Foliar pathogens are unlikely to stabilize coexistence of competing species in a California grassland

Erin R Spear,Erin A Mordecai

doi:10.1002/ecy.2427

Abstract

Pathogen infection is common in wild plants and animals, and may regulate their populations. If pathogens have narrow host ranges and increase with the density of their favored hosts, they may promote host species diversity by suppressing common species to the benefit of rare species. Yet, because many pathogens infect multiple co-occurring hosts, they may not strongly respond to the relative abundance of a single host species. Are natural communities dominated by specialized pathogens that respond to the relative abundance of a specific host or by pathogens with broad host ranges and limited responses to the relative abundance of single host? The answer determines the potential for pathogens to promote host coexistence, as often hypothesized, or to have negligible or even negative effects on host coexistence. We lack a systematic understanding of the impacts, identities, and host ranges of pathogens in natural communities. Here we characterize a community of foliar fungal pathogens and evaluate their host specificity and fitness impacts in a California grassland community of native and exotic species. We found that most of the commonly isolated fungal pathogens were multi-host, with intermediate to low specialization. The amount of pathogen damage each host experienced was independent of host species local relative abundance. Despite pathogen sharing among the host species, fungal communities slightly differed in composition across host species. Plants with high pathogen damage tended to have lower seed production but the relationship was weak, suggesting limited fitness impacts. Moreover, seed production was not dependent on the local relative abundance of each plant species, suggesting that stabilizing coexistence mechanisms may operate at larger spatial scales in this community. Because foliar pathogens in this grassland community are multi-host and have small fitness impacts, they are unlikely to promote negative frequency dependence or plant species coexistence in this system. Still, given that pathogen community composition differentiates across host species, some more subtle feedbacks between host relative abundance and pathogen community composition, damage, and fitness impacts are possible, which could, in turn, promote either coexistence or competitive exclusion.

Highlights

Pathogens are ubiquitous in ecological communities (Burdon 1993, Gilbert 2002, Lafferty et al 2008)
We hypothesize that most plant pathogens are relatively host generalized (Spear et al 2015, Spear 2017) and, by extension, that their attack rates and impacts do not respond to the relative abundance of a single host species
In a California grassland, we found ubiquitous pathogen damage: most plants and 57% of all surveyed leaves had pathogen damage

Summary

Introduction

Pathogens are ubiquitous in ecological communities (Burdon 1993, Gilbert 2002, Lafferty et al 2008). Host-specific population regulation can promote species coexistence by suppressing species when they become common and providing a relative advantage to rare species (Fig. 1) This pathogen-mediated negative frequency-dependence, sometimes called the Janzen-Connell hypothesis (Janzen 1970, Connell 1971), has growing support in diverse plant communities, including tropical forests (Augspurger 1983, Augspurger and Kelly 1984, Gilbert 2005, Bagchi et al 2010, Bever et al 2015), temperate forests (Packer and Clay 2000), and temperate grasslands (Petermann et al 2008). We hypothesize that most plant pathogens are relatively host generalized (Spear et al 2015, Spear 2017) and, by extension, that their attack rates and impacts do not respond to the relative abundance of a single host species. Few studies have assessed whether these alternatives to pathogen-mediated stabilization occur in nature (but see Mordecai 2013)

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Discussion

Conclusion