Species-specificity of facilitative interactions is ecologically relevant for community organization and dynamics. The prevalence of heterospecific facilitation which is found between higher plants seems an evolutionary paradox since beneficiary heterospecifics can compete with, and even replace, their nurse. An extensive review on facilitative interactions for both higher plants and sessile aquatic organisms is presented. The study reports on the occurrence of positive interactions between conspecifics and heterospecifics, in relation to different growth forms and facilitative mechanisms in terrestrial and aquatic environments. Four hypotheses are considered to explain the observed rarity of conspecific compared to heterospecific facilitation: 1) the occurrence of plant-induced unsuitable recruitment conditions (negative conspecific plant-soil feedback and hump-shaped recruitment distribution), 2) higher competition levels between conspecifics, 3) the prevalence of non-facilitated regeneration niche for nurses, and, 4) differences in the research effort. Self-facilitation is reported much more rarely in terrestrial vegetation (5.3% of 1554 cases of facilitiative interactions), than in aquatic ecosystems (36.2%, n = 130). In absolute terms, far more occurrences of heterospecific facilitative interactions are reported for all growth forms. However, when the occurrences of facilitative interactions are expressed as a percentage of the total con- and heterospecific interactions, annual and perennial herbaceous nurse plants show prevailing conspecific interactions, while woody (trees and shrubs) nurse species mostly show heterospecific facilitation. Increase of soil nutrient fertility, improvement of above-ground microclimate, associational refuge and seed trapping are the most common mechanisms of heterospecific interactions. Differently, conspecific facilitation is mostly due to improved soil biotic conditions, changes in fire regimes and reduction of heterospecific competition. Given the frequently reported occurrence of non-facilitated regeneration niche for nurse species, conspecific negative feedback and hump-shaped recruitment distribution in terrestrial plants, these processes are suggested as significantly contributing to explain the observed rarity of conspecific facilitation.
The consequences of decline in biodiversity for ecosystem functioning is a major concern in soil ecology. Recent research efforts have been mostly focused on terrestrial plants, while, despite their importance in ecosystems, little is known about soil microbial communities. This work aims at investigating the effects of fungal and bacterial species richness on the dynamics of leaf litter decomposition. Synthetic microbial communities with species richness ranging from 1 to 64 were assembled in laboratory microcosms and used in three factorial experiments of decomposition. Thereafter, the functionality of the different microcosms was determined by measuring their capability to decompose materials with different chemical properties, including two species of litter (Quercus ilex L. and Hedera helix L.), cellulose strips and woody sticks. Incubation was done in microcosms at two temperatures (12°C and 24°C) for 120 days. The number of microbial species inoculated in the microcosms positively affected decomposition rates of Q. ilex and H. helix litters, while relationships found for cellulose and wood were not statistically significant. Diversity effect was greater at higher incubation temperature. We found lower variability of decay rates in microcosms with higher inoculated species richness of microbial communities. Our study pointed out that the relationships between inoculum microbial diversity and litter decomposition is dependent on temperature and litter quality. Therefore, the loss of microbial species may adversely affects ecosystem functionality under specific environmental conditions.
Ring shaped patches of clonal plants fascinated plant ecologists since long time. In this work we review the reports on the occurrence of ring pattern in different environmental conditions, the growth forms of ring-forming plants, the mechanisms underlying ring formation, and the consequences for species diversity at community scale. Rings formed by 83 species of clonal vascular plants have been found in grasslands, deserts, bare substrates of lava flow, harvested peat lands, salt marshes, and sand dunes. Four causal hypotheses have been proposed for the emergence of ring patterns: i. occurrence of architectural constraints for ramets development; ii. induction by fire, drought, trampling or overgrazing; iii. nutrient and water depletion by competition inside the ring; and iv. onset of species-specific negative plant-soil feedback in the inner zone of the clone. Since almost all the available studies are observations of ring structure or modelling exercises, none of the putative mechanisms for ring formation emerged from the literature as either generally applicable or suitable for rejection. Therefore, long-term field experiments are needed to investigate the relative prevalence of different mechanisms in different environments. Ring formation bears important consequences at community scale, because ring forming plants often act as “nurses”, enhancing the recruitment and development of different plant species. In fact, ring establishment modifies above- and below-ground environmental conditions, providing specialized safe sites for beneficiaries in the inner zone of the clones. Such interspecific facilitation by ring forming plants, particularly in chronically stressed environments, contributes to increase plant species richness and can locally promote the successional dynamics.