COS 16-4 - Uncovering intra- to interspecific variation in mutualist partner quality within a species complex of keystone seed-dispersing ants

Background/Question/Methods

Generalized mutualisms, such as seed dispersal by ants (myrmecochory), involve guilds of mutualistic partners that exchange services. Partners within guilds vary in traits that affect the quality of mutualistic services. Research aimed at uncovering within-guild variation in partner quality primarily considers the identity of partner species. However, mutualistic traits vary within species, especially for populations in states of incomplete speciation where the distinction between intra- and interspecific variation is not well defined. Ants in the genus Aphaenogaster are mutualists and the major disperser of seeds of understory plants adapted to dispersal by ants (myrmecochores) in eastern North American forests. A. rudis and A. picea are polyphyletic with overlapping morphological features. We ask if functional variation partitions discretely between named species or along a continuum in this species complex.

Results/Conclusions

We collected replicate colonies of three populations, each of named species. We performed 2D morphometrics on the thorax to quantify complex morphological variation among colonies. We performed lab behavioral trials, measuring behaviors that influence seed dispersal quality such as foraging, seed dispersal rates and preference, and intra- and interspecific aggression. Morphometrics revealed differences between species but also continuous variation in diagnostic features. We found behavioral differences between species, with A. rudis being a better disperser and having higher intraspecific aggression and A. picea being a more active forager and dominant over A. rudis. Behavior varied along a continuum between species corresponding with colony morphometrics, with intermediate phenotypes having intermediate behavior. This work shows that it is important to consider partner identity in species complexes of mutualistic partners at multiple levels of biological organization.