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Mutualisms are important structuring forces in ecological communities, influencing ecosystem functions, diversity, and evolutionary trajectories. New interactions, particularly between native and non-native species, are globally increasing in biotic communities as species introductions accelerate. Positive interactions such as novel mutualisms can affect the fitness of organisms in invaded communities. Non-natives can augment native mutualism networks, replace extinct native partners, or disrupt native mutualisms. Because they are actively forming or newly formed, novel mutualisms offer a unique opportunity to examine in real time the factors governing early mutualism formation and stability, including frequency-dependent processes and those relying on specific traits or functions. These central ecological questions have been inferred from long-formed mutualisms, but novel mutualisms may allow a glimpse of successes and failures in ecological time with insights into the relative importance of these factors as ecological systems shift. To this end, this commentary addresses how novel mutualisms inform our understanding of mutualism formation, stability, the importance of functional traits, and niche vs. neutral processes, using examples across multiple systems. Novel mutualism research thus far has been largely limited in both questions and ecosystems, but if more broadly applied could benefit both theoretical and applied ecology. [ABSTRACT FROM AUTHOR]

Question The biotic resistance hypothesis states that species diversity is a barrier for plant biological invasions because alien species are more likely to find strong competitors as diversity of native plants increases. In stressful environments, however, these negative relationships could be diluted because increased physical harshness might reduce the importance of competition as a structuring force in plant communities. Nevertheless, if nurse plants facilitate other species and create more diverse vegetation patches than those found in their surroundings, the performance of invaders should be negatively related to diversity within these patches. Conversely, diversity would not regulate invasibility in the surroundings of nurses because these habitats are subjected to higher levels of environmental harshness. Location A high-Andean landscape of central Chile, dominated by cushion plants that facilitate native species and field chickweed ( Cerastium arvense), an invasive exotic species. Methods To test our hypotheses we used observational data taken in two consecutive growing seasons by sampling plant assemblages within and outside cushion-protected patches. Additionally, a field experiment with artificially assembled plant communities was conducted to assess why the abundance and performance of the invasive species was related to native plant diversity. Results Contrary to our expectations, observational data indicated that the abundance of the invader species was positively related to native plant diversity within cushion patches, while negative relationships were found in the surrounding habitat. Similarly, the field experiment indicated that the performance of C. arvense increased with plant diversity within cushion patches but, in this case, neutral relationships were found in the surrounding open areas. Conclusions Our findings indicate that nurse plants can modulate invasibility-diversity relationships in harsh environments. Overall, the results suggest that the biotic resistance hypothesis may fail to predict the direction of these relationships within nurse-protected patches. Instead, these results suggest that the alternative biotic acceptance hypothesis could operate at small spatial scales. [ABSTRACT FROM AUTHOR]

Facilitation has been proposed to be a fundamental mechanism for plant coexistence, being particularly important in highly stressful environments such as alpine environments. In this type of environment, species called “cushion plants” can ameliorate the stressful conditions, acting as nurses for other plants. Of the several mechanisms proposed in the positive-interactions framework, plant–microorganism interaction may be one of the most common, but least documented. Here we show that the presence of endophytes isolated from the roots of cushion plants Laretia acaulis can play a fundamental role in the establishment, performance and survival of both native and exotic plant seedlings that are known to be facilitated by the cushion species. To test this, we measured survival and growth of two native and one invasive species at 3200 m in the Andes of Central Chile. Plants were grown inside artificial cushions filled with native soil, with or without sterilization, and with or without fungal endophytic inoculation to evaluate the role of fungal endophytes on survival and growth. In addition, we conducted a second experiment in a greenhouse with the invasive species to evaluate the effect of fungal endophytic infection/association on plant ecophysiological performance, dry biomass and seed output. Overall, our results showed a strong positive effect of fungal endophytes on the survival and growth of both native and invasive species. Moreover, maximum quantum efficiency (Fv/Fm), biomass accumulation and seed production were enhanced in the invasive species when soils were inoculated with endophytes. Thus, facilitation by root endophytic fungi on native and invasive alpine plants could determine survival and establishment in this harsh environment. Several studies have shown that direct facilitation by cushion plants in alpine environments improves the performance and fitness of both native and exotic plants. Our results suggest that there are indirect effects, mediated by microorganism associations that may also help to explain the successful establishment of native and invasive species in these environments. If indirect plant–plant facilitation through root fungal endophytes proves to be a widespread phenomenon in alpine ecosystems, it could be a key component in the structuring of plant communities in those stressful environments. [ABSTRACT FROM AUTHOR]

Aim The drivers of tree recruitment over large spatial scales remain unexplored. Here, we ask whether species potential for recruitment and the strength of density-dependent processes, both inferred from species relative abundances, show emerging patterns that can be explained upon the basis of information about climate and functional traits. Location Eastern forests of the USA. Methods We document the geographical distributions and magnitudes of seedling recruitment and the strength of density dependence and conspecific density dependence for the forests of the eastern USA spanning > 1.2 million km2 across 88,854 local communities comprising 164 tree species. We also compiled climatic variables and 16 traits representing several important ecological axes of tree functional strategies to assess which factors were most strongly associated with the emerging broad-scale spatial patterns. Results Strong geographical variation in the potential for seedling recruitment and a latitudinal change from negative to positive density dependence moving northward were associated with adaptation to seasonal freezing temperatures and seed size. Wood density and leaf nitrogen, in contrast, were related to the magnitude of the negative density dependence and conspecific density dependence, respectively, which were prevalent over most of the region. Main conclusions Our results provide strong evidence that tree recruitment and the strength of density-dependent processes have broad-scale patterns that can be explained by a few key species functional traits. [ABSTRACT FROM AUTHOR]

The movement of species is one of the most pervasive forms of global change, and few ecosystems remain uninvaded by nonnative species. Studying species interactions is crucial for understanding their distribution and abundance, particularly for nonnative species because interactions may influence the probability of invasion and consequent ecological impact. Interactions among nonnatives are relatively understudied, though the likelihood of nonnative species co-occurrence is high. We quantify and describe the types of interactions among nonnative plants and determine what factors affect interaction outcomes for ecosystems globally. We reviewed 65 studies comprising 201 observations and recorded the interaction type, traits of the interacting species, and study characteristics. We conducted a census of interaction types and a meta-analysis of experiments that tested nonnative competition intensity. Both methods showed that negative and neutral interactions prevailed, and a number of studies reported that the removal of a dominant nonnative led to competitive release of other nonnatives. Positive interactions were less frequently reported and positive mean effect sizes were rare, but the plant characteristics nitrogen fixation, life cycle (annual or perennial), and functional group significantly influenced positive interactions. Positive interactions were three times more frequent when a neighboring nonnative was a nitrogen fixer and 3.5 times lower when a neighboring nonnative was an annual. Woody plants were two or four times more likely to have positive interactions relative to grasses or herbs, respectively. The prevalence of negative interactions suggests that managers should prepare for reinvasion of sites when treating dominant nonnatives. Though positive interactions were infrequent, managers may be able to anticipate positive interactions among nonnatives based upon traits of the co-occurring invaders. Predicting positive nonnative interactions is an important tool for determining habitat susceptibility to a particular invasion and for prioritizing management of nonnatives with a higher likelihood of positive interactions. [ABSTRACT FROM AUTHOR]

Aim To conceptualize the mechanistic pathways of the nurse-plant syndrome by life-form and to identify the implications of positive plant-plant interactions for landscape and evolutionary ecology. Location Global. Methods We conducted a quantitative review examining 298 articles to categorize the literature on nurse-plant interactions based on geographic region, mechanism of facilitation, ecological hypothesis and nurse life-form. Results A total of nine different nurse mechanisms were identified and two were classified as meta-mechanisms. We found that shrubs were the dominant nurse life-form (46% of total studies) and that studies of positive plant interactions were most frequent in areas of high abiotic stress. Nurse-plant studies were also distributed unevenly around the globe with nearly a quarter in the South American Andes and Spain. Studies testing the direct nurse-protégé interactions were the most frequently performed, including the ecophysiological responses of protégé species (32.2%). Research gaps identified in the nurse-plant literature included indirect interactions and seed trapping as well as the large-scale implications for landscape ecology and evolution. Main conclusions Nurse plants are often considered keystone species because they commonly structure plant communities. This is an important confirmatory finding in many respects, but it is also novel in that it challenges traditional plant ecology theory and has important implications for landscape-level dynamics over time. The categorization of mechanisms proposed provides a conceptual framework useful for organizing the research to date and can accelerate linkages with theory and application by identifying important connections. It is becoming increasingly apparent that future studies of the nurse-plant syndrome must decouple and consider multiple mechanisms of interaction to explain the processes that influence community structure, particularly in high-stress conditions, given a changing climate and potential shifts in biodiversity. [ABSTRACT FROM AUTHOR]

Facilitation through physical stress amelioration has been largely overlooked in subtidal marine ecosystems, where abiotic gradients are perceived to be benign. However, seemingly subtle changes in marine environmental conditions can alter community structure across vast areas, and therefore, the type of interactions and any refuge provided by marine foundation species. This could have substantial implications for community organization., We measured net recruitment rates of subtidal seaweeds under experimentally modified canopy densities, across 1000 km of coastline, to examine the nature of interactions between seaweed canopies and recruits., Contrary to expectations, facilitation, as evidenced by higher recruitment under canopies, was observed throughout all conditions for Scytothalia dorycarpa and under partial canopies at three of four locations for Sargassum spp., whilst competitive interactions remained prevalent for Sargassum under closed canopies., Supply side dynamics were also of major importance to recruitment success for Scytothalia. For Sargassum, the interaction strength between recruits and the canopy became increasingly positive towards lower latitudes, suggesting the canopy environment was mitigating stress across the latitudinal climate gradient., Synthesis. Positive interactions and stress amelioration play an important and previously unrecognized role in determining the recruitment success and viability of seaweeds in subtidal marine ecosystems. These results challenge long held paradigms about the general importance of canopy competition and force a rethink of how seaweed interactions affect habitat resilience to disturbances in subtidal ecosystems. [ABSTRACT FROM AUTHOR]

It is well known that many plants benefit from growing beside a nurse plant of another species, but the possibility that the nurse also benefits has been rarely studied. We hypothesize that positive interactions are maintained not only because of the recruitment benefits for the facilitated plants but also because of fitness benefits for the nurse plant. We tested this hypothesis by comparing seed production, seed predation and seed viability of a dominant nurse plant species ( Mimosa luisana ) when growing alone and in patches surrounded by its facilitated species. We also tested whether fitness of the nurse species is dependent on the phylogenetic neighborhood formed by their facilitated species using an analysis that accounted for the abundance and pairwise phylogenetic similitude of all species in each patch. Nurses growing associated to their facilitated species produced more seeds (1.86 times) and these seeds were more viable (1.47 times) than those of nurses growing alone. Seed predation did not alter these fitness differences. Seed number and viability increased in phylogenetically diverse neighborhoods. We conclude that distantly related partners are more likely to cause reciprocal increases in fitness, and that such effects contribute to species coexistence. [ABSTRACT FROM AUTHOR]

Highlights: [•] Direct care staff were trained to increase positive interactions with residents. [•] A bug-in-ear device was used to train staff. [•] All staff increased rates of positive interactions and maintained rates. [•] Residents also increased rates of positive interactions. [Copyright &y& Elsevier]

At micro-site scale, the spatial pattern of a plant species depends on several factors including interactions with neighbours. It has been seen that unfavourable effects generate a negative association between plants, while beneficial effects generate a positive association. In grasslands, the presence of shrubby species promotes a particular microenvironment beneath their canopy that could affect differently the spatial distribution of plants with different tolerance to abiotic conditions. We measured photosynthetic active radiation, air temperature and wind speed under shrub canopies and in adjacent open sites and analysed the spatial distribution of four grass species (two C3 and two C4) in relation to shrub canopy in a grazed sub-humid natural grassland in southern Uruguay. Radiation, air temperature and wind speed were lower under shrubs than in adjacent open sites. The spatial distribution of grasses relative to the shrub canopy varied depending on the photosynthetic metabolism of grasses. C4 grasses showed a negative association or no correlation with the shrubs, whereas C3 grasses showed a positive association. Our results highlight the importance of the photosynthetic metabolism of the grasses in the final outcome of interactions between grasses and shrubs. Micro-environmental conditions generated underneath shrubs create a more suitable site for the establishment of C3 than for C4 grasses. These results show that facilitation could be more important than previously thought in sub-humid grasslands. [ABSTRACT FROM AUTHOR]

Question Functional differences among climber species may reflect different strategies to cope with the environment created by the set of trees and shrubs used to reach the canopy. Moreover, climber diversity may be promoted and maintained by associations with subordinate woody species. Considering that functional differences among climbers may reflect this relationship, we aimed to identify trait differences among climbers, and the traits related to the association between climbers and woody species. Location Restinga de Jurubatiba National Park, a tropical sandy coastal plant community in southeast Brazil. Methods Relative abundances of climbers and shrubs/trees were surveyed, and climber species were described by a set of traits related to dispersal, establishment and persistence. Principal coordinate analysis was used to identify trait differences among climbers. Further, we performed principal components analysis in a community-weighted mean trait values matrix ( CWM) and compared the major pattern of variation with the abundance of dominant and subordinate woody species, and with environmental variables. Results The analyses revealed functional traits behind the association between climbers and subordinate woody species. These species are mostly characterized by the presence of latex and dry fruits, anemochorous dispersal and stem twining climbing strategy. They were associated with the subordinate shrub Erythroxylum subsessile. Tendril climbers were more generalist and were also characterized by zoochoric dispersal and the presence of endosperm. Conclusions The associative behaviour between climbers and the subordinate shrub E. subsessile is explained by a specific set of traits present in species of the same plant family, Apocynaceae. While stem twiners were positively associated with the specialist nurse E. subsessile, tendril climbers were associated with the dominant nurses. These results offer clues to understand the presence of specialist and generalist groups of climber species in tropical plant communities, as well as community assembly through facilitation driven by subordinate species. [ABSTRACT FROM AUTHOR]

Although the influence of positive interactions on plant and sessile communities has been well documented, surprisingly little is known about their role in structuring terrestrial animal communities. We evaluated beneficial interactions between two distantly related herbivore taxa, large vertebrate grazers (sheep) and smaller insect grazers (grasshoppers), using a set of field experiments in eastern Eurasian steppe of China. Grazing by large herbivores caused significantly higher grasshopper density, and this pattern persisted until the end of the experiment. Grasshoppers, in turn, increased the foraging time of larger herbivores, but such response occurred only during the peak of growing season (August). These reciprocal interactions were driven by differential herbivore foraging preferences for plant resources; namely, large herbivores preferred Artemisia forbs, whereas grasshoppers preferred Leymus grass. The enhancement of grasshopper density in areas grazed by large herbivores likely resulted from the selective consumption of Artemisia forbs by vertebrate grazers, which may potentially improve the host finding of grasshoppers. Likewise, grasshoppers appeared to benefit large herbivores by decreasing the cover and density of the dominant grass Leymus chinensis, which hampers large herbivores' access to palatable forbs. Moreover, we found that large herbivores grazing alone may significantly decrease plant diversity, yet grasshoppers appeared to mediate such negative effects when they grazed with large herbivores. Our results suggest that the positive, reciprocal interactions in terrestrial herbivore communities may be more prevalent and complex than previously thought. [ABSTRACT FROM AUTHOR]

Tree recruitment in Mediterranean ecosystems is strongly limited at the seedling stage by drought. Increasing evidence shows the critical positive role of the canopy nurse effect on seedling survival which results from direct and indirect, positive and negative interactions between species., Most studies, however, have only focused on the effects of tree canopy on water and light, ignoring other critical factors affecting seedling regeneration, such as canopy effects on high temperatures and the competing herb biomass., Here, we evaluate how tree canopy cover and removal of herbs affect the survival and growth of seedlings of two dominant Mediterranean Quercus species during a 3-year study. We use an integrated model that combines several data sets to quantify and predict regeneration dynamics along environmental gradients of soil moisture, temperature and light., Low soil moisture, increased soil temperature and herb biomass negatively affected seedling survival of both Quercus species. Seedling growth was positively associated with increasing soil moisture and light., Although tree canopy cover directly facilitated seedling survival in both Quercus species, it also negatively affected herb biomass and thus indirectly facilitated the survival of Quercus suber, but not of Quercus ilex seedlings at low levels of soil moisture., Overall, tree canopies increased seedling survival but not growth during the establishment phase, mainly by ameliorating the effects of low soil moisture and high temperatures. Tree canopy indirectly facilitated survival of Q. suber seedlings by negatively affecting the competing herb layer., Synthesis and applications. To improve tree recruitment and conserve Mediterranean Quercus woodlands, the removal of herbs should be integrated into management plans for dry habitats. Interactions between abiotic and biotic factors may also effect the regeneration of these tree species. In particular, a healthy tree canopy will become important for providing conditions to facilitate seedling establishment if these habitats become drier and warmer, as predicted by some climate change scenarios. [ABSTRACT FROM AUTHOR]

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The role of negative interactions in shaping ecological communities and the realized niches of species has been a focus of considerable research for at least decades. Traditionally, the discrepancy between the size of the fundamental and realized niche of a species was attributed to the effect of negative interactions, such that the realized niche is always smaller than the fundamental niche. However, in the last decade, a series of studies have highlighted the important role that positive interactions played in shaping the structure of communities. This renewed interest in positive interactions has led to a reconsideration of the niche concept. Specifically, some investigators have suggested that positive interactions can lead the realized niche of a species to be larger than its fundamental niche. Here, we show that although positive interactions can counteract the effects of negative interactions and possibly modify the realized niche of a species, the realized niche of a species can never be larger than the fundamental niche.

The dynamic of plant communities is strongly influenced by biological interactions. We studied the effects of Eichhornia azurea on the growth of the free-floating macrophytes Eichhornia crassipes, Salvinia minima, Salvinia auriculata and Limnobium laevigatum. The experiments were carried out in three lakes of the upper Paraná river floodplain. We used 0.5 m side squares where these species grew for 11 days (S. auriculata and L. laevigatum) and 17 days (E. crassipes and S. minima), in the presence and absence of E. azurea. There were no significant differences between the relative growth rates of macrophytes growing in the absence and presence of E. azurea. The relative interaction index showed the existence of competition between E. azurea and L. laevigatum, and neutrality with the other three species. There was no evidence of facilitation related to the improvement of physico-chemistry characteristics that could enhance macrophytes growth. However, we do not discard a ‘mechanical’ facilitation between E. azurea, which offers protection against wind and waves, and the other species of macrophytes

Trophic relationships between invasive species in multiply invaded ecosystems may reduce food limitation relative to more pristine ecosystems and increase resilience to control. Here, we consider whether invasive predatory American mink Neovison vison are trophically subsidized by invasive crayfish. We collated data from the literature on density and home range size of mink populations in relation to the prevalence of crayfish in the diet of mink. We then tested the hypothesis that populations of an invasive predator reach higher densities and are more resilient to lethal control when they have access to super-abundant non-native prey, even in the absence of changes in density dependence, hence compensatory capacity. We found a strong positive relationship between the proportion of crayfish in mink diet and mink population density, and a negative relationship between the proportion of crayfish in mink diet and mink home range size, with crayfish contribution to mink diet reflecting their abundance in the ecosystem. We then explored the consequence of elevated mink density by simulating a hypothetical eradication program with a constant harvest in a Ricker model. We found that mink populations were more resilient to harvest in the presence of crayfish. As a result, the simulated number of mink harvested to achieve eradication increased by 500 % in the presence of abundant crayfish if carrying capacity increased by 630 %. This led to a threefold increase in time to eradication under a constant harvest and an approximately 20-fold increase in the cumulative management cost. Our results add to evidence of inter-specific positive interactions involving invasive species, and our simple model illustrates how this increases management cost. [ABSTRACT FROM AUTHOR]

Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a 'safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change. [ABSTRACT FROM AUTHOR]

403I.403II.405III.410IV.411V.412VI.412VII.413VIII.413413References414 Summary: Models describing the biotic drivers that create and maintain biological diversity within trophic levels have focused primarily on negative interactions (i.e. competition), leaving marginal room for positive interactions (i.e. facilitation). We show facilitation to be a ubiquitous driver of biodiversity by first noting that all species use resources and thus change the local biotic or abiotic conditions, altering the available multidimensional niches. This can cause a shift in local species composition, which can cause an increase in beta, and sometimes alpha, diversity. We show that these increases are ubiquitous across ecosystems. These positive effects on diversity occur via a broad host of disparate direct and indirect mechanisms. We identify and unify several of these facilitative mechanisms and discuss why it has been easy to underappreciate the importance of facilitation. We show that net positive effects have a long history of being considered ecologically or evolutionarily unstable, and we present recent evidence of its potential stability. Facilitation goes well beyond the common case of stress amelioration and it probably gains importance as community complexity increases. While biodiversity is, in part, created by species exploiting many niches, many niches are available to exploit only because species create them. [ABSTRACT FROM AUTHOR]

Facilitation among species may promote non-native plant invasions through alteration of environmental conditions, enemies or mutualists. However, the role of non-trophic indirect facilitation in invasions has rarely been examined., We used a long-term field experiment to test for indirect facilitation by invasions of Microstegium vimineum (stiltgrass) on a secondary invasion of Alliaria petiolata (garlic mustard) by introducing Alliaria seed into replicated plots previously invaded experimentally by Microstegium., Alliaria more readily colonized control plots without Microstegium but produced almost seven times more biomass and nearly four times as many siliques per plant in Microstegium-invaded plots. Improved performance of Alliaria in Microstegium-invaded plots compared to control plots overwhelmed differences in total number of plants such that, on average, invaded plots contained 327% greater total Alliaria biomass and 234% more total siliques compared to control plots., The facilitation of Alliaria in Microstegium-invaded plots was associated with an 85% reduction in the biomass of resident species at the peak of the growing season and significantly greater light availability in Microstegium-invaded than control plots early in the growing season., Synthesis. Our results demonstrate that an initial plant invasion associated with suppression of resident species and increased resource availability can facilitate a secondary plant invasion. Such positive interactions among species with similar habitat requirements, but offset phenologies, may exacerbate invasions and their impacts on native ecosystems. [ABSTRACT FROM AUTHOR]

There is an ongoing debate about whether communities are closely integrated and bound together via interactions such as competition or facilitation, or are disintegrated and dominated by chance. We still lack community-wide data on the intensities of interactions and randomness, and measurements of their impacts on community structure., Using a long-term data set, we sought to measure the effects of interactions and stochasticity in structuring a highly diverse (>100 species) semi-arid grassland plant community, testing for positive and negative interactions at different stages of population growth of all species., During the colonization of new patches, most species were facilitated or inhibited by several others. These opposite effects can potentially have a large effect on species abundances, but they were correlated and cancelled out at the community level. Nevertheless, competition during colonization was strong enough to cause poor competitors to have small population sizes., The subsequent phase of population growth (increase and subsequent change in numbers within occupied patches) was mainly driven by intraspecific density dependence, and we found little evidence for interspecific interactions., Model results showed that stochasticity and recurrent colonization of transient, favourable patches maintained diversity, keeping poor competitors from becoming extinct., Synthesis: Our results, taken together with recent studies on tropical forests, suggest that weak interactions among established plants may be a general phenomenon, but that local interactions during colonization are important drivers of community composition. Most of the variance in species abundance in our community was explained by intraspecific competition and stochasticity, with interspecific interactions playing a minor role due to their overall weakness, interaction changes over ontogeny, and the cancellation of opposite-sign interactions when all the species in the community are considered. Despite this, some species were rare seemingly because they cannot withstand interspecific competition. Thus, to untangle the effects of interactions on community structure, future research should focus on interactions occurring at different phases of population growth and on whole communities. [ABSTRACT FROM AUTHOR]

The spatial distribution of species is mediated by a combination of biotic interactions and environmental conditions. Understanding the relative importance of these factors and how they interact is particularly important for predicting the spread of non-native species and their impact on resident communities., We used a 3-species Lotka- Volterra model parameterized with field and experimental data to understand the potential for continued spread by an introduced, non-native, dune-building beach grass ( Ammophila breviligulata) and whether this invasion will result in coexistence or displacement in the resident beach grass communities (native Elymus mollis and introduced, non-native Ammophila arenaria) of the US Pacific Northwest., We also used the model to investigate to what extent different rates of ocean-driven sand supply mediate the outcomes of beach grass species interactions., Our model suggests that A. breviligulata could invade and dominate dunes across the range of sand supply rates observed in the region. We found that sand supply influenced intra- vs. interspecific interactions, the strength of positive and indirect effects among beach grasses and the long-term abundances of the beach grass species themselves., Of the two non-natives, A. breviligulata is the inferior dune-building species. Thus, our results suggest that further invasions by A. breviligulata could reduce the coastal protection services afforded by tall dunes currently dominated by A. arenaria., Synthesis: In systems with strong environmental forcing and stressful conditions such as coastal dunes, environmentally mediated positive and indirect species interactions can govern invasion success and long-term native-non-native coexistence. In doing so, these interactions ultimately shape community structure and ecosystem function. Understanding the joint effects of environmental forcing and species interactions on community assembly is particularly important in cases where species introductions can alter ecosystem services, such as coastal protection, which are vulnerable to the effects of climate change. [ABSTRACT FROM AUTHOR]

It is well known that species interactions between exotic and native species are important for determining the success of biological invasions and how influential exotic species become in invaded communities. The strength and type of interactions between species can substantially vary, however, from negative and detrimental to minimal or even positive. Increasing evidence from the literature shows that exotic species have positive interactions with native species more often than originally thought. Gaps in our theory for how population growth is limited when interactions are positive, however, restrict our understanding of the mechanisms by which exotic "facilitators" contribute to diversity maintenance in invaded systems. Here, we quantified interactions between seven native and four exotic (established nonnative) common annual plant species in the highly diverse, York Gum woodlands of Western Australia. We used a Bayesian demographic modeling approach that allowed for interaction coefficients to be positive or negative, and explored key sources of variation in species responses to native and exotic neighbors at per capita (individual) and neighborhood levels. We observed positive per capita effects from exotic neighbors on exotic focal species as well as on several native focal species. However, all focal species were, on average, inhibited by their interaction neighborhood, when the variance in identity and abundance of observed neighbors was considered. At the neighborhood scale, exotic species were found to suppress all focal species, particularly those with high intrinsic fecundity. Our study demonstrates that within-neighborhood heterogeneity can regulate per capita positive effects of invaders, limiting runaway population growth of both natives and exotic invaders., (© 2022 The Authors. Ecology published by Wiley Periodicals LLC on behalf of The Ecological Society of America.)

Nest predation is one of the most important factors limiting reproductive success, and antipredator behaviour can significantly reduce the loss of avian broods. I carried out field experiments on two sympatric passerines: the barred warbler and the red-backed shrike. Many authors have described the protective nature of nesting association between these species. However, we have little knowledge about the true nature of the relationships between associates. I examined (1) whether barred warblers and red-backed shrikes respond differently to an avian predator, and (2) whether males and females differ in the intensity of nest defence. Decoys of a known nest predator and a non-predatory control species were used to examine the types and relative intensity of parental response. I measured behavioural responsiveness by recording aggressive behaviour toward each model during the nestling period. Barred warblers and red-backed shrikes showed considerable variation in their response. Warblers more vigorously defended their own territories than shrikes. No differences between the sexes in antipredator behaviour in red-backed shrike were found. By contrast, in barred warbler, male was more involved in nest defence. The experimental tests provide evidence that these two species are able to differentiate between a predator and non-predator species. [ABSTRACT FROM AUTHOR]

Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrices from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a het-erospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly. [ABSTRACT FROM AUTHOR]

Chance per se plays a key role in ecology and evolution, e.g., genetic mutation, resource spatiotemporal unpredictability. In community ecology, chance is recognized as a key factor in community assemblage, but less is known about its role in intraguild processes leading to species coexistence. Here we study the relevance of resource unpredictability per se as a promoter of intraguild positive interspecific interactions and as a biodiversity enhancer in an Old World avian scavenger guild, which has evolved to feed upon spatially and temporally unpredictable resources, i.e., carcasses. We performed a large-scale field experiment in which 58 carcasses were disposed of and observed until complete consumption, either in continuously active supplementary feeding stations (predictable carcasses) or disposed of at random in the field (unpredictable carcasses). Richness of scavenger species was similar at unpredictable and predictable carcasses, but their relative abundances were highly uneven at predictable carcasses leading to higher scavenger diversity (Shannon index) at unpredictable carcasses. Facilitatory interspecific processes only occurred at unpredictable resources but were disrupted in predictable conditions because the dominant specialist species (in our case, the Griffon Vulture Gyps fulvus) arrived earlier and in larger numbers, monopolizing the resource. Small, endangered scavengers congregated at supplementary feeding stations but profited less compared to unpredictable carcasses, suggesting that they could constitute an ecological trap. Our findings offer new insights into the relevance of unpredictability of trophic resources in promoting both positive facilitatory interspecific interactions and species diversity and thus maintaining the function of guilds. Finally, the preservation of randomness in resource availability and the processes associated with its exploitation should be a major goal of conservation strategies aimed to preserve scavenger guilds evolved under naturally unpredictable trophic resources. [ABSTRACT FROM AUTHOR]

By shaping the architecture and taxonomic composition of periphyton, stream current may create periphytic mats on which some grazers can feed and forage more effectively than others. Current-mediated periphytic structure also has the potential to foster positive interactions among grazers if one grazer's foraging facilitates another's access to algal food. To examine the extent to which these indirect effects of current influenced periphytic removal and grazer interactions, we conducted a mesocosm experiment with two common grazers, the caddisfly (Trichoptera) Glossosoma verdona and the mayfly (Ephemeroptera) Drunella grandis. Periphyton was allowed to colonize ungrazed tiles for 30 d and assume its natural growth form under three ranges of near-bed current, 'slow' (1-5 cm s), 'medium' (15-20 cm s), and 'fast' (30-40 cm s). Tiles were then exposed to the two grazer species at five densities. A streambed survey quantified Glossosoma and Drunella distributions in relation to near-bed current and periphytic structure (i.e., diatom films vs. filamentous mats) in the Colorado River. After 22 days of grazing, periphytic removal by Glossosoma was influenced by near-bed current and attendant periphytic structure. In slow current, where senescent Ulothrix filaments were abundant, increased Glossosoma density was correlated with an increase in periphyton biomass. Larvae became entangled and immobilized by the diffuse and senescent Ulothrix mat that characterized slow velocity, and Glossosoma mortality and weight loss was greatest in this treatment. By contrast, Drunella reduced periphyton across all density and current treatments. Drunella density correlated with increased Glossosoma survivorship and weight gain in slow current. The driving mechanism for this facilitation appeared to be removal of entangling overstory filaments by Drunella. The streambed survey showed that Glossosoma were negatively associated with filamentous mats, lending support to the hypothesis that clearing action by Drunella in the slow current/senescent Ulothrix treatment facilitated Glossosoma growth and survival. Our study helps underscore the importance of evaluating species interactions over ranges of abiotic conditions and consumer pressure to understand the patterns and processes shaping benthic communities. [ABSTRACT FROM AUTHOR]

Positive interactions between species are known to play an important role in the structure and dynamics of alpine plant communities. The balance between negative and positive interactions is known to shift along spatial and temporal gradients, with positive effects prevailing over negative ones as the environmental stress increases. Thus, this balance is likely to be affected by climate change. We hypothesized that increases in temperature (a global warming scenario) should decrease the importance of positive interactions for the survival and growth of alpine plant species. To test this hypothesis, we selected individuals of the native grass species Hordeum comosum growing within the nurse cushion species Azorella madreporica at 3,600 m.a.s.l. in Los Andes (Chile), and performed nurse removal and seedling survival experiments under natural and warmer conditions. For warmer conditions, we used open-top chambers, which increased the temperature by 4 °C. After two growing seasons, we compared the effect of nurse removal on the survival, biomass, and photochemical efficiency of H. comosum individuals under warmer and natural conditions. Nurse removal significantly decreased the survival, biomass, and photochemical efficiency of H. comosum, demonstrating the facilitative effects of nurse cushions. Seedling survival was also enhanced by cushions, even under warmer conditions. However, warmer conditions only partially mitigated the negative effects of nurse removal, suggesting that facilitative effects of cushions do not wane under warmer conditions. Thus, facilitative interactions are vital to the performance and survival of alpine species, and these positive interactions will continue to be important in the warmer conditions of the future in high-alpine habitats. [ABSTRACT FROM AUTHOR]

Aims The process of facilitation, where a species increases the survival, growth, and fitness of another species, is becoming increasingly recognized as a critical factor in shaping the structure of plant communities. This process is particularly important in stressful environments. Yet few studies have attempted to incorporate positive interactions into community ecological theories such as the neutral theory of biodiversity. Here, we use an equalizing trade-off model as a foundation to study the potential impact of facilitation on species richness and community temporal turnover. Methods Based on a spatially explicit birth–death trade-off model, we assume that the occurrence of facilitation is dependent on the presence of interspecific neighbours. We further propose that the realized birth rate for a given individual subject to facilitation is proportional to the number of interspecific neighbours within its neighbourhood. Thus, in our model, the individuals of rare species will benefit more from the existence of heterospecific individuals than common species. Important Findings As the facilitative coefficient increased, the species richness for simulated communities at the dynamically stochastic equilibrium was also increasing. Simulations also demonstrated that facilitation could increase the replacement of species through time: communities with facilitation become more dissimilar (i.e. have smaller Bray–Curtis similarity values) than communities without or with a lower degree of facilitation after the same time interval. Facilitation from interspecific neighbours on rare species increased their population sizes and consequently made them less prone to extinction, thus enhancing species richness. Meanwhile, in a saturated community, with the increase of species richness, mean population size of entire communities decreased, making species more prone to extinction on average, and thus increased the community temporal turnover. [ABSTRACT FROM PUBLISHER]

Much is known about how environmental stress mediates the strength of facilitation, but less is known about how different plant traits affect facilitation. We examined interactions between the shrub Tamarix chinensis and two congeneric forbs (Suaeda salsa and S. glauca) on the Chinese coast. Although S. salsa and S. glauca are both annuals, morphologically similar, and have synchronous phenologies, they have contrasting adaptive strategies. S. glauca is salt intolerant but competitively superior, and S. salsa is salt tolerant but competitively inferior. Field surveys showed that S. glauca was associated with T. chinensis canopies while S. salsa was more abundant in open areas. A T. chinensis removal experiment showed that S. glauca cover was lower and soil salinity higher after two years in removal than in control plots. Transplant experiments showed that S. salsa performance under T. chinensis canopies was reduced by competition from S. glauca and T. chinensis, while in open areas S. glauca was not affected by S. salsa competition. Thus, contrasting competitive abilities and stress tolerances of S. glauca and S. salsa underlie their facilitative and competitive interactions with T. chinensis, suggesting that plant strategies are critical to the outcome of species interactions. [ABSTRACT FROM AUTHOR]

Aim Theoretical models predict nurse plant facilitation enhances species richness by ameliorating stressful environmental conditions and expanding distributional ranges of stress-intolerant species into harsh environments. We studied the role of nurse facilitation on the recruitment of perennial plants along an arid scrubland-temperate rain forest boundary to test the following predictions: (1) nurse shrub canopy increases seedling abundance and species richness along the rain forest-scrubland boundary; (2) scrubland species are less dependent on facilitative interactions than temperate rain forest species, especially at the moister, upper end of the gradient. Location Bosque Fray Jorge National Park, north-central Chile, South America (30° 39′ S - 71° 40′ W). Methods We examined seedling abundance and species richness of perennial plants in the open and under different types of patches that may facilitate species recruitment (living shrubs, dead shrubs, perennial grasses and trees) along an arid scrubland-temperate rain forest boundary. To assess whether the potential role of the shrub canopy on seedling survival in the scrubland and forest differs, we designed a 2 × 2 factorial field experiment with shrub canopy (under shrub or open patch) and small mammal herbivory (access or closed) as factors. At both sides of the scrubland-forest gradient, we planted young seedlings of scrubland ( Senna cumingii) and forest ( Myrceugenia correifolia and Griselinia scandens) species under the four treatment combinations. Results Nurse living shrubs increased seedling abundance and diversity. Under living shrubs, the number of recruiting species was 100% and 30% larger than in open patches of the arid scrubland and temperate rain forest patches, respectively. Seedling abundance was 60% and 300% larger under living shrubs compared to open patches of the scrubland and the temperate rain forest, respectively. Despite their low cover, dead shrubs were particularly important for recruitment of young seedlings in the scrubland. Seedlings of forest species were extremely rare. The field experiment demonstrated the importance of nurse plant facilitation for the survival of the forest species at both sides of the boundary. In contrast, scrubland species were less dependent on nurse plants than forest and intermediate forest-scrubland species. Conclusions Our results show that facilitative interactions increase seedling abundance and species richness along an arid scrubland-temperate rain forest boundary, notably by enhancing seedling establishment of forest species. Despite increasing nurse shrub-seedling competition in the arid scrubland, shrubs have a net positive effect on plant community diversity. [ABSTRACT FROM AUTHOR]

Co-occurring foundation species can determine biological community structure via facilitation cascades. We examined the density dependencies of facilitation cascades, including how the density of a basal foundation species influences the density of secondary foundation species, and how the density of secondary foundation species influences community structure. The system in which we assessed density dependencies was a temperate mangrove forest in which pneumatophores trap the fucoid alga Hormosira banksii and provide substrate for the oyster, Saccostrea glomerata. The alga and oyster in turn determine benthic community structure. In the field, algal biomass was positively correlated with pneumatophore density. Oysters, by contrast, were highly over-dispersed and correlated with the presence/ absence of pneumatophores. Epifaunal abundance and species richness were positively correlated with algal and oyster abundance, but their effects were independent. The positive effect of pneumatophore density on epifauna was primarily an indirect effect of trapping more algae. Pneumatophores did not directly influence invertebrate communities. Experiments revealed that, at very low pneumatophore densities, algal retention was insufficient to facilitate epifauna above that found on pneumatophores alone. At higher densities, however, increasing the density of pneumatophores increased algal retention, and the density and diversity of associated invertebrates. Shading by the mangrove canopy reduced algal biomass but did not modify the density-dependent nature of the cascade. Our results extend facilitation theory by showing that the density of both basal and secondary foundation species can be critical in triggering facilitation cascades. Our study also reveals that, where foundation species co-occur, multiple, independent cascades may arise from a single basal facilitator. These findings enhance our understanding of the role of density-dependent facilitation cascades in community assembly. [ABSTRACT FROM AUTHOR]

Abstract: Most of the studies that have evaluated the interplay between interference and facilitation have been done at the interspecific level, whereas studies at the intraspecific level are scarce. The montane sclerophyllous forests of central Chile are dominated by the tree Kageneckia angustifolia, a semi-deciduous species that lose part of its foliage during summer. It has been reported that during winter snow accumulates in lower amounts beneath the canopy of K. angustifolia favoring the recruitment of new individuals compared to open areas (i.e., facilitation effect). However, it has also been reported that the leaf litter accumulated beneath parental trees contains allelopathic compounds that decrease seed germination, suggesting that recruitment beneath parental plants can be disfavored (i.e., interference effect). Hence, this system seems appropriate to assess the net-outcome between facilitative and negative effects during the emergence and survival of seedlings during the first year. In this study, we asked (i) what is the net-outcome between facilitative and interfering effects for K. angustifolia? (ii) does this net-outcome varies with the distance to parental trees? (iii) are positive and negative effects consistent through the seedling emergence and first year seedling survival phases? (iv) what are the main mechanisms behind the observed net-outcome? and (v) which is the optimal microhabitat for successful recruitment of this species? In an experimental plot of 10,000m2, we selected ten K. angustifolia trees and evaluated the effect of leaf litter on the emergence and survival of seedlings produced by experimentally sown seed seeds in three different microhabitats: beneath adult trees, edge of canopy and in open areas. In addition, we sampled three K. angustifolia stands to evaluate the microhabitat where the natural recruitment of this species actually occurring. Results showed that (1) seedling emergence was greater beneath canopy, intermediate in canopy edge and low in open areas, (2) whilst leaf litter significantly reduced seed germination, the magnitude of this negative effect was lower than the positive effect of beneath canopy microhabitat, (3) seedling survival was affected by microhabitats but not by the presence of leaf litter, (4) that the main mechanisms behind the observed patterns are the lower and delayed emergence of seedlings in open areas due to the longer duration of snow cover, decreasing the time to growth before the onset of summer drought, and (5) the greatest natural recruitment of K. angustifolia seedlings occurs beneath parental plants. Therefore, our findings suggest that the net-outcome between facilitative and interfering effect during the first year is mostly facilitative, indicating that adult trees of K. angustifolia are exerting a conspecific nurse effect on the recruitment of new individuals, a form of parental care in plants. [Copyright &y& Elsevier]

The present study was conducted in four locations in Cyprus. These areas are included in a broad study area of approximately 6200 ha. The majority of this area belongs to the Natura 2000 Network and a small part of this area is a designated Nature Reserve. In order to determine whether facilitation is the dominant process in the regeneration of the Juniperus excelsa M. Bieb. stands in Cyprus, 120 plots of 100 m² (10 m x 10 m) were established in eight site types. In each plot the regeneration plants were graded in categories, according to their location in relation to J. excelsa trees. In four of the eight site types, that were distinguished, the density of the J. excelsa seedlings in open areas was higher than the density of the regeneration plants that have been established under the facilitation of adult trees of the species. In the remaining site types, a statistically significant difference was not recorded. In Cyprus facilitation is not the dominant process in the regeneration of J. excelsa. On the contrary, it seems that in many cases competition is the dominant force that determines the regeneration process of the species. The J. excelsa formations exhibit adequate recruitment, but in the future, the presence of J. excelsa in Cyprus will gradually be reduced in better sites, since it will be replaced by more competitive species having greater site sensitivity. [ABSTRACT FROM AUTHOR]

Plants alter environmental conditions enhancing the recruitment of other species. In spite of prior reviews of facilitation, the variability of its occurrence, mechanisms, and specificity across terrestrial ecosystems has not yet been assessed. In this article, we analyze facilitative mechanisms and the distribution of specific traits, such as nitrogen fixation and the presence of fleshy fruits, across ecosystems. A comprehensive database including 2,080 cases of facilitation among higher plants from 539 articles was analyzed with descriptive statistics for occurrences of positive interactions and underlying mechanisms in different terrestrial ecosystems. Positive interactions by plant-induced environmental changes are widespread in a range of ecosystems and not limited to conditions of chronic abiotic stress such as semiarid, alpine, and wetland ecosystems. The capability to act as nurse largely varied among different growth forms, and was observed more frequently for woody than for herbaceous plants. Nitrogen fixers occur much more frequently as nurse plants than as beneficiary plants in facilitation cases due to increasing soil fertility. As known for Mediterranean ecosystems, fleshy-fruited species appear more dependent on facilitative interactions than other plants, being more frequent among beneficiaries than among nurses. The pattern can be extended worldwide being consistent in wetland, temperate, and alpine ecosystems as well. Our description of the relationship between distribution, mechanisms, and specificity of facilitation in terrestrial ecosystems has implications for the understanding of plant community organization considering that plant nursing capacity is affected by their size, architecture, and life span. [ABSTRACT FROM AUTHOR]

Foraging niche overlap between attendant species and the nuclear species of mixed-species flocks may induce competition as well as facilitation. In this study, we hypothesized that canopy species would appear more frequently in flocks where nuclear species foraged in the canopy and that species with greater niche overlap with nuclear species would have lower departure rates. Flocking and foraging behaviors of flocks led by the Grey-cheeked Fulvetta Alcippe morrisonia were studied in northeastern Taiwan for two consecutive non-breeding seasons from 1998 to 2000. The foraging height spectrum increased with increasing group size of fulvettas because more fulvettas were found foraging across different strata. The number of species and number of individuals of canopy attendants in a flock increased more rapidly than understory attendants as the group size of fulvettas increased. The more the foraging height of an attendant species overlapped with that of the nuclear species, the lower its departure rate. Specifically, canopy attendant species that usually had a lower niche overlap with the nuclear species dropped out of the flock more frequently than understory attendant species. These observations indicate that foraging niche overlap between attendant species and the nuclear species may facilitate the formation of mixed-species bird flocks. It is therefore important to incorporate the role of positive interactions when trying to understand the formation and maintenance of biological communities. [ABSTRACT FROM AUTHOR]

Positive interactions between cushion plant and associated plants species in the high Andes of central Chile should also include the effects of fungal root symbionts. We hypothesized that higher colonization by arbuscular mycorrhizal (AM) fungi exists in cushion-associated (nursling) plants compared with conspecific individuals growing on bare ground. We assessed the AM status of Andean plants at two sites at different altitudes (3,200 and 3,600 m a.s.l.) in 23 species, particularly in cushions of Azorella madreporica and five associated plants; additionally, AM fungal spores were retrieved from soil outside and beneath cushions. 18 of the 23 examined plant species presented diagnostic structures of arbuscular mycorrhiza; most of them were also colonized by dark-septate endophytes. Mycorrhization of A. madreporica cushions showed differences between both sites (68% and 32%, respectively). In the native species Hordeum comosum, Nastanthus agglomeratus, and Phacelia secunda associated to A. madreporica, mycorrhization was six times higher than in the same species growing dispersed on bare ground at 3,600 m a.s.l., but mycorrhiza development was less cushion dependent in the alien plants Cerastium arvense and Taraxacum officinale at both sites. The ratio of AM fungal spores beneath versus outside cushions was also 6:1. The common and abundant presence of AM in cushion communities at high altitudes emphasizes the importance of the fungal root symbionts in such situations where plant species benefit from the microclimatic conditions generated by the cushion and also from well-developed mycorrhizal networks. [ABSTRACT FROM AUTHOR]

In multiply invaded ecosystems, introduced species should interact with each other as well as with native species. Invader-invader interactions may affect the success of further invaders by altering attributes of recipient communities and propagule pressure. The invasional meltdown hypothesis (IMH) posits that positive interactions among invaders initiate positive population-level feedback that intensifies impacts and promotes secondary invasions. IMH remains controversial: few studies show feedback between invaders that amplifies their effects, and none yet demonstrate facilitation of entry and spread of secondary invaders. Our results show that supercolonies of an alien ant, promoted by mutualism with introduced honeydew-secreting scale insects, permitted invasion by an exotic land snail on Christmas Island, Indian Ocean. Modeling of land snail spread over 750 sites across 135 km² over seven years showed that the probability of land snail invasion was facilitated 253-fold in ant supercolonies but impeded in intact forest where predaceous native land crabs remained abundant. Land snail occurrence at neighboring sites, a measure of propagule pressure, also promoted land snail spread. Site comparisons and experiments revealed that ant supercolonies, by killing land crabs but not land snails, disrupted biotic resistance and provided enemy-free space. Predation pressure on land snails was lower (28.6%), survival 115 times longer, and abundance 20-fold greater in supercolonies than in intact forest. Whole-ecosystem suppression of supercolonies reversed the probability of land snail invasion by allowing recolonization of land crabs; land snails were much less likely (0.79%) to invade sites where supercolonies were suppressed than where they remained intact. Our results provide strong empirical evidence for IMH by demonstrating that mutualism between invaders reconfigures key interactions in the recipient community. This facilitates entry of secondary invaders and elevates propagule pressure, propagating their spread at the whole-ecosystem level. We show that identification and management of key facilitative interactions in invaded ecosystems can be used to reverse impacts and restore resistance to further invasions. [ABSTRACT FROM AUTHOR]

To test the hypothesis that the species could be more beneficial in association with two-nurse species than with single-nurse species, we surveyed an arid shrub-herbaceous community primarily composed of two dominant species, Achnatherum splendens and Nitraria tangutorum and two associate species, Phragmites australis and Reaumuria soongorica in Northwest China. The performance (frequency, abundance, and size) of the two associate species in neighbored patches (patches formed by a dominant species which were closely adjacent to the patches formed by the other dominant species), isolated patches (patches formed by one dominant species with no neighbor), and open areas was compared to analyze the individual and combined effects of the two dominant species. A. splendens and N. tangutorum appeared to have reciprocal facilitation effects when growing adjacent to one another, as evidenced by the increased size of neighbored patches over isolated ones. The individual effects of A. splendens on the associate species should be generally neutral, since the frequency, abundance, size, and co-occurrence frequency of the two associate species in isolated A. splendens patches and open areas were the same (except for the significantly larger size of R. soongorica in isolated A. splendens patches). The individual effects of N. tangutorum were positive to the associate species R. soongorica, as evidenced by the fact that the frequency and abundance of R. soongorica was significantly higher ( P < 0.05), and the size of two associate species was significantly larger ( P < 0.05), in isolated N. tangutorum patches than in open areas. However, the frequency and abundance of P. australis, and the co-occurrence frequency of the two associate species in isolated N. tangutorum patches and in open areas were the same. In comparison to the isolated patches, there were significantly higher frequency and abundance ( P < 0.05), larger sizes ( P < 0.05), and higher co-occurrence frequency ( P < 0.05) of the two associate species in neighbored N. tangutorum patches. Since the neighbored patches could be influenced by both dominant species, the combined effects of A. splendens and N. tangutorum were identified as positive and over-additive. We suggested that the indirect facilitation or synergistic effects could account for the combined effects of the two dominant species. [ABSTRACT FROM AUTHOR]