembly mechanisms appear to simultaneously influence sapling distribution. The different strategies of various species may result in a trade‐off between the importance of microhabitat conditions and distance mechanisms, with one prevailing over the other, depending on species and forest structure conditions.

Zeta diversity as a concept and metric that unifies incidence-based biodiversity patterns.

Patterns in species incidence and compositional turnover are central to understanding what drives biodiversity. Here we propose zeta (ζ) diversity, the number of species shared by multiple assemblages, as a concept and metric that unifies incidence-based diversity measures, patterns, and relationships. Unlike other measures of species compositional turnover, zeta diversity partitioning quantifies the complete set of diversity components for multiple assemblages, comprehensively representing the spatial structure of multispecies distributions. To illustrate the application and ecological value of zeta diversity, we show how it scales with sample number, grain, and distance. Zeta diversity reconciles several different biodiversity patterns, including the species accumulation curve, the species-area relationship, multispecies occupancy patterns, and scaling of species endemism. Exponential and power-law forms of zeta diversity are associated with stochastic versus niche assembly processes. Zeta diversity may provide new insights on biodiversity patterns, the processes driving them, and their response to environmental change.

Metagenetic analysis of patterns of distribution and diversity of marine meiobenthic eukaryotes

AbstractAimMeiofaunal communities that inhabit the marine benthos offer unique opportunities to simultaneously study the macroecology of numerous phyla that exhibit different life‐history strategies. Here, we ask: (1) if the macroecology of meiobenthic communities is explained mainly by dispersal constraints or by environmental conditions; and (2) if levels of meiofaunal diversity surpass existing estimates based on morphological taxonomy.LocationUK and mainland European coast.MethodsNext‐generation sequencing techniques (NGS; Roche 454 FLX platform) using 18S nuclear small subunit ribosomal DNA (rDNA) gene. Pyrosequences were analysed using AmpliconNoise followed by chimera removal using Perseus.ResultsRarefaction curves revealed that sampling saturation was only reached at 15% of sites, highlighting that the bulk of meiofaunal diversity is yet to be discovered. Overall, 1353 OTUs were recovered and assigned to 23 different phyla. The majority of sampled sites had c. 60–70 unique operational taxonomic units (OTUs) per site, indicating high levels of beta diversity. The environmental parameters that best explained community structure were seawater temperature, geographical distance and sediment size, but most of the variability (R2 = 70%–80%) remains unexplained.Main conclusionsHigh percentages of endemic OTUs suggest that meiobenthic community composition is partly niche‐driven, as observed in larger organisms, but also shares macroecological features of microorganisms by showing high levels of cosmopolitanism (albeit on a much smaller scale). Meiobenthic communities exhibited patterns of isolation by distance as well as associations between niche, latitude and temperature, indicating that meiobenthic communities result from a combination of niche assembly and dispersal processes. Conversely, isolation‐by‐distance patterns were not identified in the featured protists, suggesting that animals and protists adhere to radically different macroecological processes, linked to life‐history strategies.

The role of abiotic environmental conditions and herbivory in shaping bacterial community composition in floral nectar.

Identifying the processes that drive community assembly has long been a central theme in ecology. For microorganisms, a traditional prevailing hypothesis states that “everything is everywhere, but the environment selects”. Although the bacterial community in floral nectar may be affected by both atmosphere (air-borne bacteria) and animals as dispersal vectors, the environmental and geographic factors that shape microbial communities in floral nectar are unknown. We studied culturable bacterial communities in Asphodelus aestivus floral nectar and in its typical herbivorous bug Capsodes infuscatus, along an aridity gradient. Bacteria were sampled from floral nectar and bugs at four sites, spanning a geographical range of 200 km from Mediterranean to semi-arid conditions, under open and bagged flower treatments. In agreement with the niche assembly hypothesis, the differences in bacterial community compositions were explained by differences in abiotic environmental conditions. These results suggest that microbial model systems are useful for addressing macro-ecological questions. In addition, similar bacterial communities were found in the nectar and on the surface of the bugs that were documented visiting the flowers. These similarities imply that floral nectar bacteria dispersal is shaped not only by air borne bacteria and nectar consumers as previously reported, but also by visiting vectors like the mirid bugs.

Ecology and niche assembly of campanula tommasiniana, a narrow endemic of mt učka (Liburnian Karst, north-western Adriatic)

Abstract Campanula tommasiniana is a typical chasmophyte occupying calcareous rock crevices and cracks along a wide range of ecological gradients, demonstrating a high degree of ecological plasticity and stress tolerance with regards to abiotic factors. Generally, three ecologically and floristically distinct groups of stands were recognized and typified according to a sigmatistic approach: (a) Seslerio juncifoliae-Campanuletum tommasinianae ass. nov., with stands occupying higher elevated sites fully exposed to sun and strong winds; (b) Seslerio autumnalis-Campanuletum tommasinianae ass. nov., representing stands predominantly developed within thermophytic beech stands, semi- to fully- -shaded by the tree canopy; (c) Cystopteri fragilis-Campanuletum tommasinianae, sciophytic, stands adapted to moisture and coldwith high frequency and coverage of bryophytes. Results of DCA analyses using a unimodal model suggest that Campanula tommasiniana is primarily a plant of open and exposed sites of higher elevation despite being most frequently found in rock crevices within thermophytic and altimontane beech forests.

Distinguishing the importance between habitat specialization and dispersal limitation on species turnover.

Understanding what governs community assembly and the maintenance of biodiversity is a central issue in ecology, but has been a continuing debate. A key question is the relative importance of habitat specialization (niche assembly) and dispersal limitation (dispersal assembly). In the middle of the Loess Plateau, northwestern China, we examined how species turnover in Liaodong oak (Quercus wutaishanica) forests differed between observed and randomized assemblies, and how this difference was affected by habitat specialization and dispersal limitation using variation partitioning. Results showed that expected species turnover based on individual randomization was significantly lower than the observed value (P < 0.01). The turnover deviation significantly depended on the environmental and geographical distances (P < 0.05). Environmental and spatial variables significantly explained approximately 40% of the species composition variation at all the three layers (P < 0.05). However, their contributions varied among forest layers; the herb and shrub layers were dominated by environmental factors, whereas the canopy layer was dominated by spatial factors. Our results underscore the importance of synthetic models that integrate effects of both dispersal and niche assembly for understanding the community assembly. However, habitat specialization (niche assembly) may not always be the dominant process in community assembly, even under harsh environments. Community assembly may be in a trait-dependent manner (e.g., forest layers in this study). Thus, taking more species traits into account would strengthen our confidence in the inferred assembly mechanisms.

Latitudinal phytoplankton distribution and the neutral theory of biodiversity

AbstractAimRecent studies have suggested that global diatom distributions are not limited by dispersal, in the case of both extant species and fossil species, but rather that environmental filtering explains their spatial patterns. Hubbell's neutral theory of biodiversity provides a framework in which to test these alternatives. Our aim is to test whether the structure of marine phytoplankton (diatoms, dinoflagellates and coccolithophores) assemblages across the Atlantic agrees with neutral theory predictions. We asked: (1) whether intersite variance in phytoplankton diversity is explained predominantly by dispersal limitation or by environmental conditions; and (2) whether species abundance distributions are consistent with those expected by the neutral model.LocationMeridional transect of the Atlantic (50° N–50° S).MethodsWe estimated the relative contributions of environmental factors and geographic distance to phytoplankton composition using similarity matrices, Mantel tests and variation partitioning of the species composition based upon canonical ordination methods. We compared the species abundance distribution of phytoplankton with the neutral model using Etienne's maximum‐likelihood inference method.ResultsPhytoplankton communities are slightly more determined by niche segregation (24%), than by dispersal limitation and ecological drift (17%). In 60% of communities, the assumption of neutrality in species’ abundance distributions could not be rejected. In tropical zones, where oceanic gyres enclose large stable water masses, most communities showed low species immigration rates; in contrast, we infer that communities in temperate areas, out of oligotrophic gyres, have higher rates of species immigration.ConclusionsPhytoplankton community structure is consistent with partial niche assembly and partial dispersal and drift assembly (neutral processes). The role of dispersal limitation is almost as important as habitat filtering, a fact that has been largely overlooked in previous studies. Furthermore, the polewards increase in immigration rates of species that we have discovered is probably caused by water mixing conditions and productivity.

Spatial distribution and interspecific associations of tree species in a tropical seasonal rain forest of China.

Studying the spatial pattern and interspecific associations of plant species may provide valuable insights into processes and mechanisms that maintain species coexistence. Point pattern analysis was used to analyze the spatial distribution patterns of twenty dominant tree species, their interspecific spatial associations and changes across life stages in a 20-ha permanent plot of seasonal tropical rainforest in Xishuangbanna, China, to test mechanisms maintaining species coexistence. Torus-translation tests were used to quantify positive or negative associations of the species to topographic habitats. The results showed: (1) fourteen of the twenty tree species were negatively (or positively) associated with one or two of the topographic variables, which evidences that the niche contributes to the spatial pattern of these species. (2) Most saplings of the study species showed a significantly clumped distribution at small scales (0–10 m) which was lost at larger scales (10–30 m). (3) The degree of spatial clumping deceases from saplings, to poles, to adults indicates that density-dependent mortality of the offspring is ubiquitous in species. (4) It is notable that a high number of positive small-scale interactions were found among the twenty species. For saplings, 42.6% of all combinations of species pairs showed positive associations at neighborhood scales up to five meters, but only 38.4% were negative. For poles and adults, positive associations at these distances still made up 45.5% and 29.5%, respectively. In conclusion, there is considerable evidence for the presence of positive interactions among the tree species, which suggests that species herd protection may occur in our plot. In addition, niche assembly and limited dispersal (likely) contribute to the spatial patterns of tree species in the tropical seasonal rain forest in Xishuangbanna, China.

Environment versus dispersal in the assembly of western Amazonian palm communities

AbstractAim It is a central issue in ecology and biogeography to understand what governs community assembly and the maintenance of biodiversity in tropical rain forest ecosystems. A key question is the relative importance of environmental species sorting (niche assembly) and dispersal limitation (dispersal assembly), which we investigate using a large dataset from diverse palm communities.Location Lowland rain forest, western Amazon River Basin, Peru.Methods We inventoried palm communities, registering all palm individuals and recording environmental conditions in 149 transects of 5 m × 500 m. We used ordination, Mantel tests and indicator species analysis (ISA) to assess compositional patterns, species responses to geographical location and environmental factors. Mantel tests were used to assess the relative importance of geographical distance (as a proxy for dispersal limitation) and environmental differences as possible drivers of dissimilarity in palm species composition. We repeated the Mantel tests for subsets of species that differ in traits of likely importance for habitat specialization and dispersal (height and range size).Results We found a strong relationship between compositional dissimilarity and environmental distance and a weaker but also significant relationship between compositional dissimilarity and geographical distance. Consistent with expectations, relationships with environmental and geographical distance were stronger for understorey species than for canopy species. Geographical distance had a higher correlation with compositional dissimilarity for small‐ranged species compared with large‐ranged species, whereas the opposite was true for environmental distance. The main environmental correlates were inundation and soil nutrient levels.Main conclusions The assembly of palm communities in the western Amazon appears to be driven primarily by species sorting according to hydrology and soil, but with dispersal limitation also playing an important role. The importance of environmental characteristics and geographical distance varies depending on plant height and geographical range size in agreement with functional predictions, increasing our confidence in the inferred assembly mechanisms.

Community Assembly in the Salamander Genus Desmognathus

In this paper, I present a model of community assembly in the salamander genus Desmognathus based on a survey of assemblage composition throughout the range of the genus. The 21 species of Desmognathus can be sorted into three life-history categories, namely, stream, streamside, and forest, based on duration of the larval phase and a suite of other life-history and morphological traits that are correlated with habitat use of the several life-history stages. In most assemblages having all three life-history categories, stream species are larger than streamside species, and the latter species are larger than forest species. An evaluation of the literature on interspecific competition and predation (i.e., intraguild predation) in Desmognathus indicates that these processes are important in structuring assemblages of these salamanders. Thus, niche assembly, as opposed to dispersal assembly (i.e., neutral model), seems to be a valid model of community assembly in Desmognathus. Only streamside species occur throughout the range of the genus, and these forms alone are found around the periphery of the range. One to three streamside species form the base of extant assemblages of Desmognathus, with stream and forest species contributing to the more diverse assemblages of the southern Appalachian region. The maximum numbers are two or three streamside species, two or three stream species, and two forest species, although assemblages of more than six species are undocumented. I suggest that the rapid evolutionary diversification in body size and life history in Desmognathus that has generated the complex assemblages of this genus in the Appalachians has been facilitated by a high level of life-history symmetry in these salamanders.

Recruitment in species-rich grasslands: the effects of functional traits and propagule pressure

Aims The assembly of plant communities is a complex process which combines impacts from the species pool, dispersal and propagule pressure, niche requirements of colonizing species and the niche structure of the community. Recent theory development has incorporated all these aspects, e.g. in ‘stochastic niche theory’. We investigated recruitment into a species-rich grassland community, using an experimental approach where we manipulated the trait composition of the community and examined the success of colonizing species entering with various propagule pressure. Specifically, we examined two predictions: (i) colonization success increases with increasing difference between traits of the colonizing species and the trait profile of the community and (ii) colonization success increases with increasing propagule pressure. Methods The examined communities were species-rich semi-natural grasslands located in southern Sweden. After a careful documentation of the composition of the plant communities at the experimental sites, we manipulated the trait profile of species-rich grassland plots based on the plant functional trait specific leaf area (SLA), which is correlated with several key life history functions. In addition to SLA, seed mass was also used to describe the trait profile of grassland plots. Seeds of 12 plant species from the regional species pool, varying in SLA and seed mass, were sown into plots using four different levels of propagule pressure. Recruitment was examined after 1 year. We also planted juvenile ‘plug plants’ of the same species which allowed us to examine survivorship and growth beyond the seedling stage. Important Findings Overall we found very limited evidence for relationships between the traits of the colonizing species and the trait profile of the community and for recruitment after sowing these relationships were contrary to the prediction. Survival of plug plants after two seasons of growth was high irrespective of the trait profile of the community, but growth of plug plants was affected by the trait profile of the surrounding community. For four of the species there was a positive effect of increased propagule pressure on colonization. The results suggest that species assembly in species-rich grasslands is not strongly dependent on the niche structure of the community. However, the finding that colonization of only a third of the species responded positively to increased propagule pressure indicates that there might be niche-related effects that were not captured by our treatments. Overall, our results indicate that the factors determining colonization in this community are species specific. Some species are able to colonize irrespective of niche relationships, provided that the propagule pressure is sufficiently high to overcome stochastic mortality after seed arrival. For other species, however, we cannot exclude that niche assembly occurred, but we failed to identify the relevant niche factor.

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

Ecological approaches to community assembly have emphasized the interplay between neutral processes, niche-based environmental filtering and niche-based species sorting in an interactive milieu. Recently, progress has been made in terms of aligning our vocabulary with conceptual advances, assessing how trait-based community functional parameters differ from neutral expectation and assessing how traits vary along environmental gradients. Experiments have confirmed the influence of these processes on assembly and have addressed the role of dispersal in shaping local assemblages. Community phylogenetics has forged common ground between ecologists and biogeographers, but it is not a proxy for trait-based approaches. Community assembly theory is in need of a comparative synthesis that addresses how the relative importance of niche and neutral processes varies among taxa, along environmental gradients, and across scales. Towards that goal, we suggest a set of traits that probably confer increasing community neutrality and regionality and review the influences of stress, disturbance and scale on the importance of niche assembly. We advocate increasing the complexity of experiments in order to assess the relative importance of multiple processes. As an example, we provide evidence that dispersal, niche processes and trait interdependencies have about equal influence on trait-based assembly in an experimental grassland.

Ank3-Dependent SVZ Niche Assembly Is Required for the Continued Production of New Neurons

The rodent subventricular/subependymal zone (SVZ/SEZ) houses neural stem cells (NSCs) that generate olfactory bulb interneurons. It is unclear how the SVZ environment sustains neuronal production into adulthood. We discovered that the adapter molecule Ankyrin-3 (Ank3) is specifically upregulated in ventricular progenitors destined to become ependymal cells, but not in NSCs, and is required for SVZ nicheassembly through progenitor lateral adhesion. Furthermore, we found that Ank3 expression is controlled by Foxj1, a transcriptional regulator of multicilia formation, and genetic deletion of this pathway led to complete loss of SVZ niche structure. Interestingly, radial glia continued to transition into postnatal NSCs without this niche. However, inducible deletionof Foxj1-Ank3 from mature SVZ ependyma resultedindramatic depletion of neurogenesis. Targeting a pathway regulating ependymal organization/assembly and showing its requirement for new neuron production, our results have important implications for environmental control of adult neurogenesis and harvesting NSCs for replacement therapy.

Niche differentiation and neutral theory: an integrated perspective on shrub assemblages in a parkland savanna

Investigations of structure in ecological communities need to move beyond the dichotomy between niche and neutral theory to broader conceptual frameworks that accommodate both neutral stochastic and biological structuring processes in organizing species assemblages. We propose and test a framework that integrates niche and neutral-assembly perspectives and determines their relative contributions in structuring diverse shrub species assemblages in a parkland savanna. Our approach proposes that stochastic dispersal processes initially govern the assemblage of species in discrete shrub clusters developing in grassland, but that community structure subsequently develops through the progressive action of first positive, then negative interactions among species. A comparison of observed patterns of occurrence and niche models for 12 shrub cluster species against neutral predictions revealed that neutral stochastic, island biogeographic processes accounted for most patterns of species occurrence. One species showed strong evidence of successional differentiation, whereas evidence of slight recruitment biases for five others was equivocal. Our results demonstrate the usefulness of an approach that accommodates contributions of both neutral and niche assembly rather than assuming either process alone is sufficient to account for community structure. Further development and testing of robust and falsifiable neutral theory will allow ecologists to critically evaluate the relative roles of niche differentiation and neutral, stochastic processes in structuring communities.

Soil‐based habitat partitioning in understorey palms in lower montane tropical forests

AbstractAim Dispersal assembly and niche assembly are two competing theories proposed to explain the maintenance of species diversity in tropical forests. Dispersal theory emphasizes the role of chance colonization events and distance‐limited seed dispersal in explaining species abundance and distribution, whereas niche theory emphasizes differences among species in requirements for potentially limiting resources. Species distribution patterns in tropical forests often correlate with geology and topography, but tests of the relative importance of dispersal and niche partitioning have been hampered by an inadequate characterization of resource availability. The aim of this study was to explore how soil chemical and physical properties, climate, and geographic distance affect understorey palm communities in lower montane forests.Location Fortuna Forest Reserve, Chiriqui Province, and Palo Seco Forest Reserve, Bocas del Toro Province, in western Panama.Methods Understorey palms and soil nutrient concentrations were surveyed within 10 sites on different soil types across a 13‐km transect. Variation in palm community composition was examined in relation to spatial and environmental variables.Results The 25 understorey palm species recorded in the study were non‐randomly distributed among forests differing in soil nutrient availability. In support of dispersal theory, floristic similarity decreased predictably with increasing geographic distance. However, environmental and soil variables were also correlated with geographic distance. Floristic similarity was also highly associated with a subset of environmental variables. Variation in palm community similarity was most strongly correlated with inorganic nitrogen availability and cation concentration. A subset of soil variables had a stronger relationship with floristic similarity when geographic distance was controlled for than did geographic distance when differences in soils were controlled for.Main conclusions Both dispersal and niche processes affect palm species distribution patterns. Although spatially limited dispersal may influence species distribution patterns, soil‐based habitat associations, particularly with respect to soil nitrogen, cation availability and aluminium concentrations, remain important factors influencing palm community composition at the mesoscale level in this tropical montane forest.

Niche assembly of epiphytic bryophyte communities in the Guianas: a regional approach

AbstractAim Epiphytic bryophyte communities of tropical forests show a gradient in species composition from the base to the top of the host trees, indicating a strong role of niche assembly. This pattern, however, has never been tested at a regional scale. The aim of this study was to test whether niche assembly, rather than dispersal limitation, predominantly drives species composition of bryophyte communities across large spatial scales.Location Three lowland forests in the Guianas: one near Saul, French Guiana; and two near Mabura Hill, Guyana.Methods Communities of epiphytic bryophytes were sampled from six different height zones of several trees in three lowland forests. We analysed the composition of these communities using detrended correspondence analysis in order to find the best explanatory variable for the variation in community composition. A multi‐response permutation procedure was used to test the significance of grouping communities by height zone. We conducted an indicator species analysis to classify species as specialists or generalists and then tested, through weighted averaging, if specialists would indeed maintain their preferred height zone across the Guianas.Results Community composition was explained mainly by height zone. The similarity among communities inhabiting the same height zone of trees, across a distance of up to 640 km, was higher than the similarity among communities established along the vertical gradient of a single standing tree (30–50 m). More than half (57%) of the species had a preferred height zone, and the preference was consistent: species occupied roughly the same height zone on host trees in the different localities. The three local communities investigated were found to belong to the same regional species pool.Main conclusions Throughout the Guianas, epiphytic bryophyte communities are drawn from the same regional species pool, and their composition is shaped by micro‐environmental conditions. The predominance of niche assembly over dispersal assembly rules is consistently found at both local and regional scales.

Local immigration, competition from dominant guilds, and the ecological assembly of high‐diversity pine savannas

In high-diversity communities, rare species encounter one another infrequently and therefore may compete more intensely with common species or guilds for limiting space and resources. In addition, rare species may be strongly recruitment limited because of their low abundances. Under these conditions, stochastic dispersal and immigration history can have an important influence on community structure. We tested the hypothesis that local immigration and competition from common, large-stature guilds interact to structure local biodiversity in high-diversity longleaf pine savanna groundcover assemblages (>30 species/m2). In two factorial field experiments, we increased local immigration by adding seeds of 38 mostly rare, small-stature forbs and sedges to plots physically dominated by either a common, large-stature bunchgrass or shrub species and to plots in which competition from these dominant guilds was reduced. We measured species richness and abundance at two spatial scales (0.01 and 0.25 m2) over two years. Immigration increased total species richness and richness of focal seed addition species regardless of levels of competition with bunchgrasses and shrubs, indicating that many rare, small-stature species can recruit in the face of potential competition from dominant guilds. Removal of dominant guilds increased total and focal species richness in shrub-dominated but not bunchgrass-dominated plots. In addition, competition from both dominant guilds had no clear effect on rank-abundance distributions of focal species. Our results suggest a key role for dispersal assembly in structuring local biodiversity in this high-diversity plant community, but the importance of this mechanism depends on the strength of local niche assembly involving competition from some, but not all, dominant guilds.