Mutualisms Alter Legume Niches At A Global Scale

Although mutualism is widely expected to broaden the realized niche of participating species, there have been no global syntheses examining how mutualism affects species niches. Here, we combine mutualism trait data on 2,668 legume species with geographic occurrence, climate, and soil nitrogen data. We use these data to investigate the effects of generalized defensive ant-plant mutualisms and more specialized nutritional legume-rhizobium mutualisms on the range of temperature, precipitation, and soil nitrogen values where legumes occur. Legumes engaged in mutualisms with ants or rhizobia occupied significantly more biomes and generally larger niches than non-mutualist species. Legumes with extrafloral nectaries, a trait that attracts a generalized guild of defensive ant partners, had wider niches across all three niche axes, regardless of latitude. Effects of interacting with rhizobia--a more specialized mutualism--on niche breadth were also mainly positive, but depended on latitude, with this mutualism increasing precipitation and soil nitrogen niche breadth only at high latitude, and increasing temperature niche breadth at low latitude, but decreasing temperature niche breadth at high latitude. Mutualism therefore affects niche breadth, but in ways that depend on interaction type and biogeographic trends that vary latitudinally.