The effect of soil-borne pathogens depends on the abundance of host tree species.

Yu Liu,Suqin Fang,Fangliang He,Peter Chesson

doi:10.1038/ncomms10017

Abstract

The overarching issue for understanding biodiversity maintenance is how fitness advantages accrue to a species as it becomes rare, as this is the defining feature of stable coexistence mechanisms. Without these fitness advantages, average fitness differences between species will lead to exclusion. However, empirical evidence is lacking, especially for forests, due to the difficulty of manipulating density on a large-enough scale. Here we took advantage of naturally occurring contrasts in abundance between sites of a subtropical tree species, Ormosia glaberrima, to demonstrate how low-density fitness advantages accrue by the Janzen–Connell mechanism. The results showed that soil pathogens suppressed seedling recruitment of O. glaberrima when it is abundant but had little effect on the seedlings when it is at low density due to the lack of pathogens. The difference in seedling survival between abundant and low-density sites demonstrates strong dependence of pathogenic effect on the abundance of host species.

Highlights

The overarching issue for understanding biodiversity maintenance is how fitness advantages accrue to a species as it becomes rare, as this is the defining feature of stable coexistence mechanisms
The decreasing function of per capita population growth with abundance that reflects fitness advantages at low density inversely implies disadvantages at high density. This high-density disadvantage prevents individual species from overwhelming other species and promotes species diversity in communities. These advantages at low density and disadvantages at high density appear from the perspective of an individual species as negative density dependence (NDD) but reflect stabilizing niche differences between species in a community[1], and in some older literature have been referred to as a community compensatory trend (CCT)[7,8]
Controversy arises from recent studies about the strength of NDD in rare species versus common species, which appears to contradict the predictions of the CCT9–11

Summary

Introduction

The overarching issue for understanding biodiversity maintenance is how fitness advantages accrue to a species as it becomes rare, as this is the defining feature of stable coexistence mechanisms. Stabilization mechanisms are essential for maintaining diversity in natural communities Such mechanisms require the per capita growth rate of a population to be a decreasing function of abundance so that a species is able to increase at low abundance. One of the major mechanisms hypothesized to be widely responsible for maintaining species diversity in forests is specialized or at least partially specialized natural enemies, originally proposed independently by Janzen and Connell[10,12,13,14,15,16,17,18,19,20,21,22] The key to this hypothesis is that host-specific pathogens or herbivores are maintained by or near adult trees, inhibiting the establishment and later success of conspecific seedlings in the vicinity. A sufficiently low adult density might lead to very little effect of natural enemies even near a conspecific adult tree

Methods

Results

Conclusion