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Plants that live in association with ants

Myrmecophytes (/m//ər/'/m//ɛ//k//ə//f//aɪ//t/; literally "ant plant") are plants that live in a mutualistic association with a colony of ants. There are over 100 different genera of myrmecophytes. These plants possess structural adaptations that provide ants with food and/or shelter. These specialized structures include domatia, food bodies, and extrafloral nectaries.

Myrmecophytes share a mutualistic relationship with ants, benefiting both the plants and ants. This association may be either facultative or obligate.

In obligate mutualisms, both of the organisms involved are interdependent; they cannot survive on their own. An example of this type of mutualism can be found in the plant genus Macaranga. All species of this genus provide food for ants in various forms, but only the obligate species produce domatia.

Facultative mutualism is a type of relationship where the survival of both parties (plant and ants, in this instance) is not dependent upon the interaction. Both organisms can survive without the other species. Facultative mutualisms most often occur in plants that have extrafloral nectaries but no other specialized structures for the ants. Different Acacia species provide a variety of resources needed for their codependent counterparts. One of these resources is the need for shelter. Acacia have enlarged thorns on their stems that are excavated by ants for use as housing structures. Tubers form when the hypocotyls of a seedling swells to form a hollow, chambered structure that can become inhabited by ants. Food bodies are identified by the main nutrient they contain and by the genus of plant producing them. In most cases of ant pollination, the ants are one of multiple pollinators, meaning that the plants are not completely dependent on ants for pollination. However, the orchid Leporella fimbriata can only be pollinated by its winged male ant partner (Myrmecia urens).

Myrmecochory, "ant dispersal," is the collection and dispersal of seeds by ants. Ants disperse more than 30% of the spring flowering herbaceous plants in eastern North America. The tropical tree Cecropia peltata obtains 98% of its nitrogen from the waste deposited by its ant counterparts. A recent study by Chanam et al. showed that plants bearing domatia can be favoured even before the establishment of a specialised protection based symbiosis, as nutritional benefits can be provided by a motley set of domatia residents that could include multiple species of ant (including protective, non protective and even plant damaging species such as Crematogaster dohrni) as well as other invertebrates, including as arboreal earthworms. Only some individuals of the myrmecophyte Humboldtia brunonis (found in the Western Ghats of India) bear domatia on some of their branches, while all individuals produce extrafloral nectar. Each domatium is formed by modified swollen and hollow internodes. These domatia have a self opening slit that allows access to the domatium interior and are prone to interloping residents (including many species of non protective ants and the arboreal earthworm Perionyx pullus) in addition to the protective ants. Earlier studies established that domatia bearing H. brunonis plants have greater fruit set, hence greater reproductive success, than H. brunonis plants without domatia. Plant tissues near domatia received 17% and 9% of their nitrogen from the ants (protective and non protective) and the earthworm respectively. The absorbed nutrients also travelled to distant branches; hence, fruit set was not different between branches with and without domatia. This study demonstrated that non protective interlopers in the domatia still contribute to the greater wellbeing of the plant by contributing to plant nutrition.

Since plants provide essential resources for ants, the need to protect the plant and those resources is extremely important. Many myrmecophytes are defended from both herbivores and other competing plants by their ant symbionts. Acacia cornigera, for example, is thoroughly guarded by its obligate ant partner, Pseudomyrmex ferruginea. A single colony of P. ferruginea may contain more than 30,000 ants, and can tend multiple Acacia trees. The soldier ants are extremely aggressive, patrolling the trees twenty four hours a day. Any disturbance to the tree alerts ants, who then recruit more workers from inside the horn domatia. These ants defend the Acacia by biting, violently stinging, and pruning any trespassers. The ants keep the plant free from other insects and vertebrate herbivores, from invading fungi and also from other plants.