Testing the Janzen‐Connell mechanism: pathogens cause overcompensating density dependence in a tropical tree

Abstract

The Janzen-Connell hypothesis is a leading explanation for plant-species diversity in tropical forests. It suggests that specialized natural enemies decrease offspring survival at high densities beneath parents, giving locally rarer species an advantage. This mechanism, in its original form, assumes that density dependence is overcompensating: mortality must be disproportionately high at the highest densities, with few offspring recruiting below their parents. We tested this assumption using parallel shadehouse and field density-series experiments on seedlings of a tropical tree, Pleradenophora longicuspis. We found strong, overcompensating mortality driven by fungal pathogens, causing 90% (shadehouse) or 100% (field) mortality within 4 weeks of germination, and generating a negative relationship between initial and final seedling densities. Fungicide treatment led to much lower, density-independent, mortality. Overcompensating mortality was extremely rapid, and could be missed without detailed monitoring. Such dynamics may prevent dead trees from being replaced by conspecifics, promoting coexistence as envisioned by the Janzen-Connell hypothesis.

Interspecific Functional Convergence and Divergence and Intraspecific Negative Density Dependence Underlie the Seed-to-Seedling Transition in Tropical Trees

“…Despite these challenges, ecologists have made progress through analyzing long-term forest dynamics data sets (e.g., Condit et al 2006;Wills et al 2006;Swenson et al 2012b;Muscarella et al 2013). Recent studies have shown that nonrandom mortality is particularly high in the smallest size classes in tropical tree communities, and this leaves a disproportionally large imprint on patterns of coexistence through to adulthood (Bagchi et al 2010(Bagchi et al , 2014Metz et al 2010;Paine et al 2012;Green et al 2014). Uncovering the ecological mechanisms that determine the seed-to-seedling transition and their effects on tropical tree coexistence and community dynamics (Levine and Murell 2003) is an essential goal.…”

Species richness and species identity effects on occurrence of foliar fungal pathogens in a tree diversity experiment

“…Pathogens can promote coexistence among host species by regulating the relative abundance of hosts. According to the Janzen-Conell hypothesis, pathogens can cause local negative density dependence of host species, and thus, affect the spatial host abundance patterns (Janzen 1970, Connell 1971, Bagchi et al 2010. However, the situation in most ecosystems might be even more complex, as beside hosts and pathogens further components are involved in food-webs (Duffy 2002).…”

Spatial analysis of phylogenetic community structure: New version of a classical method

“…Their results indicate a general occurrence of litter autotoxicity related to the exposure to fragmented extracellular DNA. Hence, while the effects of negative plant-soil feedback on the spatial structure of individual plants depend on the intensity of the underlying density-dependent mechanisms (Bagchi et al 2010), its ubiquitous presence suggests that plant-soil feedback may play an important role in the maintenance of plant diversity in natural (cm) communities (Mazzoleni et al 2015b). In a review on direct evidences on conspecific negative plant-soil feedback from both terrestrial and aquatic plants, Mazzoleni et al (2007) reported 138 cases of conspecific negative plant-substrate interactions, 96 of which are associated with grassland species, including Erigeron canadensis, one species of Veronica and one species of Viola, thus reinforcing the hypothesis that plant-soil feedbacks may contribute to the disruption of shortrange phylogenetic clustering (see also Anacker et al 2014).…”