Investment in reward by ant-dispersed plants

Myrmecochory is a mutualistic interaction in which ants disperse the seeds of plants. The defining feature of such plants is the elaiosome, a lipid rich seed appendage that serves as a reward to the ants. It is estimated that ~11,000 plant species worldwide display adaptations for dispersal of seeds by ants and ~100 ant species are considered effective seed dispersers. Myrmecochorous plants make up 40 % of the herbaceous flora in some regions that are considered “myrmecochory hotspots”, such as eastern North America, South-African fynbos and Australia. The strength of the interaction between plant and ant varies with the quality of the reward, the quality of dispersal provided by the ants and the environmental context. It is however unclear whether myrmecochorous specialization is consistent along geographic gradients.

Levine et al. investigate this uncertainty in a recent study published in AoBP. First, they estimated variation in reward investment by co-occurring myrmecochores along a steep environmental gradient in a Mediterranean region. Second, they tested whether variation in plant investment in reward was positively and consistently correlated with the quality of dispersal plant received along the same gradient. The authors found that among co-occurring plant species, those that produced larger rewards were preferred by high-, but not by low-quality seed-dispersing ant species. This preference pattern was consistent along a sharp geographic gradient in a Mediterranean ecosystem,even in the presence and dominance of apparently low- quality dispersers, which remove seeds regardless of the plant investment. This may indicate that the selection by scavenging ants is sufficiently strong to justify the investment in reward even when low-quality partners are common. Alternatively, the investment in reward observed in this system may result from and reflect selection pressures that operated elsewhere at other times, but are fixed at the species level. A study that focussing on spatial heterogeneity in the functional benefits of myrmecochory along the gradient will be needed to tease apart these explanations.