Symbiotic interactions are those in which two or more species interact very closely. These associations may be beneficial or negative to one or the other party, or neutral. There are innumerable instances of symbiotic interactions in rainforests, of which a few are mentioned here.
a. Ants and epiphytes: In the Neotropics some ant species collect the seeds of epiphytes and plant them in ant “gardens” which they fertilize with feces that they collect. The plants in turn provide starch and sugar secretions for the ants. The swollen tubers of some epiphytes (Rubiaceae, for example) provide housing chambers for other ants, while the ants in turn provide excrement and humus for the nourishment of the plant.
b. Ants and Macaranga trees: In Southeast Asian forests, Macaranga trees provide shelter for ants and entice them with starch grains, while the ants repel insect predators and cut off encroaching climbing plants. The small caterpillars of the Lycaenid butterfly Arhopala are tolerated in small numbers because they produce a sugary solution when they are touched by the ants, and so they eat tree leaves in safety.
c. Azteca ants and Cecropia trees: Most trees of the genus Cecropia are associated with ants, which live in their hollow stems and feed on glycogen-rich compounds exuded from organs at the bases of the leaf petioles. The most common ants found are leaf-cutter ants of the genus Azteca, which protect the trees against encroaching vines and against the invasion of other leaf-cutter ants (such as of the genus Atta). Cecropia trees which are home to ants are attacked less frequently than others, even if their leaves are more palatable than other species of Cecropia.
d. Leaf-cutter (attine) ants and fungi: For a long time we have known that certain species of ants have a “partnership” with fungi. One of the most complex associations of this type exists between attine ants and their fungi, associations which apparently have evolved over 50 million years (Currie, 2003). Ant species specialize in particular groups of fungi. Some species of Attini ants cut large numbers of leaves, carrying them long distances to chambers in their underground nests, which may extend over a considerable area and contain more than one thousand chambers. (Other ant species utilize instead vegetation, flowers, insect remains, or discarded matter such as dead grass.) The leaf-cutter ants chew the leaves into pulp and scrape away the leaves’ waxy coating, which is a defense against fungi. The leaves are then used to provide food for a leucocoprinid fungus, which the ants prune and fertilize, and which compliantly produces structures (“gongylidia”) which the ants eat. The fungus is grown from a small pellet which the queen ant carries from her mother’s nest to a new nest. The fungi normally contain insecticides as a defense mechanism, but when in the garden, they degrade these toxic compounds, removing them from the fungal tissue eaten by the ants. Recently, it has been realized that the ant-fungal association is even more complex. Yet another parasitic fungus of the genus Escovopsis inhabits fungus gardens and is apparently accidentally carried to the nest by hitching a ride on the ants’ bodies. When the garden is healthy, these parasitic fungi are restrained by an antibiotic exuded from bacilli of the genus Streptomyces, members of which live on certain surfaces of the ants’ bodies. But when the garden is stressed, or if the ants are removed, the Escovopsis fungi explode in numbers and overwhelm the fungal garden. Then the ant population will decline due to lack of food, at least until another garden can be established. It appears that still other compounds produced by the ants may act to inhibit the growth of alien bacteria and fungi which might invade the garden, although the exact roles of these secretions are not yet known (Ariniello, 1999; Currie, 1999, 2003).
e. Plants and butterflies: Certain Passifloraceae plants have odd relationships with Heliconiine butterflies. The butterflies lay their eggs on the tips of the plant shoots which the caterpillars like to eat. When there are no eggs on the shoots, the plant produces yellow nectaries which mimic eggs, or other structures (stipules) which look like young caterpillars. Thus the “occupied” leaves are ignored by butterflies and the plant is spared. However, some butterflies will probe to see whether or not eggs are actually present and thereby circumvent the plants’ defenses.
f. Figs and wasps: Very common are highly specific relationships between a pollinator species and a plant, such as those between figs and their wasp pollinators. Figs are dioecious, that is, they have separate male and female plants. Each species of fig has its own species of pollinating wasp, both sexes of which develop within “gall (male) flowers,” from which they escape and then mate inside a fig fruit. The male dies, and the female wasp leaves the fruit, picking up pollen from the male flower within the fig. She then flies to another tree which has young figs, and enters a fruit. If the fig is female, and contains female flowers, pollen on her body will fertilize them; seeds will subsequently form. Or, the fig may, rather, be “male” and have sterile gall flowers, in which she lays an egg. The wasp grub developing from this egg consumes the ovary of the gall flower and develops into an adult wasp. And so the cycle repeats itself.
g. Termites and pitcher plants: Flesh-eating is common among animals, rare among plants. Among the best-known of the carnivorous plants are the pitcher plants, which drown insects and other very small prey in their pitchers. The pitcher, which contains nectar or some sweet substance, generally has a slippery edge, so that insects attracted to the plant’s nectar will slip into the pitcher and not be able to climb out. The Southeast Asian pitcher plant, Nepenthes albomarginata, like some animals, has a distinct preference for what it eats – it likes termites. Thousands of termites, mainly of the genus Hospitalitermes, can be found in the pitchers of this plant in Brunei. The termite lives on fungi and algae, and also likes the rim hairs of pitchers, which attraction leads to the insects’ demise. The lead termites of a column return to the column after they have contacted the hairs (trichromes) of the pitcher, and their message entices the follower termites to the pitcher (Merbach, et al., 2002). In this case the termites, at least the ones which escape, get food, and so do the pitcher plants, although the advantage definitely seems to lie with the plant!