Friday, September 5, 2014

Myrmecochory: ant distribution of seeds

Myrmecochory (literally ‘ant borne’) is a process whereby certain plants ensure the survival of their species after a fire. The seeds or fruits of literally hundreds of fynbos plants have a fleshy covering or attachment that acts as an ‘elaiosome’ or ant-attractant. When the seeds fall to the ground certain ant species, especially ants of the genus Anoplolepis (Pugnacious ants) rush to the seeds, attracted by pheromone-imitating scents. The ants sink their jaws into the soft, fleshy covering and tug the seeds into their nests, where the sweet-tasting reward is consumed. The hard, smooth nuts that are left cannot be gripped by the ants’ tiny mandibles, and so remain buried in their nests, safe from fire and animal predators. The seeds’ longevity is also enhanced by the anti-bacterial and fungicidal substances which the ants secrete to keep their crowded nests healthy. The fruits of Mimetes, Leucospermum, Paranomus and several other Proteaceous genera, all the Buchus or Rutaceae, many legumes and scores of other genera are involved in this important ecological process.
Serotinous fynbos plants have a creaky way of surviving fire, says William Bond. Serotiny means that they don’t release any seed until they’re burnt. When the plants die, the seed is released from cones or other structures. This works well unless the fire-interval is too short. If a second fire occurs before a population of serotinous proteas or other plants has matured, the species will become locally extinct. There is no safe seed store, and the species has to repopulate the area from outside—if it can. To achieve this it has to produce lots of seeds that can be carried for many miles on the wind, a pretty random process.
The re-emergence of Mimetes stokoei, thought to be extinct for nearly fifty years, dramatically demonstrates the success of myrmecochory, especially for large-seeded plants. Using ants to store seed safely in the ground is a process that is 180̊ different from serotiny. The seed does not have to travel at all, it drops straight to the ground and is buried by ants within a few minutes in the same optimal soil as its parent. These plants need to produce fewer seeds, because the chance of successful regeneration after fire is much less random. There is a safe underground seed store and not all the seeds germinate after each fire. A very short interval between two fires may have no effect. Finally, we now know that such seeds can remain viable for long periods—probably a century or longer. Tiny populations of rare species can survive in specialised habitats, apparently indefinitely—which is good news for the future of fynbos diversity. 

Mimetes stokoei

The unusual case of the extremely rare Mimetes stokoei shows how sustained interest by careful and dedicated observers over many decades can be very rewarding. If, for example, a common Leucospermum had disappeared from that habitat for over fifty years, it might not have been noticed —unlike in the case of the Mimetes. The longevity of its seeds would never have been recorded.

Invasive ants such as the Argentine ant – Linepithema humile – massively disrupt these processes. They not only completely eliminate most of the indigenous ants involved in myrmecochory, they don’t bury the affected seeds. They merely eat the elaiosomes off in situ, leaving the seed unburied and unprotected, at the mercy of rodents, birds, etc. Preliminary studies have shown that if the spread of these ants into wild habitats is not checked, the future of thousands of fynbos plant species will be at risk.

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