Strong pairwise ant-plant mutualism has limited spillover effects on an
ant community
Abstract
Pairwise mutualisms are embedded within rich communities of co-occurring
species. Mutualism by definition benefits partner species and theory
predicts these effects can spill over into broader communities, with
consequences for abundance, diversity, and composition. Specifically,
positive feedbacks between partner species may shift competitive
hierarchies and drive reductions in non-partner species. We used
long-term monitoring data spanning 16 years to determine the ant partner
species of tree cholla cacti (Cylindriopuntia imbricata), which reward
ants with extrafloral nectar in exchange for anti-herbivore defense. We
coupled these long-term data with short-term characterization of the ant
community via pitfall trapping, which sampled partner and non-partner
species across ten plots that varied naturally in cactus density. The
long-term data revealed one dominant ant partner (Liometopum apiculatum)
and two other less common partners (Crematogaster opuntiae and Forelius
pruinosus). In the short-term census, the dominant ant partner occupied
more cacti in plots of higher cactus density, and was also found at
higher occurrence within the pitfall traps in the high cactus density
plots, suggesting strong positive feedbacks that promote ant occurrence
where plant partners are available. Despite the consistency of L.
apiculatum partnership with cacti through time and the dominance of this
partnership in areas of high cactus density, spillover effects from this
mutualism appear limited. Of the common ant species, a single
non-partner ant species showed a modest reduction in occurrence at high
cactus density, possibly in response to 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 on the dominant ant partner did not
have cascading impacts on the ant community. This study provides novel
evidence that pairwise mutualisms, even those with strong positive
feedbacks, may be limited in the scope of their community-level effects.