Accounting for environmental variation in co-occurrence modelling reveals the importance of positive interactions in root-associated fungal communities.

Nerea Abrego,Gleb Tikhonov,Björn D. Lindahl,Ayco J. M. Tack,Tomas Roslin,Panu Somervuo,Otso Ovaskainen,Tea Huotari,Niels Martin Schmidt

doi:10.1111/mec.15516

Abstract

Understanding the role of interspecific interactions in shaping ecological communities is one of the central goals in community ecology. In fungal communities, measuring interspecific interactions directly is challenging because these communities are composed of large numbers of species, many of which are unculturable. An indirect way of assessing the role of interspecific interactions in determining community structure is to identify the species co-occurrences that are not constrained by environmental conditions. In this study, we investigated co-occurrences among root-associated fungi, asking whether fungi co-occur more or less strongly than expected based on the environmental conditions and the host plant species examined. We generated molecular data on root-associated fungi of five plant species evenly sampled along an elevational gradient at a high arctic site. We analysed the data using a joint species distribution modelling approach that allowed us to identify those co-occurrences that could be explained by the environmental conditions and the host plant species, as well as those co-occurrences that remained unexplained and thus more probably reflect interactive associations. Our results indicate that not only negative but also positive interactions play an important role in shaping microbial communities in arctic plant roots. In particular, we found that mycorrhizal fungi are especially prone to positively co-occur with other fungal species. Our results bring new understanding to the structure of arctic interaction networks by suggesting that interactions among root-associated fungi are predominantly positive.

Highlights

Understanding the role of interspecific interactions in shaping ecological communities is one of the central goals in community ecology (Bairey, Kelsic, & Kishony, 2016; Werner & Peacor, 2003; Wootton, 1994)
Our results from arctic plant root systems suggests that positive interactions play an important role in shaping microbial communities
By showing that co-occurrences among fungal species are predominantly positive, our results have important consequences for understanding the structure of arctic interaction networks, where associations with symbiotic fungi play a pivotal role in plant nutrient acquisition (Hobbie & Hobbie, 2006)

Summary

| INTRODUCTION

Understanding the role of interspecific interactions in shaping ecological communities is one of the central goals in community ecology (Bairey, Kelsic, & Kishony, 2016; Werner & Peacor, 2003; Wootton, 1994). We focus on root-associated fungi, which form highly interactive communities with important effects on host plant fitness (Rodriguez, White, Arnold, & Redman, 2009; Smith & Read, 2008). Among root-associated fungi, mycorrhizal fungi and endophytes are usually studied separately, and their interactions and the effects of their interaction outcomes on plant fitness are largely unknown. We investigated co-occurrences among root-associated fungi, asking whether mycorrhizal and endophytic fungi co-occur with each other more or less strongly than expected based on their responses to environmental conditions and the host plant species. We expected positive co-occurrences to be prevalent between mycorrhizal and endophytic fungi, reflecting facilitative interactions as those previously shown for mycorrhizal fungi and bacteria (FreyKlett et al, 2011; Johansson et al, 2004)

| MATERIALS AND METHODS

| Study design

Findings

| DISCUSSION