Can fungal symbionts shift host niche dimensions to promote species coexistence?

Abstract

Defining the mechanisms that enable species to coexist is a fundamental goal of community ecology. Attention to this question has traditionally focused on antagonistic species interactions, namely competition and predation, as controls on species coexistence. However, positive species interactions may rival antagonistic interactions in their importance. We examined the potential role of endophytic fungi in enabling species coexistence, considering whether the presence of the fungal endophyte Neotyphodium sp. may alter the niche of marsh bluegrass, Poa leptocoma, and affect its coexistence with a closely related but endophyte-free grass species, nodding bluegrass, Poa reflexa, in subalpine meadows of the Rocky Mountains, USA. We tested two questions: 1) Do P. leptocoma and P. reflexa occupy different ecological niches? and 2) Does endophyte presence affect P. leptocoma and P. reflexa seed germination and seedling growth in different putative niches? Poa leptocoma grew closer to water sources and in wetter soils than P. reflexa, although the two species can be found intermixed in certain environments. Endophyte-symbiotic P. leptocoma seeds germinated with greater frequency in P. leptocoma niches relative to P. reflexa niches, whereas there was no difference in percent germination of endophyte-free (experimentally removed) P. leptocoma seeds or P. reflexa seeds between the two habitats. Endophyte-symbiotic P. leptocoma seeds germinated marginally more quickly in P. leptocoma niches relative to P. reflexa niches, and P. reflexa seeds germinated more quickly in P. reflexa niches. Our study indicates that P. leptocoma and P. reflexa occupy different ecological niches in the habitats that we examined, but that niche overlap does exist between the two species. Our results suggest that the fungal endophyte Neotyphodium sp. constrains the niche of Poa leptocoma to wetter microsites during early life history stages, thus contributing to partitioning the niches of the two species by limiting its host rather than providing it with greater niche opportunities. While endophyte symbiosis has been documented to convey tolerance to abiotic and biotic stresses, it may also play a more complicated role in influencing community composition in certain systems.

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