Abstract:

Background & Aim: Since the Janzen-Connell (J-C) hypothesis was proposed half a century ago, a mounting number of studies have been conducted to test the hypothesis in tropical and subtropical forests. These studies have since greatly improved our understanding of how high biodiversity is maintained. In particular, the pathogenic fungi-induced J-C effect, a type of negative plant-soil feedback (PSF), has been well-recognized as a mechanism to maintain biodiversity and structure community composition, though the overall contribution of PSF to the persistence of a large number of rare species in nature remains controversial. As predicted by the modern species coexistence theory, the “invasion criterion” should be met for rare species to co-exist with other species such that one species will increase in abundance when rare. However, previous studies show results contrary to the prediction of such theory and have thus sparked debates on the mechanism underlying rare species maintenance.
Progresses: In this work, we review PSF and the potential factors associated with PSF, including mycorrhizal fungi, soil nutrient content, and fine root functional traits. We discuss their contributions in maintaining rare species and determining species abundance via PSF. In addition to PSF, some other perspectives about rare species maintenance are also covered in this review.
Prospects: We propose that the advantages in maintaining the long persistence of rare species and the limitations in restricting population expansion of rare species may be of equal importance for rare species. The combination of modern species coexistence theory and new techniques and methodologies provide promising future directions to fully understand rare species and to better conserve rare species in the future.

Key words: conspecific negative density dependence, forest community, invasion criterion, Janzen-Connell hypothesis, plant-soil feedback, rare species, species abundance