Frequency and intensity of facilitation reveal opposing patterns along a stress gradient

Disentangling the different processes structuring ecological communities is a long‐standing challenge. In species‐rich ecosystems, most emphasis has so far been given to environmental filtering and competition processes, while facilitative interactions between species remain insufficiently studied. Here, we propose an analysis framework that not only allows for identifying pairs of facilitating and facilitated species, but also estimates the strength of facilitation and its variation along environmental gradients. Our framework combines the analysis of both co‐occurrence and co‐abundance patterns using a moving window approach along environmental gradients to control for potentially confounding effects of environmental filtering in the co‐abundance analysis. We first validate our new approach against community assembly simulations, and exemplify its potential on a large 1,134 plant community plots dataset. Our results generally show that facilitation intensity was strongest under cold stress, whereas the proportion of facilitating and facilitated species was higher under drought stress. Moreover, the functional distance between individual facilitated species and their facilitating species significantly changed along the temperature–moisture gradient, and seemed to influence facilitation intensity, although no general positive or general negative trend was discernible among species. The main advantages of our robust framework are as follows: It enables detecting facilitating and facilitated species in species‐rich systems, and it allows identifying the directionality and intensity of facilitation in species pairs as well as its variation across long environmental gradients. It thus opens numerous opportunities for incorporating functional (and phylogenetic) information in the analysis of facilitation patterns. Our case study indicated high complexity in facilitative interactions across the stress gradient and revealed new evidence that facilitation, similarly to competition, can operate between functionally similar and dissimilar species. Extending the analyses to other taxa and ecosystems will foster our understanding how complex interspecific interactions promote biodiversity.     

Mycorrhizal symbiosis increases the benefits of plant facilitative interactions

The diversity of pathways through which mycorrhizal fungi alter plant coexistence hinders the understanding of their effects on plant‐plant interactions. The outcome of plant facilitative interactions can be indirectly affected by mycorrhizal symbiosis, ultimately shaping biodiversity patterns. We tested whether mycorrhizal symbiosis enhances plant facilitative interactions and whether its effect is consistent across different methodological approaches and biological scenarios. We conducted a meta‐analysis of 215 cases (involving 21 nurse and 29 facilitated species), in which the performance of a facilitated plant species is measured in the presence or absence of mycorrhizal fungi. We show that mycorrhizal fungi significantly enhance plant facilitative interactions mainly through an increment in plant biomass (aboveground) and nutrient content, although their effects differ across biological contexts. In semiarid environments mycorrhizal symbiosis enhances plant facilitation, while its effect is non‐significant in temperate ecosystems. In addition, arbuscular but not ecto‐mycorrhizal (EMF) fungi significantly enhance plant facilitation, particularly increasing the P content of the plants more than EMF. Some knowledge gaps regarding the importance of this phenomenon have been detected in this meta‐analysis. The effect of mycorrhizal symbiosis on plant facilitation has rarely been assessed in other ecosystems different from semiarid and temperate forests, and rarely considering other fungal benefits provided to plants besides nutrients. Finally, we are still far from understanding the effects of the whole fungal community on plant‐plant interactions, and on plant species coexistence.     

Independent and interactive effects of two facilitators on their habitat‐providing host plant,Spartina alterniflora

The role of habitat‐providing species in facilitating associated species abundance and diversity is recognized as a key structuring force in many ecosystems. Reciprocal facilitation by associates, often involving multiple species, can be important for the maintenance of the host species. As with other multi‐species interactions (e.g. multiple predator effects), non‐additive relationships may be common among these associates, yet relatively few studies have examined potential interactions among multiple facilitator species. We combined field surveys and a mesocosm experiment to examine the independent and interactive effects of two co‐occurring facilitator species, ribbed mussels Geukensia demissa and fiddler crabs Uca pugilator, on their host salt marsh plant species, cordgrass Spartina alterniflora. We also experimentally examined how these relationships varied across different host plant genotypes. Overall, facilitator effects increased with increasing facilitator density. There was a significant interaction between mussel and fiddler crab presence, indicating that the effects of each species on cordgrass were dependent on the presence of the other facilitator species. In addition, there were strong interactions among mussels, fiddler crabs, and plant genotype, with greater variation in the performance of individual genotypes when fiddler crabs were absent. Our work reinforces the importance of considering multiple responses when assessing the functional redundancy of co‐occurring facilitators, as species are seldom completely redundant across the range of services they provide. It also highlights that the strength and direction of species interactions can vary due to genetic variation within the interacting species.     

Burial disturbance leads to facilitation among coastal dune plants

There is growing evidence that interactions among plants can be facilitative as well as competitive, but knowledge of how disturbances influence these interactions and how they vary with species diversity is lacking. We manipulated plant density, species diversity (richness), and a burial disturbance in a controlled, complete factorial experiment to test theories about the relationships among species interactions, disturbance, and richness. The hypotheses tested were 1) burial disturbance reduces plant performance at all levels of density and richness, 2) burial disturbance can cause net plant interactions to become more facilitative, and 3) facilitation increases with species richness. Burial decreased plant survival by 60% and biomass by 50%, supporting the hypothesis that burial reduces plant performance. In the control (unburied) treatment, there was no difference in proportion survival or per plant biomass between low and high density plots, meaning that neither competition nor facilitation was detected. In the buried treatment, however, high density plots had significantly greater survival and greater per plant biomass than the low density plots, indicating net facilitative interactions. Thus facilitation occurred in the buried treatment and not in the unburied control plots, supporting the hypothesis that facilitation increases with increasing disturbance severity. The hypothesis that facilitation increases with increasing species richness was not supported. Richness did not affect survival or biomass, and there was no richness by burial treatment interaction, indicating that richness did not influence the response of the community to burial. The influence of the disturbance on plant interactions was thus consistent across levels of richness, increasing the generality of the relationship between disturbance and facilitation.     

Do biotic interactions shape both sides of the humped‐back model of species richness in plant communities?

A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity-diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severity.     

Competition,facilitation and the Allee effect

Individuals of different species may interact in many different ways, such as competition, mutualism, or predation, to name but a few. Recent theory and experiments reveal that whether an interaction is beneficial or detrimental to the dynamics of a population often depends on species densities and other environmental factors. Here, we explore how, for suitable densities, facilitation may arise between two competing species with an Allee effect. We consider two different mechanisms for the Allee effect: 1) plant species with obligate insect pollination, and 2) generalist predation. In the first case, a second plant species, competing for nutrients, may have a facilitative effect by attracting more pollinators. In the second case, another potentially competing species may serve to satiate the same generalist predator and thereby have a facilitative effect. We explore three aspects of facilitation in each of the two systems. The focal species may benefit from the presence of a ‘competitor’ if it experiences 1) the removal of the Allee threshold, 2) a lowering of the Allee threshold, or 3) an increase in carrying capacity. We find that the latter two effects occur in both study systems whereas the first only occurs for the generalist predation system but not for the plant‐pollination system. We give precise conditions on when such a facilitative effect can be expected. We also demonstrate several unexpected outcomes of these two‐species interactions with multiple steady states, such as obligate co‐occurence; we draw parallels to the dynamics of species known as ‘ecosystem engineers’, and we discuss implications for conservation and management.     

Changes in Patterns of Species Co‐occurrence across Two Tropical Cloud Forests Differing in Soil Nutrients and Air Temperature

Patterns of co‐occurrence of species are increasingly used to examine the contribution of biotic interactions to community assembly. We assessed patterns of co‐occurrence at four scales, in two types of tropical cloud forests in Hainan Island, China (tropical montane evergreen forests, TMEF and tropical dwarf forests, TDF) that varied significantly in soil nutrients and temperature. We tested if the patterns of co‐occurrence changed when we sorted species into classes by abundance and diameter at breast height (dbh). Co‐occurrence differed by forest type and with plot size, with significant species aggregation observed across larger plots in TDF and patterns of species segregation observed in smaller plots in TMEF. Analyses of differential abundance and dbh classes also showed that smaller plots in TMEF tend to have negative co‐occurrence patterns, but larger plots in TDF tend to show patterns of aggregation, suggesting competitive and facilitative interactions. This underscores the scale‐dependence of the processes contributing to community assembly. Furthermore, it is consistent with predictions of the stress gradient hypothesis that facilitation will be most important in biological systems subject to abiotic stress, while competition will be more important in less abiotically stressful habitats. Our results clearly demonstrate that these two types of tropical cloud forest exhibit different co‐occurrence patterns, and that these patterns are scale‐dependent, though independent of plant abundance and size class.     

Tree invasion in managed tropical forests facilitates endemic species

Aim To determine whether different abundances of introduced species of Cinchona (Rubiaceae) affect species composition and facilitate species richness in managed tropical forests, to test whether any facilitative effects on understorey species depend on forest type, and to investigate whether facilitative effects can be attributed to the ‘substitutive facilitation model’. Location Makawao Forest Reserve on Maui, Hawai’i, USA. Methods Cinchona species (Cinchona pubescens and Cinchona calisaya) were mapped within various forest types. In three forest types (ageing Eucalyptus and Pinus plantations, and near‐natural Acacia koa forests), we analysed environmental parameters (e.g. canopy cover, litter cover, pH value and soil depth) and the species composition of Cinchona‐invaded and non‐invaded plots; data were compared based on Cinchona cover and forest types. Habitat modelling for several endemic species and tree ferns was carried out to test whether Cinchona cover is an important variable for the probability of occurrence of these endemics. Results Cinchona species have naturalized mainly in Eucalyptus and Pinus plantations and Acacia koa forests and here add an additional shrub layer. In contrast to other studies, we revealed facilitative effects of Cinchona on native species within all forest types. Species richness is about 20% higher in invaded plots than in non‐invaded plots, and these show a nearly 50% higher proportion of endemic species, including tree ferns. The proportion of endemics even increases with increasing Cinchona cover. For several endemics, Cinchona is found to be an important variable for the probability of occurrence, and the removal of Cinchona cover as an explanatory variable lowers the model fit. In addition to Cinchona, variables delineating vegetation structure and light availability have a strong effect on the model fit. Main conclusions In the structurally simplified Hawaiian forests studied, Cinchona facilitated endemic species in accordance with the ‘substitutive facilitation model’. This contrasts with the results of an earlier study in the naturally treeless Galápagos highlands, which revealed a sharp decrease in the abundance of endemics under Cinchona canopy. These results illustrate that, through the same structural change (addition of a vegetation layer), an invasive species may exert divergent effects across different ecosystem types. The facilitation of endemic understorey species by invasive tree species in managed forests leads to a dilemma in conservation but also to new perspectives for ecosystem restoration.     

Stress‐Driven Changes in the Strength of Facilitation on Tree Seedling Establishment in West African Woodlands

The strength of competitive and facilitative interactions in plant communities is expected to change along resource gradients. Contrasting theoretical models predict that with increasing abiotic stress, facilitative effects are higher, lower, or similar than those found under more productive conditions. While these predictions have been tested in stressful environments such as arid and alpine ecosystems, they have hardly been tested for more productive African woodlands. We experimentally assessed the strength of tree seedling facilitation by nurse trees in mesic and dry woodlands in Benin, West Africa. We planted seedlings of the drought‐sensitive Afzelia africana and the drought‐tolerant Khaya senegalensis under three microsite conditions (closed woodland, woodland gap, and open fields). Seedling survival was greater within woodlands compared with open fields in both the mesic and dry woodlands. The relative benefits in seedling survival were larger at the dry site, especially for the drought‐sensitive species. Nevertheless, plant interactions became neutral or negative during the dry season in the drier woodland, indicating that the net positive effects may be lost under very stressful abiotic conditions. We conclude that facilitation also occurs in the relatively more productive conditions of African woodlands. Our results underscore the role of environmental variation in space and time, and the stress tolerance of species, in explaining competitive and facilitative interactions within plant communities. Abstract in French is available at http://www.blackwell‐synergy.com/loi/btp .     

The good with the bad: when ecological restoration facilitates native and non‐native species

Organisms interact with each other along a spectrum ranging from competition to facilitation. A theme in restoration ecology is tipping the balance of these interactions to favor desired species and site conditions, exemplified by restoring fertile islands and their nurse plant effects to encourage plant recruitment. We tested the effectiveness of outplanting nursery‐grown native perennials and vertical mulching (placing dead plant material upright in soil) for stimulating annual plant recruitment in a disturbed Mojave Desert shrubland in Joshua Tree National Park, California, U.S.A. Over 9 years, differences in annual species richness and cover between interspaces and below outplants and vertical mulch varied among years, potentially via inter‐annual fluctuations in precipitation or maturation of restoration sites. In the ninth year, which was the wettest, both native and non‐native cover averaged 3× higher below outplants than in interspaces. Overall among years at the microsite scale, non‐native annual plants more consistently exploited environments provided by outplants and vertical mulch structures than did native annuals. However, these restoration structures were important for native annual diversity. At the 40‐m² plot scale, disturbed plots that received outplanting supported greater richness of native annual species than disturbed unrestored plots. By facilitating both non‐native and native plants, reestablishing fertile islands to restore dryland ecosystems is a conundrum for restoration. Treatments reducing non‐native plants may need to accompany fertile island restoration to tip the balance of facilitative plant interactions in favor of native species.     

Disentangling biotic interactions,environmental filters,and dispersal limitation as drivers of species co‐occurrence

A key focus in ecology is to search for community assembly rules. Here we compare two community modelling frameworks that integrate a combination of environmental and spatial data to identify positive and negative species associations from presence–absence matrices, and incorporate an additional comparison using joint species distribution models (JSDM). The frameworks use a dichotomous logic tree that distinguishes dispersal limitation, environmental requirements, and interspecific interactions as causes of segregated or aggregated species pairs. The first framework is based on a classical null model analysis complemented by tests of spatial arrangement and environmental characteristics of the sites occupied by the members of each species pair (Classic framework). The second framework, (SDM framework) implemented here for the first time, builds on the application of environmentally‐constrained null models (or JSDMs) to partial out the influence of the environment, and includes an analysis of the geographical configuration of species ranges to account for dispersal effects. We applied these approaches to examine plot‐level species co‐occurrence in plant communities sampled along a wide elevation gradient in the Swiss Alps. According to the frameworks, the majority of species pairs were randomly associated, and most of the non‐random positive and negative species associations could be attributed to environmental filtering and/or dispersal limitation. These patterns were partly detected also with JSDM. Biotic interactions were detected more frequently in the SDM framework, and by JSDM, than in the Classic framework. All approaches detected species aggregation more often than segregation, perhaps reflecting the important role of facilitation in stressful high‐elevation environments. Differences between the frameworks may reflect the explicit incorporation of elevational segregation in the SDM framework and the sensitivity of JSDM to the environmental data. Nevertheless, all methods have the potential to reveal general patterns of species co‐occurrence for different taxa, spatial scales, and environmental conditions.     

Ecological assembly rules in plant communities—approaches,patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta‐analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co‐occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta‐analyses suggest that non‐random co‐occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non‐random aspect of assembly in plant communities.     

Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This type of indirect facilitation impacts species coexistence and ecosystem functioning in a large array of ecosystems worldwide. It has nonetheless generally been understudied so far in comparison with the role played by direct facilitation among plants. We aimed at providing original data on indirect facilitation at the community scale to determine the extent to which indirect facilitation mediated by grazers can shape plant communities. Such experimental data are expected to contribute to refining the conceptual framework on plant–plant–herbivore interactions in stressful environments. We set up a 2‐year grazing exclusion experiment in tropical alpine peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring cushion‐forming plants (hereafter referred to as “nurse” species), in which associated plant communities develop. Fences have been set over two nurse species with different strategies to cope with grazing (direct vs. indirect defenses), which are expected to lead to different intensities of indirect facilitation for the associated communities. We collected functional traits which are known to vary according to grazing pressure (LDMC, leaf thickness, and maximum height), on both the nurse and their associated plant communities in grazed (and therefore indirect facilitation as well) and ungrazed conditions. We found that the effect of indirectly facilitated on the associated plant communities depended on the functional trait considered. Indirect facilitation decreased the effects of grazing on species relative abundance, mean LDMC, and the convergence of the maximum height distribution of the associated communities, but did not affect mean height or cover. The identity of the nurse species and grazing jointly affected the structure of the associated plant community through indirect facilitation. Our results together with the existing literature suggest that the “grazer–nurse–beneficiary” interaction module can be more complex than expected when evaluated in the field.     

A dimmer shade of pale: revealing the faint signature of local assembly processes on the structure of strongly filtered plant communities

Trait‐based ecology suggests that abiotic filtering is the main mechanism structuring the regional species pool in different subsets of habitat‐specific species. At more local spatial scales, other ecological processes may add on giving rise to complex patterns of functional diversity (FD). Understanding how assembly processes operating on the habitat‐specific species pools produce the locally observed plant assemblages is an ongoing challenge. Here, we evaluated the importance of different processes to community assembly in an alpine fellfield, assessing its effects on local plant trait FD. Using classical randomization tests and linear mixed models, we compared the observed FD with expectations from three null models that hierarchically incorporate additional assembly constraints: stochastic null models (random assembly), independence null models (each species responding individual and independently to abiotic environment), and co‐occurrence null models (species responding to environmental variation and to the presence of other species). We sampled species composition in 115 quadrats across 24 locations in the central Pyrenees (Spain) that differed in soil conditions, solar radiation and elevation. Overall, the classical randomization tests were unable to find differences between the observed and expected functional patterns, suggesting that the strong abiotic filters that sort out the flora of extreme regional environments blur any signal of other local processes. However, our approach based on linear mixed models revealed the signature of different ecological processes. In the case of seed mass and leaf thickness, observed FD significantly deviated from the expectations of the stochastic model, suggesting that fine‐scale abiotic filtering and facilitation can be behind these patterns. Our study highlights how the hierarchical incorporation of ecological additional constraints may shed light on the dim signal left by local assembly processes in alpine environments.     

植物邻体间的正相互作用

植物间的正负相互作用是构建植被群落的重要因素,也是群落生态学研究的中心内容之一。近20a来,植物间正相互作用的研究得到快速发展。综述了正相互作用的定义,不同植物群落中的直接、间接正相互作用及其发生机制,正相互作用研究的实验和模型方法,正负相互作用随胁迫梯度的变化及正相互作用对群落构建的影响。探讨了正相互作用研究前景:(1)进一步理解正负相互作用的平衡及其对群落构建的影响;(2)加深对全球变暖背景下的正相互作用的认识;(3)需把正相互作用研究同进化联系起来;(4)充分发挥正相互作用在生态系统中的推动力作用,把正相互作用应用到生态恢复中,为恢复退化生态系统服务。     

Counterbalancing effects of competition for resources and facilitation against grazing in alpine snowbed communities

Alpine snowbeds are habitats where the major limiting factors for plant growth are herbivory and a small time window for growth due to late snowmelt. Despite these limitations, snowbed vegetation usually forms a dense carpet of palatable plants due to favourable abiotic conditions for plant growth within the short growing season. These environmental characteristics make snowbeds particularly interesting to study the interplay of facilitation and competition. We hypothesised an interplay between resource competition and facilitation against herbivory. Further, we investigated whether these predicted neighbour effects were species‐specific and/or dependent on ontogeny, and whether the balance of positive and negative plant–plant interactions shifted along a snowmelt gradient. We determined the neighbour effects by means of neighbour removal experiments along the snowmelt gradient, and linear mixed model analyses. The results showed that the effects of neighbour removal were weak but generally consistent among species and snowmelt dates, and depended on whether biomass production or survival was considered. Higher total biomass and increased fruiting in removal plots indicated that plants competed for nutrients, water, and light, thereby supporting the hypothesis of prevailing competition for resources in snowbeds. However, the presence of neighbours reduced herbivory and thereby also facilitated survival. For plant growth the facilitative effects against herbivores in snowbeds counterbalanced competition for resources, leading to a weak negative net effect. Overall the neighbour effects were not species‐specific and did not change with snowmelt date. Our finding of counterbalancing effects of competition and facilitation within a plant community is of special theoretical value for species distribution models and can explain the success of models that give primary importance to abiotic factors and tend to overlook interrelations between biotic and abiotic effects on plants.     

The interplay between species' positive and negative interactions shapes the community biomass–species richness relationship

We used an individual-based spatially-explicit model to assess the role of facilitation and plant strategies in shaping the 'community biomass–species richness' relationship. Facilitation had few impacts on community's richness under both the most benign (high community biomass) and the most severe (low community biomass) environments where its intensity was weak. From medium to high environmental severity, facilitation increased community richness, because all plant strategies were facilitated. In contrast, from low to medium environmental severity facilitation decreased community richness, because only the most competitive species were facilitated, which induced a decrease in the richness of the stress-tolerant species overwhelming the increase in richness of the competitive species. Above all, our simulations show how 'strategy-dependent' interactions among species combine to shape the humped-back biomass–species richness relationship. It also demonstrates that facilitative effects might have long-term negative effects on species richness, which result is not included in current facilitation models.     

Shared environmental responses drive co‐occurrence patterns in river bird communities

Positive or negative patterns of co‐occurrence might imply an influence of biotic interactions on community structure. However, species may co‐occur simply because of shared environmental responses. Here, we apply two complementary modelling methodologies – a probabilistic model of significant pairwise associations and a hierarchical multivariate probit regression model – to 1) attribute co‐occurrence patterns in 100 river bird communities to either shared environmental responses or to other ecological mechanisms such as interaction with heterospecifics, and 2) examine the strength of evidence for four alternative models of community structure. Species co‐occurred more often than would be expected by random community assembly and the species composition of bird communities was highly structured. Co‐occurrence patterns were primarily explained by shared environmental responses; species’ responses to the environmental variables were highly divergent, with both strong positive and negative environmental correlations occurring. We found limited evidence for behaviour‐driven assemblage patterns in bird communities at a large spatial scale, although statistically significant positive associations amongst some species suggested the operation of facilitative mechanisms such as heterospecific attraction. This lends support to an environmental filtering model of community assembly as being the principle mechanism shaping river bird community structure. Consequently, species interactions may be reduced to an ancillary role in some avifaunal communities, meaning if shared environmental responses are not quantified studies of co‐occurrence may overestimate the role of species interactions in shaping community structure.