Missing checkerboards? An absence of competitive signal in Alnus-associated ectomycorrhizal fungal communities.

Peter Kennedy,Nhu Nguyen,Hannah Cohen,Kabir Peay

doi:10.7717/peerj.686

Abstract

A number of recent studies suggest that interspecific competition plays a key role in determining the structure of ectomycorrhizal (ECM) fungal communities. Despite this growing consensus, there has been limited study of ECM fungal community dynamics in abiotically stressful environments, which are often dominated by positive rather than antagonistic interactions. In this study, we examined the ECM fungal communities associated with the host genus Alnus, which live in soils high in both nitrate and acidity. The nature of ECM fungal species interactions (i.e., antagonistic, neutral, or positive) was assessed using taxon co-occurrence and DNA sequence abundance correlational analyses. ECM fungal communities were sampled from root tips or mesh in-growth bags in three monodominant A. rubra plots at a site in Oregon, USA and identified using Illumina-based amplification of the ITS1 gene region. We found a total of 175 ECM fungal taxa; 16 of which were closely related to known Alnus-associated ECM fungi. Contrary to previous studies of ECM fungal communities, taxon co-occurrence analyses on both the total and Alnus-associated ECM datasets indicated that the ECM fungal communities in this system were not structured by interspecific competition. Instead, the co-occurrence patterns were consistent with either random assembly or significant positive interactions. Pair-wise correlational analyses were also more consistent with neutral or positive interactions. Taken together, our results suggest that interspecific competition does not appear to determine the structure of all ECM fungal communities and that abiotic conditions may be important in determining the specific type of interaction occurring among ECM fungi.

Highlights

A common way to assess the role of interspecific competition or facilitation in determining community structure is experimental manipulation involving the removal of neighboring individuals
We found that the ECM fungal communities in A. rubra forests displayed a different pattern of taxon co-occurrence compared to those seen for other ECM fungi
The results we obtained are consistent with the ‘stress gradient hypothesis’, which posits that species interactions shift from negative to positive as environmental conditions become harsher (Bertness & Callaway, 1994)

Summary

Introduction

A common way to assess the role of interspecific competition or facilitation in determining community structure is experimental manipulation involving the removal of neighboring individuals. The presence of certain bird species on a given island was associated with the absence of other species (and vice versa on other islands), resulting in a series of ‘forbidden species combinations’ or ‘checkerboard distributions’, which were posited to be the result of competitive exclusion (Diamond, 1975) This technique provided an important step forward in assessing the role of species interactions in field-based studies at the community level, but it has been frequently noted that analyses of species co-occurrence patterns need to include comparisons with patterns generated from communities assembled randomly to maximize inference (Connor & Simberloff, 1979; Gotelli & Graves, 1996)

Methods

Results

Conclusion