If a coral reef is exposed to stressful conditions that are known to cause bleaching, its fate is influenced by three key ecological attributes:
- Extent to which corals can withstand elevated stress without bleaching (resistance)
- Ability of corals to survive bleaching (tolerance)
- Ability of coral communities to be replenished (recovery) should significant coral mortality occur
An understanding of these attributes and the factors that influence them can inform management responses that aim to maximize the resilience of reefs to climate change.
The variability that characterizes bleaching events points to an important fact: individual corals vary in their bleaching responses to light and heat stress. Variability in the severity of bleaching response has been observed within individual coral colonies, among colonies of the same species, and across different species. These taxonomic variations are further compounded by spatial patterns, with corals of the same species often showing different bleaching responses at different locations. These patterns have been observed at scales ranging from meters to thousands of kilometers. Some of the factors that influence the coral response include:
- Sea-surface temperature patterns, especially at large scales
- Regional and local differences in weather
- Proximity to upwelling of cooler water
- Water currents and flow regimes
- Genetic identity of corals
- Genetic variation in zooxanthellae
- Severity of local stressors due to human activities
For corals that survive a bleaching event, characteristics that influence a return to a healthy coral reef community include:
- Severity of bleaching event: The severity of a bleaching event affects whether a coral loses zooxanthellae in response to thermal stress and also affects the amount of zooxanthellae lost.
- Immune system response: A weakened immune system is less capable of fighting off disease. Because coral reefs are weakened by a bleaching event, their ability to reproduce and fight diseases may be impaired.
- Metabolic adaptations: Persistence of these coral reefs may depend in part on the differential ability of some corals to acclimatize to warmer temperatures.
Reefs that suffer substantial mortality face different challenges than those where the majority of corals manage to survive the bleaching event. The biggest difference is the need for a much longer time period until returning to pre-bleaching structure. How long it takes a coral community to recover from bleaching related mortality depends on a variety of factors, including:
Connectivity and Recovery (2:20)
David Obura discusses connectivity and recovery of coral communities.
- Favorable recruitment conditions: These include good water quality, open hard substrate for settlement, presence of coralline algae (provide settlement substrate and chemical cues to facilitate coral settlement), and healthy herbivore populations.
- Larval supply: Regardless of how good recruitment conditions are (e.g., availability of substrate, presences of important herbivores), reefs require a robust supply of larvae from source reefs to recover following a disturbance event.
- Connectivity: Reefs with high mortality after bleaching depend on connectivity to other sources of live corals for re-seeding. For example, it is possible for reefs receiving great numbers of larvae from other source reefs to recover in a relatively short time span (~10 years), provided that recruitment conditions are favorable.
- Grazing: If important herbivores are missing, overgrowth by algae can slow reef recovery by taking up space that would otherwise be available to coral recruits.
- Natural selection: The recovery of coral reefs may be facilitated by settlement of larvae from nearby, more heat-resistant corals that survived the temperature-driven bleaching event. Over time, this could lead to heat-tolerant species increasing their distribution range into habitats previously dominated by other species.
- Synergistic effects: Factors not previously recognized as important to resilience, such as robust tissue regeneration, high competitive ability of the corals, seasonal dieback in a seaweed bloom, protection afforded by an effective marine protected area system, and moderate-to-good water quality, can result in rapid coral recovery.1