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Summary After a period of heavy emphasis on negative interactions, such as predation and competition, the past two decades have seen an explosion of literature on the role of positive interactions in ecological communities. Such positive interactions can take many forms. One possibility is that amelioration of environmental stress by plants or sessile animals enhances growth, reproduction and survival of others, but many more intricate patterns exist. Importantly such positive interactions may contribute to creating a positive feedback. For instance, biomass can lead to improved environmental conditions causing better growth and therefore leading to more biomass. A positive feedback is a necessary (but not sufficient) condition for the emergence of alternative stable states at the community scale. However, the literature on positive interactions in plant and animal communities rarely addresses this connection. Here, we address this gap, asking the question of when positive interactions may lead to alternative stable states, and hence set the stage for catastrophic transitions at tipping points in ecosystems. We argue that, although there are a number of now classical examples in the literature for which positive interactions are clearly the main actors of positive feedback loops, more empirical and theoretical research scaling up from the individual‐level interactions to the community and the ecosystem scale processes is needed to further understand under which conditions positive interactions can trigger positive feedback loops, and thereby alternative stable states. Lay Summary

Viewing facilitation through the lens of the niche concept is one way to unify conceptual and empirical advances about the role of facilitation in community ecology. We clarify conceptually and through examples from marine and terrestrial environments how facilitation can expand species’ niches and consider how these interactions can be scaled up to understand the importance of facilitation in setting a species’ geographic range. We then integrate the niche‐broadening influence of facilitation into current conceptual areas in ecology, including climate change, diversity maintenance and the relationship between diversity and ecosystem functioning. Because facilitation can influence the range of physical conditions under which a species can persist, it has the potential to mitigate the effects of climate change on species distributions. Whereas facilitation has mostly been considered as a diversity‐promoting interaction by ameliorating abiotic stresses, if facilitated species’ niches expand and become less distinct as a result of habitat amelioration, the forces that maintain diversity and promote coexistence in regions or habitats dominated by the facilitator could be reduced (i.e. the sign of the effects of facilitation on populations could be species‐specific). Finally, shifting or broadening ecological niches could alter the relationship between diversity and ecosystem functioning. A niche‐based perspective on the effects of facilitation can foster a greater mechanistic understanding of the role played by facilitation in regulating species coexistence, range shifts and ecosystem functioning in a changing world.

Positive interactions have started to gain a place as important drivers of community structure and biological diversity. Defined as non‐trophic interspecific interactions that increase the average individual fitness of one species, by definition, the presence of one plant species enhances the chances that another species co‐occur in the same place, indicating that positive interactions may determine biological diversity. However, this has been poorly explored. The majority of the studies addressing community‐level consequences of facilitation have compared the diversity of the plant assemblages growing within nurses vs. those growing outside them, reporting contrasting results among them. Nonetheless, nurses and their alternative microhabitats (open areas among nurses) are part of the same community. Thus, if nurses allow for the persistence of species that otherwise would be excluded from the community, a net increase in the species diversity at the entire community level will be generated even though nurse plants contained fewer species than open areas. Here, we conducted a bibliographic search using the ISI Web of Knowledge data base and reviewed the literature conducted on alpine plant communities where assessments of the diversity of plants growing within and outside a nurse species were available. In most cases nurse species substantially increased species richness at the community level, despite the fact that in some cases, they contained lower species numbers than open areas. Nurse species enhanced species richness more in systems with impoverished local diversity, suggesting that facilitative interactions in alpine habitats act as an insurance policy that sustains diversity under very harsh conditions.

Food security is currently considered a major global problem. However, increasing intensity of food production in agricultural systems has driven reductions in farmland biodiversity. A major challenge is to enable biodiversity conservation whilst addressing the problem of food security. Here we describe how facilitative plant–plant interactions in crop systems could be used to help strike this balance. An obvious example is that of intercropping systems, where combinations of crop species can – under some circumstances – deliver reduced inputs of agrochemicals (fertilizers, pesticides) per unit yield, with potential knock‐on benefits for biodiversity conservation. Other facilitative processes can also play a role in biodiversity conservation. Increased intraspecific crop genetic diversity can help protect crops from pests and diseases. Although overlooked in facilitation research, we argue that the mechanisms operate in a manner which is directly analogous to associational defence against herbivores, a process well recognized in the facilitation literature. As with intercropping, the benefits to nature conservation arise from reduced pesticide use per unit harvested crop. Crops may have facilitative effects on some arable weed species, particularly those that are currently considered rare in intensive farming systems. Work is in its early stages to understand the underlying mechanisms, but it appears that crops might create niche space to which some weed species are adapted. Increasing plant species diversity through niche space creation may then have cascading benefits for other components of farmland biodiversity. Our new understanding of facilitative processes arising from work on crop systems has lessons for the study of facilitative interactions in natural and semi‐natural communities. We argue that, although easier to identify and quantify in crop systems, some of these facilitative processes have to date been overlooked in studies of non‐crop systems and certainly deserve further consideration. Finally, we discuss what steps may be needed to move from our understanding of the role of facilitation to the development of new agricultural practice. In some cases the challenge may be one of the encouraging uptake of existing practices, and in others more research is needed to understand how new ecological understanding might deliver more sustainable agricultural practice.

Summary Facilitation by nurse plants is a key process involved in the organization of plant communities and maintenance of biodiversity, particularly in harsh environments. Nurse plants increase plant diversity and productivity in these ecosystems, but our knowledge on the mechanisms through which such facilitation operates is still expanding. Despite growing evidence that soil microbiota impact plant fitness and community dynamics, their role in plant facilitation has been little explored. Here, we synthesize available evidence on the effect of nurse plants on the abundance, composition and activity of soil microbial communities, and the effect of these soil communities on beneficiary plant species. Studies conducted mostly in arid and semi‐arid systems show that nurse plants promote the development of differentiated soil microbial communities characterized by a higher microbial abundance and activity, the dominance of competitive bacteria and larger mycorrhizal networks, compared to gaps and to coexisting non‐nurses. There is also evidence that differentiated soil microbiota associated with nurse plants has positive effects on the establishment, growth and fitness of beneficiary plant species, although the mechanisms involved remain unclear. We suggest that they include increased nutrient availability for plants, a better use of resources through functional complementarity in the microbial community, soil stabilization and also direct molecular signalling between soil microbes and plants that affect plant defence and plant interactions. Evidence for the role of soil microbiota as mediators of facilitation by nurse plants is growing, but there are still too few studies on which to draw generalizable conclusions. Future studies are needed to assess the effect of plant ontogeny and environmental conditions on soil microbial communities under nurse plants and other coexisting species, and to determine the microbial groups and specific mechanisms involved in facilitation by nurse plants. Lay Summary

Summary Plant–plant interactions may critically modify the impact of climate change on plant communities. However, the magnitude and even direction of potential future interactions remains highly debated, especially for water‐limited ecosystems. Predictions range from increasing facilitation to increasing competition with future aridification. The different methodologies used for assessing plant–plant interactions under changing environmental conditions may affect the outcome but they are not equally represented in the literature. Mechanistic experimental manipulations are rare compared with correlative approaches that infer future patterns from current observations along spatial climatic gradients. Here, we utilize a unique climatic gradient in combination with a large‐scale, long‐term experiment to test whether predictions about plant–plant interactions yield similar results when using experimental manipulations, spatial gradients or temporal variation. We assessed shrub–annual interactions in three different sites along a natural rainfall gradient (spatial) during 9 years of varying rainfall (temporal) and 8 years of dry and wet manipulations of ambient rainfall (experimental) that closely mimicked regional climate scenarios. The results were fundamentally different among all three approaches. Experimental water manipulations hardly altered shrub effects on annual plant communities for the assessed fitness parameters biomass and survival. Along the spatial gradient, shrub effects shifted from clearly negative to mildly facilitative towards drier sites, whereas temporal variation showed the opposite trend: more negative shrub effects in drier years. Based on our experimental approach, we conclude that shrub–annual interaction will remain similar under climate change. In contrast, the commonly applied space‐for‐time approach based on spatial gradients would have suggested increasing facilitative effects with climate change. We discuss potential mechanisms governing the differences among the three approaches. Our study highlights the critical importance of long‐term experimental manipulations for evaluating climate change impacts. Correlative approaches, for example along spatial or temporal gradients, may be misleading and overestimate the response of plant–plant interactions to climate change. Lay Summary

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. Lay Summary

The nature of the relationship between water limitation and facilitation has been one of the most contentious debates surrounding the stress‐gradient hypothesis (SGH), which states that plant‐plant interactions shift from competition to facilitation with increasing environmental stress. We take a closer look at the potential role of soil moisture in mediating plant‐plant interaction outcomes by assessing effects of climate and soil texture on plant modulation of soil moisture. Using an empirically‐parameterized soil moisture model, we simulated soil moisture dynamics beneath shrubs and in un‐vegetated coarse and fine soils for 1000 sites in the Western United States with <700 mm mean annual precipitation. This threshold reflects the transition from dryland (<600 mm precipitation) to mesic ecosystems. Positive effects of shrubs on shallow soil moisture (i.e. the difference between shrub and interspace soil moisture) decreased along the aridity gradient when long‐term average conditions were considered, contrary to expectations based on the SGH. Negative effects of shrubs on deeper soil moisture also increased with aridity. However, when extreme years were considered, positive effects of shrub on soil moisture were greatest at intermediate points along the spatial aridity gradient, consistent with a hump‐backed model of plant‐plant interactions. When viewed through time within a site, shrub effects on shallow soil moisture were positively related to precipitation, with more complex relationships exhibited in deeper soils Taken together, the results of this simulation study suggest that plant effects on soil moisture are predictable based on relatively general relationships between precipitation inputs and differential evaporation and transpiration rates between plant and interspace microsites that are largely driven by temperature. In particular, this study highlights the importance of differentiating between temporal and spatial variation in weather and climate, respectively, in determining plant effects on available soil moisture. Rather than focusing on the somewhat coarse‐scale predictions of the SGH, it may be more beneficial to explicitly incorporate plant effects on soil moisture into predictive models of plant‐plant interaction outcomes in drylands.

Facilitating effects of benefactor plants on plant species richness have been commonly tested in stressful habitats because competitive effects are assumed to predominate in more productive habitats. Here, we examine this assumption by testing whether benefactor plants can nonetheless be facilitating in competitive environments. We provide a conceptual framework describing how a trait of benefactor plants, canopy height of shrubs, can have a portfolio of facilitative effects on species richness in more competitive environments, and we provide an empirical assessment of this portfolio effect in tundra plant communities. Across tundra plant communities representing an extensive gradient in aboveground live biomass ranging from 11 to above 800 grams per m², we found that species richness exhibited a humped‐back relationship. Increasing canopy height of shrubs to a maximum height of what defines the dwarf shrub tundra, that is 40 cm, consistently and significantly increased species richness along the entire biomass gradient tested. The positive effect of shrub canopy height was not confounded with herbivore intensity, competitive interference or abiotic factors such as bedrock‐weathered mineral availability, moisture availability or temperature. However, we cannot rule out that the general presence of large mammalian herbivory may have been central to the positive effect of shrub canopy height in reducing herbivore impacts on species richness. In this study, conceptual and empirical evidence support that increasing canopy height of shrubs facilitates species richness regardless of relative abiotic stress levels within tundra ecosystems. We propose that positive interactions can play an important ecological role in systems where competitive effects are observed or assumed. For tundra plant communities where climate change is currently causing encroachment of shrub species, the effects of increasing canopy height may have unprecedented effects on plant species richness.

In facilitative interactions, the beneficiary feedback effect (BFE) has been defined as the effect of beneficiary species (facilitated species) on their benefactor. BFEs have been shown to be dependent on environmental conditions and the composition of the beneficiary community. In alpine cushion systems, BFEs are more negative with more abundant, diverse and phylogenetically aggregated communities of beneficiary species. We tested the hypothesis that the functional composition of the beneficiary communities correlates with the direction and strength of BFE received by alpine cushion benefactors and specifically that a more negative BFE would occur with increasing density of graminoids and a more positive BFE would occur with increasing density of forbs and legumes. Additionally, we predicted that the negative BFE of graminoids would increase with increasing summer aridity. We used a data base of alpine cushion communities from 30 sites throughout the world to assess the overall relationship between the composition of beneficiary communities and the total flower density of cushion benefactors, and its variation with increasing drought. Additionally, in order to assess more precisely the role of the functional composition of the beneficiary communities on BFE in a very dry site with cushion benefactors exhibiting contrasting functional compositions of beneficiary communities, we also designed a field study in the Qilian Shan mountain range (China). At this site with a highly continental climate, we compared the number of flowers and fruits of different phenotypes of the alpine cushion species Thylacospermum caespitosum hosting numerous graminoids, numerous forbs or very few beneficiary species. In the intercontinental study, we found a negative relationship between graminoids and cushion benefactor flower density but no effect of other functional groups. The negative BFE of graminoids increased with increasing summer drought. In the dry Qilian Shan range, we found both a negative effect of graminoids on total flower density and a positive effect of forbs on flower density and fruit set. Our study indicates that the context dependence of BFE may be partially explained by the composition of beneficiary communities and in particular the negative effect of graminoids.