Close Encounters

The pros and cons of ant mutualism in epiphytic Tillandsia species

Ants provide protection and nutrients to some plant species but are there downsides to this mutualistic relationship?

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Myrmecophytes are plants that share a mutualistic relationship with ants. In tropical forests, these relationships can be highly diverse. Ants can protect the plants by deterring herbivores, they can help to disperse seeds and can also supply nutrients to the plants. Meanwhile, the plants provide ants with food and shelter. In tropical forests, ants are the dominant arthropods and can represent up to 30 % of arthropods’ biomass in the canopy.

In the Neotropics, at least 379 myrmecophytic plant species are known (including trees, shrubs, terrestrial herbs, lianas and epiphytes), represented by around 22 families. Several epiphytic Tillandsia species of the Neotropical family Bromeliaceae are myrmecophytes. Compared to terrestrial plant species, there have been few studies on the effects of insects on epiphytes. However, there has been some evidence that suggests the presence of myrmecophytic Tillandsia species could prevent the establishment of competing epiphytes because the ants associated with myrmecophytes could be lowering seed establishment.

The ants associated with the myrmecophyte Tillandsia caput-medusae reduce seed abundance of all the Tillandsia species and diminishes their probability of establishing new plants. Image credit: Vergara-Torres et al.

In their new study published in AoBP, Vergara-Torres et al. investigated whether ants associated with the myrmecophyte Tillandsia caput-medusae remove its seeds and the seeds of other sympatric non-myrmecophyte Tillandsia species in a tropical dry forest in south central Mexico. In the study, they recorded the diversity of ants associated with the species, and experimentally tested whether ants were associated with the removal of its seeds and the seeds of other sympatric non-myrmecophyte species of Tillandsia. They also investigated whether seed remotion by ants corresponded with epiphyte load in the preferred (Bursera copallifera) and limiting phorophyte species (B. fagaroides, Ipomoea pauciflora and Sapium macrocarpum).

In their study, they found that ants associated with the myrmecophyte Tillandsia caput-medusae lowered the density of seeds of all the Tillandsia species studied, including those of the myrmecophyte T. caput-medusae. This ant activity is harmful to the myrmecophyte and is more intense where ant diversity is higher, probably because competition between ants increases. Seed remotion by ants was found to be independent of phorophyte species. The authors concluded that although ants can provide benefits to T. caput-medusae they could also be lowering their abundance. They hope that future research will help to explain the effect of ants on other aspects of this species’ growth and reproduction.