Myrmecochory ( (sometimes myrmechory); from ("ant") and khoreíā ("circular dance") is seed dispersal by ants, an ecologically significant ant–plant interaction with worldwide distribution. Most myrmecochorous plants produce seeds with elaiosomes, a term encompassing various external appendages or "food bodies" rich in lipids, amino acids, or other nutrients that are attractive to ants. The seed with its attached elaiosome is collectively known as a diaspore. Seed dispersal by ants is typically accomplished when foraging workers carry diaspores back to the ant colony, after which the elaiosome is removed or fed directly to ant larvae. Once the elaiosome is consumed, the seed is usually discarded in underground middens or ejected from the nest. Although diaspores are seldom distributed far from the parent plant, myrmecochores also benefit from this predominantly mutualistic interaction through dispersal to favourable locations for germination, as well as escape from seed predation.
Distribution and diversity
Myrmecochory is exhibited by more than 3,000 plant species worldwide and is present in every major biome on all continents except Antarctica. Seed dispersal by ants is particularly common in the dry heath and sclerophyll woodlands of Australia (1,500 species) and the South African fynbos (1,000 species). Both regions have a Mediterranean climate and largely infertile soils (characterized by low phosphorus availability), two factors that are often cited to explain the distribution of myrmecochory. Myrmecochory is also present in mesic forests in temperate regions of the Northern Hemisphere (i.e. in Europe and in eastern North America), as well as in tropical forests and dry deserts, though to a lesser degree. Estimates for the true biodiversity of myrmecochorous plants range from 11,000 to as high as 23,000 species worldwide, or about 5% of all flowering plant species.
Evolutionary history
Myrmecochory has evolved independently many times in a large number of plant families. A recent phylogenetic study identified more than 100 separate origins of myrmecochory in 55 families of flowering plants. With many independent evolutionary origins, elaiosomes have evolved from a wide variety of parent tissues. Strong selective pressure or the relative ease with which elaiosomes can develop from parent tissues may explain the multiple origins of myrmecochory. These findings identify myrmecochory as a prime example of convergent evolution. In addition, phylogenetic comparison of myrmecochorous plant groups reveals that more than half of the lineages in which myrmecochory evolved are more species-rich than their nonmyrmecochorous sister groups. Not only is myrmecochory a convergent trait, but it also promotes diversification in multiple flowering plant lineages.
Ecology
Myrmecochory is usually classified as a mutualism, but this is contingent on the degree to which participating species benefit from the interaction. Several different factors likely combine to create mutualistic conditions. Myrmecochorous plants may derive benefit from increased dispersal distance, directed dispersal to nutrient-enriched or protected microsites, and/or seed predator avoidance. Costs incurred by myrmecochorous plants include the energy required to provision diaspores, particularly when a disproportionate investment is made of growth-limiting mineral nutrients. For instance, some Australian Acacia species invest a significant portion of their yearly phosphorus uptake in producing diaspores. Diaspores must also be protected from outright predation by ants. This is typically accomplished by the production of a hard, smooth testa, or seed coat.
Few studies have examined the costs and benefits to ants participating in myrmecochory. Much remains to be understood about the selective advantages conferred upon myrmecochorous ants.
No single hypothesis explains the evolution and persistence of myrmecochory. Instead, a combination of beneficial effects working at different spatiotemporal scales likely contribute to the viability of this predominantly mutualistic interaction. Three commonly cited advantages to myrmecochorous plants are increased dispersal distance, directed dispersal, and seed predator avoidance.