Why are there so many myrmecochorous species in the Cape fynbos?

Abstract

The exceptionally high incidence of myrmecochory (ant-dispersed species) in Cape fynbos and similarly fire-prone vegetation on nutrient-poor soils in Australia has aroused much interest. An ecological advantage of myrmecochory on both continents is the removal of seeds to sites safe from seed predation. By failing to address evolutionary questions, ecological hypotheses do not explain the very high numbers of myrmecochorous species in fynbos. We show that myrmecochores are seldom dominant in fynbos communities although species numbers are invariably high. We also show that myrmecochores are not a random assemblage with regard to biological traits. Most myrmecochores are dwarf to low shrubs, obligately reseeding from relatively large seeds. Limited data suggest that myrmecochores have small, transient seed banks and are vulnerable to fire-induced local extinction. Myrmecochores were also significantly over-represented amongst a lowland neoendemic flora, suggesting that lineages possessing this trait are associated with recent diversification. Finally, in a general survey of the fynbos flora, we found that myrmecochorous genera were significantly more speciose than genera with other dispersal modes.

We conclude that the major ecological advantage of myrmecochory is the removal of large, precious seeds to sites safe from predators. Large seed size ensures seedling establishment in the nutrient-poor summer dry fynbos environment. Myrmecochores produce fewer seeds, however, and have smaller, less persistent seed banks than species with other dispersal modes. These traits, in combination with obligate reseeding and short dispersal distances, result in myrmecochore lineages being vulnerable to fire-induced population reduction and fragmentation. These processes also, and incidentally, promote diversification.