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.
