Trees harbouring ants are better defended than con-generic and sympatric ant-free trees

Abstract

Plant strategies against herbivores are classically divided into chemical, physical, biotic defences. However, little is known about the relative importance of each type of plant defence, especially in the same species. Using the myrmecophyte Triplaris americana (both with and without ants), and the congeneric non-myrmecophyte T. gardneriana, we tested whether ant defence is more effective than other defences of naturally ant-free myrmecophytes and the non-myrmecophyte congeneric species, all spatially co-occurring. In addition, we investigated how plant traits vary among plant groups, and how these traits modulate herbivory. We sampled data on leaf area loss and plant traits from these tree groups in the Brazilian Pantanal floodplain, and found that herbivory is sixfold lower in plants with ants than in ant-free plants, supporting a major role of biotic defences against herbivory. Whereas ant-free plants had more physical defences (sclerophylly and trichomes), they had little effect on herbivory—only sclerophylly modulated herbivory, but with opposite effects depending on ants’ presence and species identity. Despite little variation in the chemicals among plant groups, tannin concentrations and δ¹³C signatures negatively affected herbivory in T. americana plants with ants and in T. gardneriana, respectively. We showed that ant defence in myrmecophytic systems is the most effective against herbivory, as the studied plants could not fully compensate the lack of this biotic defence. We highlight the importance of positive insect-plant interactions in limiting herbivory, and therefore potentially plant fitness.