IB DP Biology- B4.2 Ecological niches -FA 2025- IB Style Questions For HL Paper 2

Most reef-building corals contain photosynthetic algae, called Zooxanthellae, that live in their cells. Coral bleaching can occur as a result of human-induced changes leading to the Zooxanthellae being ejected from the coral.

a. State the type of interaction that occurs between Zooxanthellae and reef-building corals. [1]

b. State the trophic level of Zooxanthellae. [1]

c. When coral is bleached, certain organisms become more common in the ecosystem such as the cnidarian Gorgonia, the echinoderm Diadema, other algae and certain sponges. State the term that is used for organisms whose presence provides evidence of the existence of a particular environmental condition. [1]

d. A coat of algae builds up on coral reefs as a consequence of eutrophication. Explain the relationship between eutrophication and algal growth. [2]

e. Explain how an excessive growth of algae on coral reefs can be controlled by top-down factors. [2]