Coral reefs increasingly face a number of threats including climate

Coral reefs increasingly face a number of threats including climate change, ocean acidification, overfishing, and eutrophication. Due to a number of these threats, coral reefs have experienced a distressing decline in the past several decades, associated in many places with an increase in algal biomass. Accordingly, the loss of live coral cover is often associated with a phase-shift from a coral-dominated community structure to a fleshy algae dominated ecosystem.

There are many ways that this community shift can happen, and controversy exists over the relative importance of different factors in contributing to change. According to the bottom-up model, nutrient enrichment of coral reef areas results in an increase of benthic algae, which can grow quicker than coral in large blooms and out compete coral for resources, leading to reef degradation. In the top-down model, herbivory keeps macro algae biomass in check and allows the coral-dominated community to persist. Furthermore, disturbance events like bleaching can lead to a community shift because coral cover grows at a slower rate after a disturbance event and competitive interactions with macro algae become more frequent. The health of corals has come under more scrutiny particularly because of incidences of bleaching, which has been linked to heat stress. Thus, knowledge of coral physiology and coral-algae interaction is becoming increasingly relevant to understanding how corals can recover from stress events.

Previous studies have shown that a number of algae species have negative effects on coral health and physiology. For example, coral tissue mortality may be strongly affected by the presence of competing macro algae. However, in some cases, coral has been shown to actually overgrow and displace algae, while in other cases the algae has no significant impact on the coral. It seems the impact of macro algae on coral is dependent on both the coral and algae species, and that the competition between the two is highly variable, sometimes showing very little to no effect.

Despite the importance of understanding coral-algal competition and the phase-shifts in community structure, there has been very little previous experimental research on quantifying this interaction. Past studies have focused on the impact of herbivory or nutrient enrichment on algal growth and resulting competition with corals, but few quantify the effects of algae on coral physiology to measure the possible competition between the two.

Source - CrackVerbal - GMAT Reading Comprehension Guide

1. The primary purpose of the passage is to
(A) discuss the competition for survival that exists between the coral tissue and the macro algae.
(B) Evaluate the environmental consequences the loss of live coral cover has and suggest some reparative measures.
(C) Discuss a factor that could be causative for the loss of live coral cover and suggest the need for further research to ascertain its impact.
(D) Present a concern about the possible consequences of promoting macro algae population growth in marine ecosystems.
(E) Explain why a particular strategy to increase coral reef population has been less successful than was anticipated.

2. Which of the following can be inferred about macro algae?
(A) They are often more hardy than the coral reef during disturbance events like bleaching.
(B) They pose the biggest threat to the coral reef population.
(C) They are less likely than are the corals to be affected by competition from other marine organisms.
(D) Some species of macro algae are more competitive than others.
(E) Some macro algae are considered inedible by many herbivores resulting in their being unchecked by herbivory.

3. According to the passage, the decline of coral reef could occur because of all of the following EXCEPT?
(A) Substantial elevation of temperature
(B) Competition with other marine organisms
(C) Increased levels of acidity in water
(D) Herbivory of macro algae
(E) Unscrupulous fishing practices