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Understanding species coexistence and regulation of biodiversity are major research challenges, yet there is no consensus on the effects of diversity on diversity, including their mediation through plant-plant interactions., We examined how the diversity of recipient communities impacted on the establishment of colonising species. We ran a greenhouse-based community experiment, creating artificial arable crop communities with varying levels of barley genotype and weed species diversity, analysed with structural equation modelling of responses across trophic levels., Colonising arable weed species establishment was significantly reduced when the recipient communities' weed species richness, but not barley genotype richness, increased. Recipient plant communities with higher species richness occupied more total niche space and showed stronger competition with colonising species. In contrast, recipient plant communities with higher genotype richness, even though they had slightly greater niche space occupancy, had no significant effects on the colonisation of additional species., Responses were species specific: colonising common weed species establishment was less affected by the recipient plant communities than that of colonising rare weed species. Strong responses of colonising rare species were related to the abiotic rather than the biotic environment created by the recipient communities. The stronger response of colonising rare species suggests greater niche overlap with and competitive inferiority to the recipient plant community, whereas the relative independence of colonising common species indicates less niche overlap and/or competitive equality or superiority., We observed a negative species diversity effect on colonising species, with the effect size increasing with increasing rarity of the colonising species. This differential responsiveness of colonising rare and common arable weed species to the presence of and diversity in the recipient arable plant communities may be crucial in understanding rarity and may result from differential evolutionary pathways in response to interactions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Facilitation is known as the positive effect of one species (benefactor) on associated neighbouring species (beneficiaries). Although the beneficial part of this interaction has received considerable research interest, there is a gap of knowledge on the bidirectional nature of these interactions; in particular, the physiological and fitness consequences for both beneficiaries and benefactors., Alpine cushion plants are generally strong benefactors, increasing species richness and abundance on a global scale, and provide a suitable system to study the physiological effects of bidirectional interactions and its consequences for reproduction. Current knowledge suggests that species improve their fitness when associated to a benefactor cushion species, whereas cushions may receive predominantly negative feedbacks., We measured physiological and reproductive traits of the cushion species Arenaria tetraquetra ssp. amabilis and three other forbs ( Eryngium glaciale, Lotus corniculatus ssp. glacialis and Plantago nivalis) in the dry Sierra Nevada Mountains, southern Spain. All four species were studied either when growing alone or when the three forbs were associated with the cushion plant., The three forb species improved their water status when associated with the cushion, and Lotus and Plantago significantly increased their seed set. In contrast, Arenaria showed poorer water status and reduced flower density and seed set with increasing cover of beneficiary species. There was a clear relationship between physiological and reproductive traits in Arenaria growing without beneficiaries but not in Arenaria with beneficiaries. Control cushions (without beneficiary species) showed increased seed set and seed mass with increasing photosynthetic and water use efficiencies, respectively, the latter being positively related to leaf nitrogen content. In contrast, cushions with a large cover of beneficiary species did not show such relations. The missing relationship between physiological and reproductive traits for these facilitating cushions indicates that reproductive output in heavily colonized cushions may not be directly related to the plant physiological status and the availability of resources., Our results revealed the antagonistic behaviour underlying the interaction between beneficiary species and facilitating cushion plants, similar to parasitic interactions. They also provide indications for changes in cushions' resource allocation pattern in response to colonization by beneficiaries. [ABSTRACT FROM AUTHOR]