Benefactor Species Research Articles

Long‐term spatially‐replicated data show no physical cost to a benefactor species in a facilitative plant–plant interaction

Facilitation is an interaction where one species (the benefactor) positively impacts another (the beneficiary). However, the reciprocal effects of beneficiaries on their benefactors are typically only documented using short‐term datasets. We use Azorella selago, a cushion plant species and benefactor, and a co‐occurring grass species, Agrostis magellanica, on sub‐Antarctic Marion Island, comparing cushion plants and the grasses growing on them over a 13‐year period using a correlative approach. We additionally compare the feedback effect of A. magellanica on A. selago identified using our long‐term dataset with data collected from a single time period. We hypothesized that A. selago size and vitality would be negatively affected by A. magellanica cover and that the effect of A. magellanica on A. selago would become more negative with increasing beneficiary cover and abiotic‐severity, due to, e.g. more intense competition for resources. We additionally hypothesized that A. magellanica cover would increase more on cushion plants with greater dead stem cover, since dead stems do not inhibit grass colonization or growth. The relationship between A. magellanica cover and A. selago size and vitality was not significant in the long‐term dataset, and the feedback effect of A. magellanica on A. selago did not vary significantly with altitude or aspect; however, data from a single time period did not consistently identify this same lack of correlation. Moreover, A. selago dead stem cover was not significantly related to an increase in A. magellanica cover over the long term; however, we observed contrasting results from short‐term datasets. Long‐term datasets may, therefore, be more robust (and practical) for assessing beneficiary feedback effects than conventional approaches, particularly when benefactors are slow‐growing. For the first time using a long‐term dataset, we show a lack of physical cost to a benefactor species in a facilitative interaction, in contrast to the majority of short‐term studies.

The stress gradient hypothesis explains plant-plant interaction networks in edapho climatic gradients

The stress-gradient hypothesis (SGH) predicts a shift from facilitation to competition as the environment turns from severe to favorable. Different authors have reported conflicting results when testing the SGH, mainly concerning the role of facilitation in stressful environments. We aimed to infer the mechanisms best explaining the variation of interspecific interactions across gradients through the joint analysis of network metrics and abiotic variables. We analyzed six plant-plant interaction networks (size, number of links, connectance, average degree, nestedness, and modularity) in edapho-climatic gradients in northeastern Brazil. Nestedness occurs when many species connect both with each other and with a set of other species with few connections, indicating facilitation. Modularity occurs when the network is split into groups of species linking to each other only within the groups, indicating competition. We used correlation, multiple linear regressions, and a generalized linear model to understand how the abiotic variables could drive network alterations and if the abundance of a possible benefactor species (Fabaceae trees) could increase facilitation. All the climate, soil, and network variables varied widely across the sites. All the sites had aluminium-toxic, nutrient-poor and nitrogen-limited soils, and Holdridge's life zones varied from very dry to moist tropical forests. Climate, soil, and network variables were correlated to each other, but the soil was the main driver of variation in the network metrics. Nestedness and modularity were negatively correlated, and both were correlated with the number of Fabaceae trees in each community. Deciduous species in very dry and dry forests require more nitrogen than evergreen species in dry/moist and moist forests, so the soil was more limiting to the deciduous species, which associated with Fabaceae trees and increased network nestedness. Our data corroborated the SGH, which, in this case, was mediated by the soil nutrient status and had Fabaceae as the benefactor species.

The importance of facilitation on community assembly disappears under severe drought stress

Abstract Despite the recognition of positive interactions as an important driver of species coexistence and community structure, the underlying mechanism of how facilitation affects assembly processes along stress gradients is poorly explored. Understanding the responses of functional diversity to benefactor species at the extreme end of the stress gradient could provide valuable insight about facilitation‐involved assembly mechanisms and contribute to the predictions of species coexistence under climate change. In the drought‐stressed community in the Badain‐Jaran Desert, the responses of the local community to the nurse shrub species Calligonum mongolicum Turcz. were evaluated using hierarchical Bayesian models. For the 3‐year experiment, summer rainfall in each year formed a natural gradient of drought stress. To evaluate the shrub's effects on the assembly process along that gradient, individual samples were collected in pairwise under‐shrub and open habitats, and four traits related to stress tolerance and resource acquisition were measured simultaneously. Under moderate drought stress, we observed shifting community‐weighted means, broadening ranges and reducing overlaps of functional traits under shrubs. These effects were partly driven by a distinct microenvironment created by shrub plants, in particular the improvement and heterogeneity of soil moisture conditions. However, this influence on trait distributions was strongly dependent on the environmental context, and generally disappeared as drought stress shifted towards its driest end, almost in line with the decreased positive interaction assessed by plant density and species diversity. This study focused on water‐limited community that lies at the driest end of drought gradient and confirmed that facilitation can drive the assembly process through both environmental filtering and niche differentiation. More importantly, these assembly mechanisms are proven to become less efficient under extreme drought stress, which may suggest the occasionally disappearing role of benefactor plants on community assembly and an increasing risk of biodiversity loss in the context of climate change. Read the free Plain Language Summary for this article on the Journal blog.

Managing biotic interactions during early seagrass life stages to improve seed‐based restoration

Abstract Seagrasses are declining globally, and effective restoration actions to promote the recovery of degraded meadows are urgently needed. Harnessing positive plant interactions during early life stages is considered a valuable strategy to improve terrestrial and coastal habitat restoration. Yet, its application to seagrass restoration is still in infancy, and very little is known on the role played by biotic interactions in shaping newly established populations. We assessed the feasibility of manipulating intraspecific and interspecific plant interactions to enhance seed‐based restoration success using the seagrass Posidonia oceanica as a model. Specifically, we investigated in mesocosm whether seed germination increased with increasing seed density and whether increasing seedling density and planting a pioneer seagrass promoted facilitation among seedlings. To do this, seedlings were grown either as a single individual or in clumps of medium and high density, with and without Cymodocea nodosa, for 2 years encompassing their most critical growth stage. Germination of P. oceanica seeds was not affected by seed density. Posidonia oceanica seedlings planted at medium and high density showed higher survival than those planted individually but only in the presence of C. nodosa. Seedlings planted at medium and at high density with C. nodosa performed better than those grown at low density or without C. nodosa due to a positive joint effect of intraspecific and interspecific interaction. Cymodocea nodosa plants grown with P. oceanica seedlings at high density were larger than those grown alone, indicating a mutualistic relationship. Synthesis and applications. Our results show that in seagrasses positive interactions during early life stages can be promoted by planting seedlings in dense clumps in association with an early successional species. The incorporation of this novel nature‐based approach in seed‐based restoration could accelerate the recolonization of degraded seagrass habitats. In planning future seed‐based restoration interventions, managers should assess both intraspecific and interspecific interactions established by seedlings of target species for identifying proper planting density/spatial configuration and potential benefactor species promoting facilitative mechanisms to maximize seedling planting success.

Interspecific facilitation mediates the outcome of intraspecific interactions across an elevational gradient.

Where interspecific facilitation favors the establishment of high densities of a beneficiary species, strong intraspecific competition may subsequently impede beneficiary performance. Consequently, the negative influence of intraspecific competition between beneficiary individuals could potentially outweigh the positive influence of interspecific facilitation when, for example, higher densities of a beneficiary are negated by the negative effect of crowding on beneficiary reproduction. The aim of this study was, therefore, to examine the impact of an interspecific interaction on the outcome of intraspecific interactions within the context of plant-plant facilitation. We used the cushion-forming Azorella selago and a commonly co-occurring dominant perennial grass species, Agrostis magellanica, on sub-Antarctic Marion Island as a model system. We assessed the impact of an interspecific interaction (between A. selago and A. magellanica) on the outcome of intraspecific interactions (between A. magellanica individuals), by testing if the impact of A. magellanica density on A. magellanica performance is mediated by its interaction with A. selago. We observed evidence for competition among A. magellanica conspecifics, with a decreasing proportion of A. magellanica individuals being reproductive under higher conspecific density. This negative intraspecific effect was greater on A. selago than on the adjacent substrate, suggesting that the facilitative effect of A. selago changes the intensity of intraspecific interactions between A. magellanica individuals. However, experimentally reducing A. magellanica density did not affect the species' performance. We also observed that the effect of A. selago on A. magellanica was positive, and despite the negative effect of intraspecific density on the proportion of reproductive A. magellanica individuals, the net reproductive effort of A. magellanica (i.e., the density of reproductive individuals) was significantly greater on A. selago than on the adjacent substrate. These results highlight that, in abiotically severe environments, the positive effects of interspecific facilitation by a benefactor species may outweigh the negative effects of intraspecific competition among beneficiaries. More broadly, these results suggest that both positive inter- and intraspecific biotic interactions may be key to consider when examining spatial and temporal variation in species' performance.

Testing the role of functional trait expression in plant–plant facilitation

AbstractPositive biotic interactions between plant species may strongly affect species and community‐level patterns, but the processes through which benefactor species alter the performance of interacting species (via, e.g. beneficial mechanisms like resource provisioning) are still inadequately understood. One poorly explored potential explanation is that plant–plant facilitation could occur through the impact of benefactor species on the functional trait expression of beneficiary species. Indeed, plant species that affect local conditions can modify functional trait expression of interacting species, thereby improving their performance and resulting in a facilitative interaction. However, the response of intraspecific trait variation to biotically driven microhabitat modification, and its role in determining the outcome of plant–plant interactions, has rarely been explored.Here, we test whether growing with benefactor species affects the expression of functional traits of eight species, encompassing different plant growth forms, in two contrasting study systems. This is achieved using a paired sampling approach to compare values of seven functional traits of conspecific individuals growing within and adjacent to cushion plants (i.e. benefactor species which are known to strongly alter microhabitat conditions and to have positive effects on some of the focal species). In addition, we test whether the effect of biotic interactions on functional trait expression changes along elevational gradients, as the outcome of biotic interactions is expected to vary with elevation.Contrary to predictions, in both systems, intraspecific trait variation was not well explained by the biotic interaction with the cushion plant species or the variation in abiotic conditions associated with elevational gradients. Where biotic interactions did affect functional trait expression and bivariate trait relationships, traits responded variably between species, suggesting that context specificity may be a constraint to predicting how intraspecific trait variation responds to plant–plant interactions, adding to the growing body of literature that challenges the generality and predictability of the drivers of intraspecific trait variation.This research, therefore, suggests that benefactor species’ facilitative process is likely not through an impact on intraspecific trait expression, and that instead other processes may be more important for translating beneficial microhabitat modification or increased resource availability by benefactor species into positive impacts on beneficiary species.A freePlain Language Summarycan be found within the Supporting Information of this article.

Community‐scale effects and strain: Facilitation beyond conspicuous patterns

Abstract The large majority of ‘facilitation studies’ focus on benefactor–beneficiary interactions. However, this may skew our view of the role of facilitation toward systems where it is most conspicuous, and perhaps toward harsh environments where discrete vegetation cover and pattern suggestive of facilitation are more common. Here we argue that a particular benefactor species is not required for facilitation to occur. Where vegetation cover is continuous, the community as a whole can ameliorate the abiotic conditions (community‐scale facilitative effect) and any species from the assembly experiencing stress (strain) could be facilitated. As a consequence, community‐scale facilitative effect might be generally common but often inconspicuous. Whether this community‐scale facilitative effect is influenced by particular characteristics of the vegetation cover is still largely unknown (e.g. biomass, species richness, functional composition and diversity). Expanding our research on facilitation beyond benefactor–beneficiary interactions and considering together the concept of community‐scale effect and strain might be key to progress our understanding of the general role of facilitation for species coexistence, ecosystem function, species distribution or climate change impact.

Resource conversion: a generalizable mechanism for resource‐mediated positive species interactions

Positive species interactions are ubiquitous in natural communities, but the mechanisms through which they operate are poorly understood. One proposed mechanism is resource conversion – the conversion by a benefactor species of a resource from a resource state that is inaccessible to a potential beneficiary species into a resource state that is accessible. Such conversion often occurs as a byproduct of resource consumption, and sometimes in exchange for non‐resource benefits to the benefactor species. At least five known classes of interactions, including both facilitative and mutualistic ones, may be classified as resource conversion interactions. We formulated a generalizable mathematical model for resource conversion interactions and examined two model variants that represent processing chain and nurse plant interactions. We examined the conditions under which these conformed to the stress‐gradient hypothesis (SGH), which predicts increased interaction benefits in more stressful environments. These yielded four key insights: 1) resource conversion interactions can be positive (towards the resource recipient) only when facilitator‐mediated resource conversion is more efficient than the baseline, spontaneous, facilitator‐independent resource conversion; 2) the sign of resource conversion interaction outcomes never switches (e.g. from net positive to net negative) with changing levels of resource availability, when all other parameters are kept constant; 3) processing chain interactions at equilibrium can never be positive in a manner that conforms to the SGH; 4) nurse plant interactions can be positive and conform to the SGH, although the manner in which they do depends largely on how resource stress is defined, and the environmental supply rate of surface soil moisture. The first two insights are likely to be generalizable across all resource conversion interactions. The general agreement of the model with empirical studies suggest that resource conversion is the mechanism underlying the aforementioned interactions, and an ecologically meaningful way of classifying these previously unassociated positive species interactions.

The shape is more important than we ever thought: Plant to plant interactions in a high mountain community

Abstract Plant to plant interactions are probably the most important driver of species coexistence at fine spatial scales, but their detection represents a challenge in Ecology. Spatial point pattern analysis (SPPA) is likely the approach most used to identify them, however, it suffers from some limitations related to the over‐simplification of individuals to points. Here, we propose a new approach called Overlapping Area Analysis (OAA) to test whether the consideration of the shape and orientation of the individuals reveal signs of interactions between species that would remain undetected with SPPA. We used this approach to analyse a fully mapped cryophilic grassland in the Sierra de Guadarrama National Park (Spain), where the crown of each individual plant (i.e. the species canopy) was approximated by a polygon. We then computed and compared the total overlapping area between the canopy of a focal species and that of any other species in the community with the expectations of a null model of random rotation of each plant around its centroid. We complemented the results of our new approach by comparing with that of SPPA of plants’ centroids. Results of OAA showed that up to 41% of species pairs had less canopy overlap than expected, suggesting that many interspecific canopy associations in this plant community were significantly negative at the finest spatial scale. Contrarily, SPPA estimated that 12% of species pairs were positively associated at spatial scales up to 20 cm, confirming the facilitative effect displayed by the main engineer in the community (Festuca curvifolia Lag.) and by some other dominant species. Our new approach quantifying canopy associations provides new insights into the processes guiding community assembly. While the results of SPPA suggested the prevalence of traditional ‘stress gradient hypothesis’ (i.e. prevalence of positive interactions under stressful abiotic conditions), OAA revealed that many interspecific canopy associations were significantly negative. Overall, most facilitated species optimized this positive effect by placing their centroids as close to the benefactor species as their foraging behaviour allowed while avoiding crown overlap. The method proposed is available in a dedicated r‐package that will facilitate its application by other ecologists.

Density-Dependent Plant-Plant Interactions Triggered by Grazing.

Plant species performance in rangelands highly depends on the effect of grazing and also on the occurrence of unpalatable benefactor species that can act as biotic refuges protecting neighboring plants from herbivores. The balance between facilitation and competition may changes with the benefactor density. Despite the high number of studies on the role of biotic refuges, the density dependent effects of unpalatable herbaceous plants on the performance of other species, and on the habitat heterogeneity of rangelands are still unclear. Therefore, we performed a study to test the following hypotheses: (i) Performances of understory species follow a humped-back relationship along the density gradient of the unpalatable benefactor species. (ii) Small-scale heterogeneity of the vegetation decreases with increasing benefactor density. We studied meadow steppes with medium intensity cattle grazing in Hungary. We surveyed understory species’ performance (number of flowering shoots and cover scores) along the density gradient of a common, native unpalatable species (Althaea officinalis). Our findings supported both hypotheses. We found unimodal relationship between the benefactor cover and both the flowering success and richness of understory species. Moreover, small-scale heterogeneity declined with increasing benefactor cover. In this study we detected a humped-back pattern of facilitation along the density gradient of an herbaceous benefactor in pastures. Indeed, this pattern was predictable based on such conceptual models like “consumer pressure-abiotic stress model,” “humped-back model,” “intermediate disturbance hypothesis,” and “disturbance heterogeneity model”; but until now the validity of these relationships has not been demonstrated for herbaceous species. By the demonstration of this effect between herbaceous species we can better forecast the responses of grasslands to changes in management.

Nurse plants and the regeneration niche of tree seedlings in wood-pastures from Western and North-Western Romania

Facilitation is a positive interaction demonstrated to be one of the important factors shaping the regeneration niche of trees, mostly under stressful conditions which is currently studied in the frame of complex ecological networks. The protection provided by benefactor plants for tree seedlings playing the role of beneficiaries is documented mainly in arid and semi-arid habitats or in situations where herbivores’ pressure constitutes the main stressful factor for tree regeneration. One of the iconic Transylvanian landscapes is the wood-pasture, also one of the oldest agro-forestry systems to which recent forest expansion in abandoned agricultural fields or pastures is added. The proposed work represents a preliminary investigation on the association between benefactor plants, mostly spiny shrubs (Rosa canina, Crataegus monogyna, Prunus spinosa as the most frequently encountered benefactors) and tree seedlings (Quercus spp., Tilia spp., Carpinus betulus, Fraxinus angustifolia, Pyrus pyraster as most frequently encountered beneficiaries), in four different locations from North-Western and Western Romania, wood-pastures, abandoned pastures and abandoned agricultural fields under the consideration that the main stressful factor is represented by livestock grazing. Bipartite, qualitative merged network was generated depicting the interaction between beneficiaries and benefactors. Commonly used metrics were calculated: connectivity, nestedness, modularity, betweenness centrality and centralization compared to similar facilitation networks presented in the literature. Facilitation network is characterized by high nestedness (N=0.896), lack of modularity, relatively high connectance (C=0.233), features encountered in mutualistic networks also. Betweenness centrality scores highlighted the keystone benefactor and beneficiary species, while betweenness centralization score (0.192) indicates the fact that there are several species sharing the dominant position in terms of interactions. The analysis of measurement data (seedlings’ and benefactor plants’ heights, distance from focal seedlings to nearest benefactor species and orientation) showed that there is common pattern in orientation (most of the benefactor species oriented toward South or South West) also in dimensional variability (MANOVA results).

Geographic location, local environment, and individual size mediate the effects of climate warming and neighbors on a benefactor plant.

Predictions of plant responses to global warming frequently ignore biotic interactions and intraspecific variation across geographical ranges. Benefactor species play an important role in plant communities by protecting other taxa from harsh environments, but the combined effects of warming and beneficiary species on their performance have been largely unexamined. We analyzed the joint effects of elevated temperature and neighbor removal on the benefactor plant Silene acaulis, in factorial experiments near its low- and high-latitude range limits in Europe. We recorded growth, probability of reproduction and fruit set during 3 years. The effects of enhanced temperature were positive near the northern limit and negative in the south for some performance measures. This pattern was stronger in the presence of neighbors, possibly due to differential thermal tolerances between S. acaulis and beneficiary species in each location. Neighbors generally had a negative or null impact on S. acaulis, in agreement with previous reviews of overall effects of plant-plant interactions on benefactors. However, small S. acaulis individuals in the north showed higher growth when surrounded by neighbors. Finally, the local habitat within each location influenced some effects of experimental treatments. Overall, we show that plant responses to rising temperatures may strongly depend on their position within the geographic range, and on species interactions. Our results also highlight the need to consider features of the interacting taxa, such as whether they are benefactor species, as well as local-scale environmental variation, to predict the joint effects of global warming and biotic interactions on species and communities.

Females engage in stronger relationships: positive and negative effects of shrubs are more intense for Poa ligularis females than for males.

Dioecious plants are of particular concern in view of global environmental changes because reproductive females are more sensitive to abiotic stresses, thus compromising population viability. Positive interactions with other plants may counteract the direct effects of any abiotic environmental stress, allowing them to thrive and maintain a viable population in suboptimal habitats, although this process has not been tested for dioecious species. Furthermore, almost no data are available on the outcome of such species interactions and their link with local spatial patterns and sex ratios. We set up a field experiment with Poa ligularis, a dioecious native grass from the arid grasslands of South America. We studied the interaction of male and female plants with cushion shrubs of contrasting ecological strategies. We experimentally limited direct shrub-grass competition for soil moisture and transplanted plants to evaluate the amelioration of abiotic stress by shrub canopies (i.e. sun and wind) on grasses. We also studied the distribution of naturally established female and male plants to infer process-pattern relationships. Positive canopy effects as well as negative below-ground effects were more intense for females than for males. Deep-rooted Mulinum spinosum shrubs strongly facilitated survival, growth and reproduction of P. ligularis females. Naturally established female plants tended to distribute more closely to Mulinum than co-occurring males. Female growth suffered intensive negative root competition from the shallow-rooted Senecio filaginoides shrub. Interactions with other plants may reduce or enhance the effect of abiotic stresses on the seemingly maladapted sex to arid environments. We found that these biased interactions are evident in the current organization of sexes in the field, confirming our experimental findings. Therefore, indirect effects of climate change on population sex ratios may be expected if benefactor species abundances are differentially affected.

Modelling the niche space of desert annuals needs to include positive interactions

The niche is a necessary consideration when estimating habitable area and geographic range of a species. Modellers often examine the fundamental niche and the environmental requirements for plant species, ignoring interactions among species. In deserts, positive plant interactions are important drivers of biodiversity and structure communities through many mechanistic pathways including modifying environmental conditions. Thus, we tested the hypothesis that desert shrubs increase the geographical extent of some annual species because, through modifying the microclimate, they match the niche requirements of beneficiary species. We used the database of the Global Biodiversity Information Facility to construct MaxEnt species distribution models (SDM) with and without reported benefactor species within the Mojave Desert in California. We chose 20 annual species to be modeled including 10 species that had been previously reported in the literature as being facilitated (beneficiary) and 10 that had no record of being facilitated (unreported). Beneficiary annuals co‐occurred significantly more with benefactor shrubs than the unreported annual species. The inclusion of shrubs into SDMs significantly improved model predictability and geographic range for all the beneficiary annual species, but not for the unreported annual species. Thus, positive interactions are species specific and it is possible to determine annual species dependency on benefactor shrubs at the regional scale. The co‐occurrence of benefactor shrubs and annual species can be used as a proxy for facilitation and recent developments in SDM techniques encourage the inclusion of biotic interactions. Species distribution models should include estimates of facilitation because biotic interactions determine the niche of species and can have implications with a changing climate.

A cost–benefit model for plant–plant interactions: a density-series tool to detect facilitation

Generally, only the net outcome of plant–plant interactions is measured in population and community ecology research, with few attempts to determine the relative importance of negative (competition) and positive (facilitation) interactions between subordinate species. Changes in the intensity of interactions along gradients, between life-stages, or with changing densities, and the use of selective removals enhance our capacity to infer positive and negative interactions. However, the most powerful examples at least in detecting facilitation typically involve measuring changes with or without a nurse-plant or benefactor species and often involve only a very limited numbers of species. In plant competition studies, however, greater number of species are commonly tested and density-dependent series are not an uncommon tool to test for net negative interactions. Here, we develop a cost–benefit model that can be used to comprehensively calculate the average expected net gain per individual at every point in a density series provided several response variables are recorded at each density. The utility of this model is demonstrated using both hypothetical data and several empirical data sets, and it is used to infer either both positive and negative net effects. Expected net gain can also serve as an accurate estimate of mean fitness per individual at a given density provided appropriate performance measures were recorded within the primary study. Within a single density series, both facilitation and competition can occur and were detectable using this method. This approach emphasizes the current view that both negative and positive interactions play a role in shaping plant communities. Furthermore, it is evident that facilitation can be detected using the manipulative density series typically associated with competition studies and not just using the typical target nurse-plant methodology. Finally, this method is a significant advance over the current practice of tallying up single responses within a study to estimate outcomes by providing a single, synthetic measure of the net gain or cost of interactions.

Functional assessment of animal interactions with shrub‐facilitation complexes: a formal synthesis and conceptual framework

Summary Facilitation studies focus primarily on plants often neglecting the extended effects that cascade through ecological networks. Plants interact with other organisms through consumptive effects and a myriad of non‐trophic effects such as habitat amelioration or pollination. Shrubs are a dominant benefactor species frequent in plant‐facilitation studies but can also have direct and indirect interactions with animals. Herein, we use a systematic review to address the following two objectives: (i) to propose a conceptual framework that explores these interactions including the functional roles of the interacting species, and (ii) to quantitatively summarize the current state of this field examining effects beyond plant–plant interactions. To date, a relatively limited number of studies have examined the importance of coupled benefactor‐subordinate plant positive interactions with animals (79 studies in total). From this set of studies, 36 studies documented positive plant interactions generating a total of 53 independent instances of either shrub–plant–animal or shrub–animal–plant interactions. These interaction pathways were evenly split between direct (49%) and indirect (51%) interactions of shrubs with animals. Hypotheses frequently tested included seed trapping, herbivore protection, magnet pollination and facilitation‐mediated secondary seed dispersal. The most common functional role of shrubs was protection from herbivory, and the most common animal role associated with plant‐facilitation complexes was that of a consumer. None of these studies explored bidirectional plant–animal interactions, used a network approach to describe the interaction sets, nor contrasted interaction strengths. Multitrophic, integrated sets of experiments incorporating plant facilitation into community dynamics are thus critical in advancing management of high‐stress ecosystems wherein positive interactions are commonly reported.

Ubiquitous germination among common perennial species in response to facilitated and unfacilitated microhabitats

We present results from a study that examines species-specific facilitation of germination in an arid perennial community. This system suffered 64% adult mortality and the local extinction of two common species as a consequence of drought that occurred between 1999 and 2004. Ambrosia dumosa, Larrea tridentata, and Tetracoccus hallii are included as candidate benefactor species based on their high abundance and broad distribution at the study site. Using complementary field and greenhouse experiments, we measured the effects of facilitation, light and nitrogen on germination of four species common to the region, L. tridentata, A. dumosa, Sphaeralcea ambigua and Eriogonum fasciculatum.Our results show that benefactor species do not uniquely influence germination. Species-specific effects among beneficiaries largely explained differences in percent germination in the field, and the interaction between beneficiary species and microhabitat was significant for only one species. E. fasciculatum consistently showed significantly higher germination than more abundant species and showed reduced germination in the interspace. In the greenhouse, germination responses differed at each light level, beneficiaries responding most uniquely at full light. These results refine expectations from previous studies of facilitation at this site by showing that facilitation does not strongly enhance germination.

Soil nutrient availability determines the facilitative effects of cushion plants on other plant species at high elevations in the south-eastern Himalayas

Background: Cushions are the typical nurse species of high-elevation environments. However, few studies have explored the differences in facilitative power and environmental amelioration between nurse plants of different species that share a common cushion morphology.Aims: To compare the nurse effects of different alpine cushion plants and their mechanism.Methods: We compared the effects of two co-occurring cushion species, Potentilla articulata and Arenaria polytrichoides on species richness, Shannon–Wiener diversity and the evenness of vascular plant assemblages in the Himalayas. In addition, we compared the temperature and the soil nutrients within and outside the two cushion species.Results: The presence of either cushion species significantly increased the species richness and diversity in comparison with the areas outside the cushions; P. articulata cushions were associated with greater species richness than A. polytrichoides. Substrate temperatures were similar under the two species and in open areas, but nitrogen and potassium levels were higher under either of the two cushion species than they were outside. Soil phosphorus and potassium concentrations were significantly higher beneath P. articulata than beneath the A. polytrichoides cushions.Conclusions: Our results indicate that facilitation of local species richness by cushion plants contributes to the structure of the alpine plant community in the eastern Himalayas. The intensity of the nurse effect varies with the soil nutrient level beneath different benefactor species.

A role for below‐ground biota in plant–plant facilitation

Summary Plant–plant facilitation is an important driver of plant diversity, which in turn maintains ecosystem multifunctionality and can buffer some negative effects of climate change. Facilitation is classically attributed to the amelioration of environmental extremes and resource availability. Integrating below‐ground biota into the positive plant interactions framework should provide a more realistic understanding of this process, enabling us to gain insights into the dynamics of below–above‐ground communities. We tested the effect of below‐ground biota on the performance of a plant community and of individual species using soil extracts from the understorey of a benefactor plant species and adjacent open spaces. Soil bacteria from extracts and experimental microcosms were analysed using pyrosequencing. Soil biota had a significant effect on the abundance, growth, functional traits and reproductive output of beneficiary plant species through processes that are independent of the direct influence of the benefactor species. Different soil bacterial communities were associated with the benefactor species, the individual beneficiary plant species and the plant community, revealing complex below–above‐ground links between plants and soil microbiota. Synthesis. The below‐ground biota cultivated by benefactor plant species play a fundamental role in positive interactions between plant species contributing to the preservation of diversity and the evolution of plant communities.

Differential facilitative and competitive effects of a dominant macrophyte in grazed subtropical wetlands

Summary 1. Plant–plant interactions fluctuate between competition and facilitation depending upon ecological conditions and species traits. Facilitative interactions are expected to increase in frequency via associational defences with increasing consumer pressure. The ability of species to cope with competition and ⁄ or ecological stressors may alter the outcome of plant–plant interactions. 2. We conducted a transplant experiment to determine if native and non-native grasses and forbs respond similarly to interactions with Juncus effusus L., an unpalatable benefactor species, along a grazing intensity gradient in two contrasting pasture types: intensively managed and semi-natural. We expected competitive taller, erect species (grasses) and non-natives to obtain stronger facilitative effects than shorter, sprawling species (forbs) and natives. 3. We transplanted two species each of grasses and forbs, including one native and one non-native species of each, into 1-m 2 grazed plots or exclosure plots either with or without Juncus in wetlands experiencing a range of grazing intensities across the two pasture types. 4. Interactions with Juncus varied from facilitation in grazed plots to competition in exclosures for the two grasses and the non-native forb. Juncus did not facilitate the native forb in grazed conditions. Grazing negatively influenced all species. For the grasses, facilitation intensity increased as grazing intensity increased, while facilitation intensity for forbs was unrelated to grazing intensity. 5. Synthesis. Facilitation intensity of a dominant macrophyte increased with consumer pressure for tall grasses in productive subtropical wetlands. These findings suggest that facilitation can allow native and non-native species to pass through human-induced ecological filters, such as cattle grazing, with implications for understanding forces governing community composition in disturbed environments.