What type of association does an epiphyte on a tree trunk illustrate?

1. Analyze the question An epiphyte (for example, an orchid, a fern, or Platycerium, the "elephant ear" fern) grows on the trunk or branches of a tree. We need to name the type of ecological association this shows.

2. Explain the concept

• Commensalism: one organism benefits and the other is neither harmed nor helped.
• Parasitism: one organism benefits and the other is harmed, because the parasite takes nutrients from its host.
• Saprophytism: an organism feeds on dead or decaying matter.
• Cannibalism: an animal feeds on others of its own species.
• Symbiosis: a close living-together relationship. In many textbooks it means mutualism, where both partners benefit.

3. Solution

• An epiphyte uses the tree only as a support. It gets more light and a higher position for its leaves, and it collects water and nutrients from rain, air, and debris, not from the tree's tissues.
• The tree is not harmed and gains nothing.
• So one partner benefits and the other is unaffected. This is commensalism.

Why not the others?

• Parasitism is wrong because the epiphyte does not draw food from the tree (unlike mistletoe, which is a true parasite).
• Saprophytism is wrong because the epiphyte makes its own food by photosynthesis.
• Cannibalism does not apply to plants.
• Symbiosis is too general. Strictly, commensalism is a type of symbiosis, but the more specific answer is the better one here. Since the tree gains nothing, it is not mutualism.

4. Answer The correct option is B. Commensalism.

5. Exam tip Remember the "+ / 0" rule: commensalism means one partner gains (+) and the other is unaffected (0). Epiphytes on trees, such as orchids and ferns, are the classic example. If the host is harmed (+ / −), it is parasitism.