Ants and Plants

Ants have coexisted with flowering plants for at least 140 million years (Moreau et al., 2006) and have developed a large range of mutualistic interactions with plants. These include plants that provide resources for ants such as nesting sites, ants that disperse the seeds of plants, and plants that reward ants for protecting them from herbivors and competitors. The details of these interactions are diverse, involve numerous ant and plant species, and make fasinating reading.

The degree of association between plants and ants varies greatly. On one end of a spectrum of ant plant interactions are ants that use a plant exploitatively or in a relatively benign manner. Plants at the opposite extreme are obligate mutualists that have co-evolved with an ant partner and provide specialized nesting and food resources. Other plants fall somewhere between these two types of interactions.

Holldobler and Wilson (1990) provided a summary of ant-plant interactions in The Ants (Chapter 14). They note that complex symbioses have been fashioned among the thousands of species of ants and plants. Often these relationships are parasitic, with one exploiting the other and giving nothing in return. In other cases they are commensalistic, with one partner making use of the other but, as in the case of ants occupying hollow stems, neither harming nor helping it. But of maximum scientific interest, some symbioses appear to be mutualistic; in other words, both partners benefit from the association. To put the matter as briefly as possible, ants use cavities supplied by the plants for nest sites, as well as nectar and nutritive corpuscles given them as food. They in turn protect their plant hosts from herbivores, distribute their seeds, and literally pot their roots with soil and nutrients. There is abundant evidence that some pairwise combinations of ants and plants have coevolved so that each is specialized to use the other's services. This mutualistic linkage has produced some of the most elaborate adaptations known in nature.

Wilson and Hölldobler (2005) argued that interactions between ants and plants led to important changes in ant diversity and the evolution of ant life histories. This was driven by ecological change arising from plant diversification and the ant's utilizing plants for food and nest resources. These emerging and evolving changes allowed ants to expand their initial ecological success beyond what was a clade of predominately ground-dwelling predators.

Nelson et al. (2018) examined aspects of the evolutionary history of the relationships between ants and plants: estimating a time-scaled phylogeny of more than 1,700 ant species and a time-scaled phylogeny of more than 10,000 plant genera, they inferred when and how interactions between ants and plants evolved and assess their macroevolutionary consequences. They estimated that ant-plant interactions originated in the Mesozoic, when predatory, ground-inhabiting ants first began foraging arboreally. This served as an evolutionary precursor to the use of plant-derived food sources, a dietary transition that likely preceded the evolution of extrafloral nectaries and elaiosomes. Transitions to a strict, plant-derived diet occurred in the Cenozoic, and optimal models of shifts between strict predation and herbivory include omnivory as an intermediate step. Arboreal nesting largely evolved from arboreally foraging lineages relying on a partially or entirely plant-based diet, and was initiated in the Mesozoic, preceding the evolution of domatia. Previous work has suggested enhanced diversification in plants with specialized ant-associated traits, but it appears that for ants, living and feeding on plants does not affect ant diversification. Together, the evidence suggests that ants and plants increasingly relied on one another and incrementally evolved more intricate associations with different macroevolutionary consequences as angiosperms increased their ecological dominance.

Ant-Plant Topics

Ant Plants

Definitions

Hollobler and Wilson's The Ants as well as Campbell et al. (2022) define a number of specialized terms that are commonly used when discussing ant-plant mutualisms.

Ant garden: A cluster of epiphytic plants inhabited by ant colonies. To qualify as a true ant garden, the plants must benefit from the association. True ant gardens are known from both the Asian and New World tropics.

Ant plant: Also known as myrmecophyte; A species of plant with domatia (specialized structures for housing ant colonies).

Carton nests: Nests actively built by the majority of arboreal ants. Carton nests are made of plant materials, other debris, and ant secretions.

Beccarian bodies: The pearl bodies (a special kind of food bodies) produced by the stipules or young leaves of the Old World tropical genus Macaranga, and consumed by resident ants.

Beltian bodies: The food bodies found on the tips of the pinnules and rachises of some New World species of Acacia, and consumed by the resident Pseudomyrmex.

Domatia: Singular domatium. Also called myrmecodomatia; Plant structures that are used by symbiotic ants for nesting. Domatia can be derived from a variety of modified plant structures including stems, leaves, rachis, thorns, root tubers and hypocotyls.

Elaiosomes: Also known as arils; Specialized nutrient-rich, fleshy structures attached to ant-dispersed seeds. Stimulated by the attractants, ants transport the seeds to new locations, discard them after feeding on the elaiosomes, and hence aid in the dispersal of the seeds.

Extrafloral nectaries: Secretory organs, often no more than small patches of tissue, that produce sugary secretions, which possibly contain amino acids, attractive to ants and other insects. By definition, extrafloral nectaries are not involved in pollination, although they may occur on the flower outside the perianth.

Food bodies: Special nutritive corpuscles evolved by ant plants to feed ants; particular kinds of food bodies include Beltian bodies, Müllerian bodies, and pearl bodies.

Müllerian bodies: Food bodies produced by Cecropia trees on the trichilium (a pad at the base of the petiole) and consumed by resident ants, which are usually Azteca but also includes Camponotus balzani and Neoponera luteola.

Myrmecochory: The dispersal of seeds by ants stimulated by nutritive bodies (elaiosomes) or special seed attractants. Dispersal of seeds by granivorous ants without the aid of such specialized attractants is not included.

Myrmecophily: The general condition of encouraging ants. In the system proposed by van der Pijl (1955) and Beattie (1985), myrmecophily is used to denote ant pollination. However, Jolivet (1986) and other authors have used the expression to refer to the condition of being an ant plant, in other words possessing domatia or the same as myrmecophytism. Since the presence of domatia is so often accompanied by extrafloral nectaries, food bodies, and other coadaptive traits, it seems more appropriate to use the expression “myrmecophilous” in the broadest sense, comprising both pollination and myrmecophytism and all the accouterments they use to attract and award ants.

Myrmecophytism: The condition of being an ant plant, in other words possessing ant shelters (“domatia”).

Myrmecotrophy: The transport of soil, litter, and other nutrient-bearing materials by ants that results in the feeding of the plant hosts.

Pearl bodies: Also known as bead glands; A heterogeneous group of food bodies with a pearl-like luster and rich in lipids, protein and carbohydrates. They are similar to extrafloral nectaries; some plants offer pearl bodies to elicit a defensive response from ants.

Post-anthetic nectar rewards: Specialised food rewards of epiphytic Rubiaceae (Hydnophytinae), consisting of the large nectary discs, which become ant rewards once the corolla has fallen off. These nectaries maintain a high sucrose content by delaying fruit development. Symbiotic ants access these rewards by biting through the thick epidermis, which reduces attraction to opportunists.

References

Campbell, L.C.E., Kiers, E.T., Chomicki, G. 2022. The evolution of plant cultivation by ants. Trends in Plant Science (doi:10.1016/j.tplants.2022.09.005).
Chomicki, G., Renner, S.S. 2015. Phylogenetics and molecular clocks reveal the repeated evolution of ant-plants after the late Miocene in Africa and the early Miocene in Australasia and the Neotropics. New Phytol. 207, 411–424.
Hölldobler, B., Wilson, E. O. 1990. The Ants. Cambridge, Mass. Harvard University Press.
Lengyel, S. et al. 2010. Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey. Perspect. Plant Ecol. Evol. Syst. 12, 43–55.
Moreau, C.S. et al. 2006. Phylogeny of the ants: diversification in the age of angiosperms. Science 312, 101–104
Nelsen, M.P., Ree, R.H., Moreau, C.S. 2018. Ant-plant interactions evolved through increasing interdependence. Proceedings of the National Academy of Sciences of the United States of America. 115:12253-12258 (doi:10.1073/pnas.1719794115 10.1073/pnas.1719794115).
Weber, M.G., Keeler, K.H. 2013. The phylogenetic distribution of extraﬂoral nectaries in plants. Ann. Bot. 111, 1251–1261.
Wilson, E.O., Hölldobler, B. 2005. The rise of the ants: a phylogenetic and ecological explanation. Proc. Natl. Acad. Sci. U.S.A. 102:7411-7414 (doi:10.1073/pnas.0502264102 10.1073/pnas.0502264102).

Ant-Plant Topics

Plants Provide Resources for Ants

Seed Dispersal by Ants

Plant Rewards

Plant cultivation

Ant Plants

Definitions

References