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Generalized mutualisms involve guilds of partners exchanging services with reciprocal fitness benefits. Partners within guilds vary in traits, affecting the quality of services and partner fitness. Research aimed at uncovering how partner interchangeability affects mutualistic outcomes largely considers variation in partner species identity. Aphaenogaster ants are mutualists, dispersing seeds of 30-40% of understory plants in North American forests that are adapted to dispersal by ants (myrmecochores). This dispersal by ants is highly specialized and disruptions due to climate change could greatly affect forest understory communities. Impacts such as changes in temperature could result in shifting functional ranges of species causing increased interspecific competition. A. rudis and A. picea are known seed dispersers that are polyphyletic and have overlapping ranges and phenology. We aim to uncover if functional variation partitions between named species and among separated populations. We collected colonies of three populations of each named species. We then performed lab behavioral trials where we measured foraging activity, rates of dispersal, and preference of seeds of four myrmecochore species with differing phenology. Using Principal component analysis and linear and generalized linear models, we found that behavior differed between the named species. A. picea had more workers actively foraging, while A. rudis moved more seeds. A. picea and A. rudis preferred seeds of different plant species, with early-foraging A. picea preferring Anemone acutiloba, a myrmecochore species that dehisces seeds earlier than other species. There was also variation in behaviors among populations, with more variation within A. rudis than within A. picea. Our findings suggest significant functional differences in this ecologically important species complex are partitioned primarily among putative species, but also varies within species. Our research also has implications for how climate change might affect this important interaction, as previous work shows that putative species have different thermal tolerances and foraging times.



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Uncovering How Partner Interchangeably Affects Mutualistic Outcomes In A Species Complex Of Seed-dispersing Ants