Ants as seed-planters (myrmecochory)

Walk through the Adelaide Hills scrub in autumn and there is a transaction going on beneath your boots that nobody sees. Fall from a wattle, a hakea, a grass-tree, and many of this country’s seeds arrive with a small parcel of fat glued to one end — an elaiosome, botanists call it, a purpose-built bribe with no other job than to smell irresistible to an ant. An ant finds the fallen seed, hauls it back to the nest — sometimes many times its own body length away — strips off and eats the elaiosome, and abandons the seed itself, now cleaned, in the rich, moist soil of the nest chamber. It is myrmecochory: seed dispersal by ants, and it is one of the quieter bargains running through the web of partnerships that hold this bush together, alongside the birds that pollinate for nectar and the fungi that trade water for sugar.

Nobody designed this as a service to plants, but the plant does remarkably well out of it. Buried in the nest, the seed escapes the beaks of seed-eating birds and the jaws of other ants, lands in soil already enriched by the colony’s own waste, and, in fire country, sits below the surface, insulated from the flames and heat that would otherwise cook it where it fell in the open. Australia has taken up this trick more than any other flora on Earth: something like 1,500 native plant species across 87 genera and 24 families have independently evolved elaiosomes, most of them concentrated in heath and dry sclerophyll shrubland — exactly the poor-soil, fire-prone country the Mount Lofty scrub exemplifies (Berg 1975).

So many unrelated plants hit on the same solution for a reason. Australia’s soils are ancient and starved of phosphorus, its summers run dry for months, and fire is a routine visitor rather than a catastrophe — three pressures that all reward a seed strategy that puts distance and depth between a parent plant and its offspring. Ants, tireless underground labourers already abundant in exactly this kind of country, were the ready-made courier service waiting to be hired. The transaction is invisible unless you know to look for it, but it is going on beneath almost every ridge and gully in the ranges, one greasy little seed at a time.