In multi-species, facultative mutualisms, benefits to one partner often vary with the density and species identity of partners in the other guild. One such mutualism is the interaction between plants with extrafloral nectaries (EFN plants), and ants that visit them. The presence of other EFN plants might affect the density of ants visiting a plant, if competition for mutualists occurs. Long-term facilitation may also occur, in which proximity to other EFN plants is beneficial because the presence of more plants and thus greater nectar resources increases local abundance of ants. I asked whether plants compete intraspecifically for ants, and how that competition (or facilitation) affects plant performance and population growth of Colubrina spinosa (Rhamnaceae), a neotropical EFN-bearing treelet visited by >70 ant species. To assess competition for mutualists, I excluded ants from different conspecific neighbors, and then followed ant abundance on the focal plant. To consider facilitation, I manipulated nectar resources in 5x5m plots by adding artificial nectar or excluding ants from plants, and estimated local ant abundance over two years. I also followed plants in these plots (n=1032) to determine how ant density affects growth and survival. I then incorporated competition for ants and ant effects on plant demographic parameters into a stage-structured matrix model to predict long-term population growth of C. spinosa.
Results/Conclusions
Short-term manipulations of ants on groups of C. spinosa juveniles produced evidence of size-dependent competition for mutualists: larger neighbors tended to draw ants away from a focal individual while smaller neighbors were neutral or contributed ants. Conversely, facilitation was evident at slightly larger time and spatial scales. Overall ant abundance tended to increase in plots where nectar was added, and decrease in ant-excluded plots relative to the control, such that there was benefit to being near other EFN plants, although this benefit did not always offset small-scale competition effects. Benefit to plants, as measured by growth and survival, depended on ant density (controlled for plant size), ant species identity, and plant size, with very small plants receiving no benefit and the largest receiving the most. The fact that both competition for mutualists and benefits to plants are size-dependent could be important for long-term population dynamics of C. spinosa. Size-based intraspecific competition for ants redistributes ants among plants of different size classes, which have differential effects on population growth. If effects of competition for mutualists are strong enough, they could introduce a new source of density dependence.