Reefs, Resilience, and Refuges - Theoretical Considerations and Real-Life Examples

If global changes impact reefs relatively uniformly, reefs with viable population dynamics will be resilient but some, and maybe eventually all, corals may be forced into refuges. Resilience or refuge character can be rooted in a more clement environment, adaptation to a harsh environment, or readjustments of population (community) dynamics caused by modified mortality-, survival- and recruitment-characteristics among species. Since many “traditional reefal settings” (tropical reef crests and slopes) seem to be degrading, one may search for sustainable dynamics in different habitats such as upwelling areas, deep reefs, or non-reefs. While these habitats may harbor less species or individuals, their population dynamics may be more advantageous. Even if the environments appear less suitable and the communities depauperate, a positive net population growth identifies potential refuges. Also, pulse-instability within dominance hierarchies may promote, or be a sign of resilience. Opportunistic species may be naturally prone to large swings in abundance and find refuge in meta-populations that allow easy expansion or restriction of range. These species may adapt to only temporarily dominate communities of more resilient species. Their adaptation to changed environments may be less likely, since their reaction to stress would be population restriction and relocation into other, presumably more favorable, habitats. Temporary local absence would be part of survival strategy. Thus monitoring and assessment programs that primarily focus on coral cover, and not the underlying dynamics, may misunderstand resilience or refuge value of a habitat – since the present abundances are variable and may be misleading. Stochasticity of timing and distribution of impacts may allow apparently rich, but increasingly maladapted communities to persist, and lead to false conclusions regarding the nature of resilience or refuge. The presently richest areas may not be those with highest survivability. We use examples from all oceans to illustrate our point.