title: Dispersal and spatial heterogeneity allow coexistence between enemies and protective mutualists.
abstract: Protective mutualisms, where a symbiont reduces the negative effects of another species on a shared host, represent a common type of species interaction in natural communities, yet it is still unclear what ecological conditions might favor their emergence. Studies suggest that the initial evolution of protective mutualists might involve closely related pathogenic variants with similar life histories, but different competitive abilities and impacts on host fitness. We derive a model to evaluate this hypothesis and show that, in general, a protective variant cannot spread from rarity or exclude a more pathogenic strain. While the conditions allowing mutualist invasion are more likely with increased environmental productivity, they still depend on initial densities in the invaded patch exceeding a threshold, highlighting the likely importance of spatial structure and demographic stochasticity. Using a numerical simulation approach, we show that regional coexistence is in fact possible in an explicitly spatial system and that, under some circumstances, the mutualist population can exclude the enemy. More broadly, the establishment of protective mutualists may be favored when there are other life-history differences from more pathogenic symbionts, such as vertical transmission or additional direct benefits to hosts.
authors.affiliation: Université Montpellier II, Institut des Sciences de l'Evolution, UMR 5554 Place Eugène Bataillon, 34095, Montpellier, CEDEX 05, France ; Département de Biologie, Université du Québec à Rimouski 300 Allée des Ursulines, Rimouski, Quebec, G5L 3A1, Canada ; Québec Centre for Biodiversity Sciences Montréal (QC), Canada ; School of Biological Sciences, University of Canterbury Private Bag, 4800, Christchurch, 8140, New Zealand.
authors.forename: Timothée
authors.initials: T
authors.lastname: Poisot
authors.affiliation: Department of Biology, Indiana University Bloomington, Indiana, 47405.
authors.forename: James D
authors.initials: JD
authors.lastname: Bever
authors.affiliation: CSIRO Plant Industry GPO Box 1600, Canberra, Australian Capital Territory, 2601, Australia.
authors.forename: Peter H
authors.initials: PH
authors.lastname: Thrall
authors.affiliation: Université Montpellier II, Institut des Sciences de l'Evolution, UMR 5554 Place Eugène Bataillon, 34095, Montpellier, CEDEX 05, France ; Santa Fe Institute Santa Fe, New Mexico, 87501 ; Wissenschaftskolleg zu Berlin Berlin, 14193, Germany.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Hochberg
keywords: Community ecology; coexistence theory; host-symbiont interactions; metacommunities; source-sink dynamics; spatial dynamics