Does ant-plant mutualism have spillover effects on the non-partner ant community?

Mutualism benefits partner species and theory predicts these partnerships
can affect the abundance, diversity, and composition of partner and
non-partner species. We used 16 years of monitoring data to determine the
ant partner species of tree cholla cacti (Cylindriopuntia imbricata),
which reward ants with extrafloral nectar in exchange for anti-herbivore
defense. This long-term data revealed one dominant ant partner (Liometopum
apiculatum) and two less common partners (Crematogaster
opuntiae and Forelius pruinosus. We then used short-term
characterization of the terrestrial ant community via pitfall
trapping to sample partner and non-partner ant species across ten plots of
varying cactus density. We found that the dominant ant partner tended a
higher proportion cacti in plots of higher cactus density, and
was also found at higher occurrence within the pitfall traps in higher
density plots, suggesting strong positive feedbacks that promote ant
partner occurrence where plant partners are available. Despite
the strong association and increased partner occurrence, ant
community-wide effects from this mutualism appear limited. Of the
common ant species, the occurrence of a single non-partner ant species was
negatively associated with cactus density and with the increased
presence of L. apiculatum. Additionally, the composition and diversity of
the ant community in our plots were insensitive to cactus density
variation, indicating that positive effects of the mutualism on the
dominant ant partner did not have cascading impacts on the ant community.
This study provides novel evidence that exclusive mutualisms, even those
with strong positive feedbacks, may be limited in the scope of their
community-level effects.