Uncovering how partner interchangeably affects mutualistic outcomes in a species complex of seed-dispersing ants

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William R. Smisko, Carmela M. Buono, Andrew J. Lupinski, Gabby I. Quartuccia, Thomas H.Q. Powell and Kirsten M. Prior, Department of Biological Sciences, Binghamton University (SUNY), Binghamton, NY

Background/Question/Methods
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. However, functionally important traits can vary within and among species, especially for populations that exist in states of incomplete speciation where the distinction between intra- and interspecific variation is not well defined. In these cases, traits that affect partner quality may vary among species, genetically distinct populations, or along a continuum. Aphaenogaster ants are mutualists, dispersing seeds of 30-40% of understory plants in North American forests that are adapted to dispersal by ants (myrmecochores). A. rudis and A. picea are known seed dispersers that are polyphyletic and have overlapping morphological features. We aim to uncover if functional variation partitions between named species or along a continuum between species. We collected colonies of three populations of each named species. We performed morphometric analysis of complex thorax variation to determine if colonies group in discrete species or along a continuum. We then performed lab behavioral trials, where we measured foraging, dispersal, and preference of seeds of four myrmecochore species.
Results/Conclusions
Our morphometric analysis revealed that two axes (Principal component analysis) explained complex thorax variation, one of which significantly varied among species and populations along a largely continuous gradient. Many colonies showed intermediate morphological phenotypes between A. picea and A. rudis. We performed linear or generalized linear models to examine if interspecific variation between putative species and colony morphology affected foraging activity, seed dispersal, and seed preference. 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. Further, intraspecific variation in the form of continuous morphological variation across the complex explained considerable variation in foraging and dispersal behaviors. Our findings suggest important functional differences in this ecologically important species complex are partitioned not only among putative species, but also along a continuum. Our research also implicates how climate change might affect this interaction, as previous work shows that putative species have different thermal tolerances and foraging times.