[1] We often think of the natural world of plants and animals as a competitive struggle for survival, but nature provides us many examples of cooperation between species. One such relationship is called mutualism, in which two species benefit from each other’s activity. And one prime example of mutualism is the relationship between ants and a group of plants known as myrmecophytes. This group of plants includes many genera of trees, shrubs, flowers, and ferns, and each one has structural adaptations that help ants in their struggle for survival.
[2] The first plant adaptation that benefits ants is called domatia, which are hollow cavities in stems, leaves, spines, and even tubers. Without harming the plant, ants utilize these cavities for shelter and nesting. Another feature of myrmecophytes that is attractive to ants is food bodies, which are nutrient-rich parts of the plant that ants eat. In many cases, the plant’s food bodies serve as the ants’ primary food source. And finally, ants benefit from “extrafloral nectaries.” While we typically associate nectar with flowers, “extrafloral” nectaries are sugar-producing structures outside of flowers, typically on leaves and stems. This nectar is another food source for ants, and another reason for the ant to provide its own services to the plant.
[3] For one, ants may provide nutrition in return. Some species of myrmecophytes obtain virtually all of their nitrogen from the debris left by ants during the course of their activities in the nest. Studies have demonstrated that H. brunonis plants that bear domatia, and thus house ants, produce more fruit than those which lack domatia, a fact which serves as proof of the ants’ positive impact. Besides nutrition, ants also assist plants with seed dispersal. Ants collect seeds because of the nutritious food bodies found on their surfaces; when they transport them back to their colonies, they remove the food bodies and place the seeds in ideal conditions for growth. In fact, ants’ role in seed dispersal is so significant that by some estimates, 30% of the spring-flowering herbaceous plants in the eastern region of North America are the result of ants’ seed dispersal activities.
[4] Of course, if these plants provide ants with a home and food, the ants will be willing to actively defend them. The insects will bite and sting other insects who encroach on their plant homes as well as herbivores that attempt to eat the plants or their leaves. This is of enormous benefit to the plant, since its reproductive success depends, of course, on its survival. Ants will also protect their host from direct competition by pruning back other plants that threaten to crowd out the home plant.
[5] The mutualistic relationship between ants and myrmecophytes was not always fully understood, or even acknowledged. It wasn’t until 1910 that a botanist named William Morton Wheeler was able to demonstrate that ants not only benefitted from extrafloral nectar but actively worked to protect the plants that secreted it.
[6] Another important scientist was Barbara Bentley, whose work in the 1970s and 1980s further proved how ants and plants assist each other. In one of her studies, Bentley compared the seed production of six plants that grew without the help of ants, and 17 plants that were assisted by ants. The former produced an average of only 45 seeds per plant, whereas the latter produced an average of 211 seeds per plant. In these instances, ants helped to drive away grasshoppers by biting on their feet; ants also helped to drive away adult seed beetles by disturbing them as they lay their eggs or by eating their eggs.
[7] Another more recent study showed how ants attracted to extrafloral nectaries protect morning glories against intruding insects. Many such invaders prefer to feed on the morning glory’s flowers and fruits rather than its leaves. One such invader, the grasshopper, feeds indirectly on the flowers and therefore blocks the pollination and production of seeds. This destroys the stigma or the corolla which receives the pollen. Without the bright attractive a, flowers cannot attract pollinators; consequently, the plants are not fertilized. However, both grasshoppers and seed beetles are may be pushed away from the morning glories by ants, and the study demonstrated greater reproductive success when ants were present.
