Plants often engage in ant-defense mutualisms, in which the plants provide extrafloral nectar (EFN) to attract the ants, and the ants remove potential herbivores. If the benefit to the plant varies with the number of mutualist ants present, and visits by ants are decreased (or increased) by the presence of neighbors, then competition (or facilitation) for ant mutualists will occur among neighboring plants. I manipulated ant presence on the EFN-bearing neotropical treelet species Colubrina spinosa (Rhamnaceae) in order to ask if EFN plants compete for mutualist ants. Using 80 pairs of C. spinosa saplings, I measured ant visits to a focal plant over 2 months in the presence of an ant-excluded or control conspecific neighbor. Additionally, I used ant exclusion and artificial nectar addition treatments to induce a wide range of values of ant presence on 300 C. spinosa saplings over 2 years. I measured herbivory damage, growth, and survival of these saplings in order to relate the level of ant attendance to plant performance.

Results/Conclusions

Results suggest C. spinosa plants may compete intraspecifically for ant mutualists. When ants were excluded from C. spinosa saplings growing near a conspecific neighbor, the number of ants on the neighbor plant increased over 2 months. Effects of sharing ants may depend on plant size. In turn, the benefit C. spinosa plants receive from mutualist ants appears to depend on the frequency of visits by ants and number of ants present. Benefit to plants may also depend on the ant species present (~60 ant species were observed to visit C. spinosa). After controlling for their size, plants with more ants exhibited increased growth and survival compared to plants with fewer ants. Therefore, the presence of a neighboring plant could lead to decreased ant visits and consequently lower plant performance. This may reduce plant fitness and thus introduce a source of density dependence mediated by competition for mutualists. EFN-bearing plants are abundant in tropical forests, suggesting that intra- and interspecific interactions among these plants could be an important influence on the diversity and distribution of tropical plants.