Myrmecophytes

An ant-plant, also known as a myrmecophyte, is a type of plant that has a symbiotic relationship with ants. These plants have evolved specialized adaptations to attract, house, and sometimes feed ants, while the ants provide various benefits to the plants. The relationship between ant-plants and ants can range from mutualism, where both parties benefit, to commensalism, where the ants benefit but the plants are neither helped nor harmed.

There are several types of interactions between ant-plants and ants:

Domatia: Many ant-plants provide shelter and nesting sites for ants through specialized structures called domatia. These are hollows, pouches, or cavities found in various parts of the plant, such as leaf bases or stems, where ants can live and raise their brood.

Food Bodies: Some ant-plants produce food bodies, which are small structures containing nutrient-rich substances that ants feed on. These food bodies can be produced on leaves or other plant parts and serve as a food source for the ants.

Extrafloral Nectaries: These are nectar-producing glands found on plant parts other than flowers, such as leaves or stems. Ants are attracted to the sugary nectar, which can provide them with a consistent food source.

Protection: In exchange for shelter and food, ants offer protection to the ant-plants. They defend the plants against herbivores, competing plants, and sometimes even prevent parasitic plants from establishing themselves on the ant-plants.

Nutrient Cycling: Ants contribute to the nutrient cycling of the ecosystem by bringing in organic matter, such as dead insects, which they use to feed their brood. These materials can enrich the soil around the ant-plants.

Examples of well-known ant-plants include acacias and certain species of epiphytic plants in the bromeliad family. Acacias have swollen thorns that provide nesting sites for ants, and in return, the ants protect the acacias from herbivores. Epiphytic ant-plants, like some bromeliads, have hollow leaf bases that provide shelter for ants and may contain food bodies as well.

Introduction

Co-evolved species typically have specialized structures and life histories that are intimately tied to their mutualistic ant-partners. One example is the epiphyte Dischidia major. Highly modified leaves of the plant (‘pitchers’) provide lodging for numerous ant species, especially those in the genus Philidris. In return, the ants bring organic debris into their nest, which provides nutrients to the plants when it is placed with branching adventitious roots that grow within the inhabited pitchers.

Cluster of pitcher leaves of Dischidia major growing on a tree trunk. Note the normal leaves at bottom. From Songkla, Thailand (Photo by Christian Peeters).

Philidris ants build soil runways to connect the entrances of adjacent pitchers of Dischidia major (the central stem can no longer be seen). (Photo by Christian Peeters).

Inhabited pitcher cut away to reveal Philidris workers and brood, as well as roots and inner partitions. (Photo by Christian Peeters).

Philidris build inner partitions by using the internal roots as framework. Ants bring debris from outside as construction material. These roots allow the epiphyte to use ant faeces and food remains as a nitrogen source. (Photo by Christian Peeters).

Myrmecophytic Plants

Acacia

The taxonomy of this genus has changed dramatically in recent decades. Once treated as a large group of species with a widespread distribution across the areas regions of the world, it was found to be polypyhletic in contemporary phylogentic studies. It was proposed to maintain Acacia for only some of the species, with others falling under other generic names, e.g., Vachellia, Mariosousa and Turnera . This proved to be controversial, there has not been a consensus as to what is the best solution, and many species are referred to by different names as there is more than one classification system in use.

The acacias of the New World tropics (Vachellia) are pioneer species that form mutualisms with Pseudomyrmex ants. The interactions between these plants and ants ants were famously written about by Belt (1874) in A Naturalist in Nicaragua. These ant-plant interactions were also the subject of Janzen's classic studies (e.g. Janzen 1966) of mutualism that are often detailed in ecology textbooks. The plants provide food and shelter for the ants and the ants in turn attack herbivores and destroy any encroaching plants around the base of the tree.

Some Pseudomyrmex species, for example Pseudomyrmex gracilis and Pseudomyrmex nigropilosus, are considered a parasite of the acacia-ant mutualism. Pseudomyrmex nigropilosus will use the hollow-thorn domatia for nesting and exploit the food produced by the plants, i.e, beltian bodies and nectar from extrafloral nectaries, but they are not effective at decreasing herbivores and do not remove encroaching vegetation.

Turnera velutina

(previously Acacia hindsii) A new world tropical and subtropical plant species. The majority of individual plants are inhabited by the aggressive Pseudomyrmex ferrugineus. The ants defend the plant from herbivores and enchroaching plants, with the plant in turn producing both hollow thorns for nesting and beltian bodies for food. Fonseca-Romero et al. (2019) found the opportunistic ant Pseudomyrmex gracilis, that can sometimes become established in this plant, effectively exploits the hollow thorns and beltian bodies without providing robust defensive functions like the very aggressive Pseudomyrmex ferrugineus. The lack of a strong defensive function alters the plant by its producing thicker leaves and reducing its production of food bodies.

Vachellia collinsii

(previously Acacia collinsii) A common new world acacia that is typically inhabitated by a single colony of the mutualistic species Pseudomyrmex spinicola, Pseudomyrmex nigrocinctus or Pseudomyrmex flavicornis

Cecropia

A neotropical genus of fast growing pioneer trees. Ants of the genus Azteca are most commonly found living in and patrolling these plants. Neoponera luteola nests exclusively inside the stems of Cecropia tessmannii trees in Peru, feeding on glycogen-rich Müllerian bodies produced by the plant to guarantee the protection of ants against herbivores.

Ant-Plant Ants

The following species have been reported associated with ant-plants.

References

Belt, T. 1874. The Naturalist in Nicaragua. E. Bumpus, London.
Janzen D.H. 1974. Epiphytic myrmecophytes in Sarawak: mutualism through the feeding of plants by ants. Biotropica 6: 237 – 259.
Hölldobler B. and Wilson E.O. 1990. The Ants. Cambridge, Mass. Harvard University Press.
Kaufmann E. and Maschwitz U. 2006. Ant-gardens of tropical Asian rainforests. Naturwissenschaften 93: 216 – 227.
Peeters, C. & D. Wiwatwitaya 2014. Philidris ants living in Dischidia epiphytes from Thailand. Asian Myrmecology 6: 49-61.
Fonseca-Romero, M. A., J. Fornoni, E. del-Val, and K. Boege. 2019. Ontogenetic trajectories of direct and indirect defenses of myrmecophytic plants colonized either by mutualistic or opportunistic ant species. Oecologia. 190:857-865. doi:10.1007/s00442-019-04469-y
Weir JS and Kiew R. 1986. A reassessment of the relations in Malaysia between ants (Crematogaster) on trees (Leptospermum and Dacrydium) and epiphytes of the genus Dischidia (Asclepiadaceae) including ‘ant-plants’. Biological Journal of the Linnean Society 27: 113 – 132.