Mutualism (biology)

Mutualism in biology is a kind of relationship shared between life forms in which all the participating organisms benefit from and may depend upon each other for survival, as defined in Herbert C. Hanson’s Dictionary of Ecology (1962). In other words, mutualism is an ideal and profitable partnership or interaction between species in the natural world. It is one of the most commonly observed, well-documented, and important phenomena in ecosystems and has provided insights into the ecosystem structure and function.

Background

Coined in 1873 by Pierre-Joseph van Beneden (1809–84), a zoologist and professor at the Catholic University of Leuven, Belgium, “mutualism” was first used to describe mutually beneficial relationships between species, particularly between “lower” and “higher animals.” The term has since been broadened to apply to any organisms that form such interdependent associations.

Mutualism can be classified into two major types: obligate mutualism and facultative mutualism. Obligate mutualism occurs when the partners or species involved in a mutualistic relationship absolutely need each other to be present in the relationship for successful survival. On the other hand, facultative mutualism occurs when one partner does not critically depend on the other and can lead an independent life on its own. In nature, many varieties of organisms exhibit mutualism.

One example of obligate mutualism is the relationship between a fig wasp, Ceratosolen solmsi, and the fig tree, Ficus moracae. The fig tree has its reproductive structure securely covered inside an unripe fig fruit. A female queen wasp carrying pollen grains (the male reproductive cells of plants) digs into the fig fruit to lay its own eggs. In the process, the pollen carried by the wasp is also released and thus fertilizes the fig. The wasp eggs hatch, and male and female wasps emerge. The newly hatched male wasps dig escape tunnels for the newly hatched female wasps to exit the fruit. The released female wasps repeat the entire process on a new tree, while the male wasps die inside the fig fruit. In this plant-insect relationship, the fig trees need the wasps for pollination and the wasps need the safety of the fig fruit to lay their eggs; thus, they critically depend on each other for their own survival. This is a demonstration of obligate mutualism.

Facultative mutualism can be seen in the interaction between the aphids, Aphis fabae, and the ants, Lasius niger. Aphids are tiny insects that survive by sucking the sap of their host plants. The aphids excrete excess sap as a sugary sticky by-product called “honeydew.” Rich in carbohydrates, this honeydew becomes an inviting food source for ants, attracting them to aphids. The ants feed off the honeydew and offer the aphids protection from predators in return. The sticky honeydew, if left to accumulate, may lead to fungal growth, which in turn may attract predators and endanger the aphids. However, the intervention of ants precludes this situation, as the ants clear off the honeydew and prevent accumulation. The aphids do not necessarily need the ants’ services for their survival, and the same is true for ants. Their interaction gives them an added advantage toward their survival and hence is seen as facultative mutualism.

Impact

Mutualism between species affects ecosystems directly, and any disruptions in these relationships may have detrimental effects on the environment. These effects may be local or global, depending on the species and the kind of mutual interaction involved.

One basic example of mutualism is the bee-flower interaction: the bee needs the flower’s nectar and the flower needs the bee for pollination. If the bee habitat is disturbed, the essential bee-flower relationship is broken. As a result, the flowers, which have no other means of pollination, are cut off from their pollinators, also affecting the livelihood of farmers dependent on those flowering plants and honey.

Another scenario involving mutualism is the host-microbe interaction of the human gut. The intestine hosts one hundred trillion microbial cells, and those microbes, through millions of years of evolution, have formed a mutually beneficial relationship with their hosts, the humans. Those microbes obtain their nourishment from the nutrient-rich environment of the human gut and, in turn, appear to offer a number of benefits to the human body, such as regulating immunity and maintaining homeostasis (internal equilibrium). An overdose of antibiotic treatment or adverse changes in food habits can put this relationship under stress and may wipe out these beneficial microbes. Consequently, diseases such as diabetes, obesity, and inflammatory bowel syndrome may develop in the body, demonstrating the importance of mutualism and its impact.

Mutualism plays a role in driving and regulating the process of evolution, and dangers arise from the destruction of these important relationships. The dangers that threaten these ecosystem relationships are primarily anthropogenic, that is, of human origin. Global climate change, pollution, and habitat fragmentation are consequences of primarily human activities, and these have a direct detrimental effect on mutualistic relationships, which are beneficial to humans and other life forms.

Bibliography

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