Commensalism is a fascinating ecological interaction where one species benefits while the other is neither helped nor harmed. This relationship plays a subtle but important role in maintaining the balance and diversity of ecosystems. Understanding commensalism helps us appreciate the complexity of nature and the many ways organisms coexist.

What Is Commensalism?

Commensalism is defined as a symbiotic relationship in which one organism (the commensal) derives a benefit, such as food, shelter, or transportation, while the other organism (the host) is unaffected—experiencing neither a positive nor negative impact. This “win-neutral” scenario distinguishes commensalism from other interactions like mutualism (where both benefit) and parasitism (where one benefits at the expense of the other)456.

Classic Examples of Commensalism

• Epiphytic plants on trees: Orchids and other epiphytes grow on the branches or trunks of trees to access sunlight and air moisture. The tree provides physical support but is not harmed or helped by the presence of the epiphyte2568.

• Barnacles on whales: Barnacles attach to the skin of whales, gaining mobility and access to nutrient-rich waters, while the whale is unaffected578.

• Remora fish and sharks: Remoras hitch rides on sharks, feeding on leftover food scraps, but do not affect the shark’s health or behavior468.

• Cattle egrets and grazing animals: Cattle egrets follow livestock, eating insects disturbed by the animals, while the livestock are unaffected27.

The Orchid and Mango Tree: A Textbook Case

Among the options given, the interaction between an orchid plant growing on the trunk of a mango tree is a textbook example of commensalism. Here’s why:

• Benefit to the orchid: The orchid gains a place to live, better access to sunlight, and improved air circulation, which are essential for its survival and reproduction.

• Effect on the mango tree: The mango tree is not harmed or helped by the presence of the orchid. The orchid does not take nutrients from the tree or cause any damage to its structure568.

This relationship is a classic example of inquilinism, a type of commensalism where one organism lives on or within the habitat of another without causing harm56.

Why Other Options Are Not Commensalism

• Nitrogen-fixing bacteria in legume roots: This is mutualism; both the bacteria and the plant benefit.

• Microbes in the human gut: This is typically mutualism, as microbes help with digestion and humans provide a habitat.

• Female mosquito feeding on human blood: This is parasitism, as the mosquito benefits and the human is harmed.

Conclusion

Commensalism is a unique ecological relationship where one species benefits and the other remains unaffected. The best example among the options provided is the orchid plant growing on the trunk of a mango tree. This interaction highlights the subtle ways organisms can coexist and thrive without competing for resources or causing harm.

Correct answer: (4) orchid plant growing on the trunk of mango tree