By Fran Zak
Symbiosis is defined as a close living association of two or more organisms from different species. The rainforests that I visited in Australia and Panama provided me with many examples of symbiosis. There are several types of symbiosis. I can use a marriage analogy to help explain the differences between these types of symbiosis:
In commensalism, one member of the association is helped while the other is neither helped nor harmed. This is like a marriage where only one partner benefits. For example, I no longer have to pay for a landscaper because my husband helps me by taking care of the lawn and landscaping. Yet, since I work long hours, my husband still takes care of his own laundry and cooking. So, I benefit from this relationship, while my husband is not helped or harmed since he was doing all 3 chores (landscaping, cooking, and laundry) before the marriage anyway.
Mutualism is a living association where both members benefit by living together. This is the ideal marriage. Here, I do the laundry and cooking for both of us (this benefits my husband since he no longer has to do "indoor chores") and my husband takes care of all the landscaping and snow shoveling (so, you see how I benefit).
Finally, there is a marriage analogy for parasitism, where one member of the pair benefits while harming the other. This is a very unfortunate marriage where, let's say, the wife assumes all the indoor and outdoor household responsibilities while working full-time. The husband in this marriage is jobless and does not do any household chores. The husband represents the parasite, while the wife is the host. The parasite must be careful not to be so harmful as to cause death to its host since the host insures the parasite's survival, the free ride.
I saw two great examples of 3-way mutualism in Australian tropical forests. There was an arboreal termite nest high up on a tree with a large, conspicuous hole (approximately 4 cm. in diameter). This hole was created by a kookaburra bird. The bird hollows out part of the termite nest to use as its own nest, where it will lay its eggs. The kookaburra lines the bottom of its nest with twigs and mud. The termites benefit from the bird nest because the hole created by the bird helps ventilate and regulate the temperature of the termite nest in this hot, tropical environment. The kookaburra benefits because it has a safe nesting location where predatory animals will not disturb its young when they hatch. The third member of this mutualistic trio is a moth who lays its eggs so they will hatch at the same time as the kookaburra eggs hatch. The moth larvae eat excrement and parasites from the young kookaburra chicks. The moths benefit because they get a plentiful supply of food and a safe living location. The birds benefit because the moths keep their nest and chicks clean and free of excrement and parasites. This is a beneficial association for all three members-termites, kookaburras, and moths.
A second example of 3-way mutualism also involves termites. This time we look at the termite's digestive system (intestines). Termites do not have the necessary enzymes to digest wood, so they encourage a mutualistic protozoan to live in their gut (intestines). The protozoan benefits by getting a stable living environment, but the protozoan is also unable to digest wood, so it encourages a mutualistic bacterium to live within its cytoplasm. The bacterium also gets a stable living environment and all the wood it can digest since it has the necessary digestive enzymes. The bacterium shares its digested products with the protozoan who shares these usable food supplies with the termite, who in turn, supplies all the wood that the bacterium can digest by chewing wood into small particles. So, all three members of this mutualistic association (termite, protozoan, and bacterium) live together better than they could without each other.
Australia is not the only tropical forest where 3-way mutualism is seen. I saw a very good example of this in Panama rainforests. This involves Virola trees, toucans, and agoutis. Agoutis are small rainforest mammals that eat fruits and seeds. A Virola tree produces fruits high in the top of the tree and they are eaten by toucans. Agoutis cannot climb and so eat Virola seeds from fruits eaten by toucans that drop the seeds. In times of plenty, when there are too many seeds to eat, agoutis bury some seeds that they plan on digging up when food is in short supply in the dry season. You can imagine that an agouti might forget the location of some of its buried Virola seeds, which are now, in effect, planted under the soil and can germinate and grow when conditions are favorable. So, the toucan helps both the tree and the agouti. The tree supplies the food (seeds) for both animal species, and the agouti helps the tree by planting its seeds so new trees will grow.
I saw several examples of mutualism between 2 organisms, specifically lichens, in the Australian forests. I saw lichens growing on tree trunks, rocks, and even leaves. A lichen is a fungus and alga living together. The alga is photosynthetic, so it supplies food when sunlight is present. The fungus supplies moisture and gives the alga a good environment to live in. Again, both organisms benefit from this mutualistic living situation and lichens often grow in environments where either one or both of its components could not survive alone. This photo shows several different types of lichens growing in close proximity on a tree trunk.
While driving through Australian grasslands, I saw an example of commensalism, but I was not able to get a photograph of it. There were farms with cattle and some of the cattle had cattle egrets (white birds) perched on their backs. The bird gets an easy food source as the cow walks and stirs up insects in the grass. The cow does not mind having the bird on its back. As a matter of fact, if the bird were to eat parasitic insects off the cow's back, then it would be a true mutualistic situation.
Like a good marriage, the above examples of symbiosis on different continents show the adaptive value of mutualistic living situations. Both organisms benefit from living together. Symbiotic living situations seem to be universal throughout the biomes on earth.
Robbins, C. et al. Birds of North America. Racine, WI: Western Publishing Co., 1966.
Simpson, K. and N. Day. Birds of Australia. Princeton: Princeton University Press, 1999.
Pascack Valley HS
Jackie Willis, Ph.D.
Montclair State University
Upper Montclair, NJ
Special thanks to Professional Resources in Science and Mathematics (PRISM) at Montclair State University and Dr. Jackie Willis for making these ecology trips possible and for sharing her wealth of knowledge, her expertise, and her photographs with us.