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seeds for ants

11.3.2. Myrmecochory : seed dispersal by ants

Many ants are seed predators that harvest and eat seeds (section 11.2.5). Seed dispersal may occur when seeds are accidentally lost in transport or seed stores are abandoned. Some plants, however, have very hard seeds that are inedible to ants and yet many ant species actively collect and disperse them, a phenomenon called myrmecochory. These seeds have food bodies, called elaiosomes, with special chemical attractants that stimulate ants to collect them. Elaiosomes are seed appendages that vary in size, shape, and color and contain nutritive lipids, proteins, and carbohydrates in varying proportions. These structures have diverse derivations from various ovarian structures in different plant groups. The ants, gripping the elaiosome with their mandibles (Fig. 11.9), carry the entire seed back to their nest, where the elaiosomes are removed and typically fed to the ant larvae. The hard seeds are then discarded, intact and viable, either in an abandoned gallery of the nest, or close to the nest entrance in a refuse pile.

Myrmecochory is a worldwide phenomenon, but is disproportionately prevalent in three plant assemblages: early flowering herbs in the understorey of north temperate mesic forests; perennials in Australian and southern African sclerophyll vegetation; and an eclectic assemblage of tropical plants. Myrmecochorous plants number more than 1500 species in Australia and about 1300 in South Africa, whereas only about 300 species occur in the rest of the world. They are distributed amongst more than 20 plant families and thus represent an ecological, rather than a phylogenetic, group, although they are predominantly legumes.

This association is of obvious benefit to the ants, for which the elaiosomes represent food; and the mere existence of the elaiosomes is evidence that the plants have become adapted for interactions with ants. Myrmecochory may reduce intraspecific and/or inter- specific competition amongst plants by removing seeds to new sites. Seed removal to underground ant nests provides protection from fire or seed predators, such as some birds, small mammals, and other insects. Postfire South African fynbos (plant) community structure varies according to the presence of different seed dispersing ants (Box 1.2). Furthermore, ant nests are rich in plant nutrients, making them better microsites for seed germination and seedling establishment. However, no universal explanation for myrmecochory should be expected, as the relative importance of factors responsible for myrmecochory must vary according to plant species and geographical location.

Myrmecochory can be called a mutualism, but specificity and reciprocity do not characterize the association. There is no evidence that any myrmecochorous plant relies on a single ant species to collect its seeds. Similarly, there is no evidence that any ant species has adapted to collect the seeds of one particular myrmecochorous species. Of course, ants that harvest elaio- some-bearing seeds could be called a guild, and the myrmecochorous plants of similar form and habitat also could represent a guild. However, it is highly unlikely that myrmecochory represents an outcome of diffuse or guild coevolution, as no reciprocity can be inferred. Elaiosomes are just food items to ants, which display no obvious adaptations to myrmecochory. Thus, this fascinating form of seed dispersal appears to be the result of plant evolution, as a result of selection from ants in general, and not of coevolution of plants and specific ants.

Myrmecochory: seed dispersal by ants / The Insects

Harvester ants farm by planting seeds to eat once they germinate

Tschinkel WR, Kwapich CL (2016), DOI: 10.1371/journal.pone.0166907

They’ve cracked it. Small ants carry home large seeds to eat all the time, but no one knew exactly how they managed to break through the seeds’ tough exterior.

It turns out that Florida harvester ants, Pogonomyrmex badius, have developed a clever farming strategy to do so – they plant seeds, wait for them to germinate and then eat the soft spoils.

Some 18 genera of ants harvest seeds, and colonies of some species can store more than 300,000 seeds in their underground granaries.

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So far, scientists thought that ants must be able to break the seeds open and just ate them as they were. “The reality is a lot more interesting,” says Walter R. Tschinkel at the Florida State University.

“There are many studies of seed choice by forager harvester ants, but none of the authors asked the question of whether the ants can open the seeds,” says Tschinkel. “This may be in part because most of these studies were done on western harvester ants whose deep nests are in hard soil, so the seed chambers are not easily excavated.”

Tschinkel WR, Kwapich CL (2016), DOI: 10.1371/journal.pone.0166907

With his team, Tschinkel excavated and studied approximately 200 P. badius nests and found that the ants mostly open and consume small seeds, which are easier to crack. Foragers collect seeds of all sizes, so this leads to the accumulation of larger seeds, which end up forming 70 per cent of stored seeds by weight.

In a series of lab and field experiments Tschinkel and colleagues showed that P. badius doesn’t seem to be able to open the large seeds unless they have germinated first. Even the caste of ants with large heads and mandibles thought to be specialised for seed opening can’t crack the big seeds.

Germination, on the other hand, splits the tough husk, making the seed contents available as food for the ants. A single large seed may have nutritional value of 15 smaller seeds, so it makes sense to collect it and wait for it to crack open. Seeds from various species germinate at different times, which may give the ants a steady supply of their “crop”.

This is the first example of ants relying on germination to consume large seeds, although some worms seem to do it, too. The only other example of ants farming plants for food is of the Fijian ant Philidris nagasau, which grows Squamellaria plants and harvests their fruit.

The ants’ unusual trick lets them snack on seeds that are too big from them to crack. They just let the seeds crack themselves