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Resilience refers to the ability of biological or social systems to overcome pressures and stresses by maintaining key functions through resisting or adapting to change.<ref>Holling, C.S. 1973. Resilience and stability of ecological systems. ''Annual Review of Ecology and Systematics'', 4: 1-23.</ref> Reef resilience is the biological ability of reefs to recover from natural disturbances (e.g. storms and bleaching episodes).<ref> "http://coralreef.noaa.gov/conservation/keythreats/"</ref> Reef resistance measures how well coral reefs tolerate changes in ocean chemistry, sea level, and sea surface temperature.<ref>http://www.reefresilience.org/Toolkit_Coral/C1_Intro.html</ref> Reef resilience and resistance are important for the recovery of coral reefs from the effects of climate change and ocean acidification. Natural reef resilience can be used as a recovery model and opportunity for management of MPAs to protect coral reefs.

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== Thermal tolerance<br> ==

Corals rely on a symbiotic algae called zooxanthellae for nutrient uptake. Increases in sea surface temperature cause zooxanthellae to die and bleach corals. Difference in symbionts, determined by genetic groupings (clades A-H), explain thermal tolerance in corals <ref>Sampayo, E.M., Ridgway, T., Bongaerts, P., Hoegh-Guldberg, O., 2008. Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type. PNAS Environmental Sciences, Vol 105, No. 30:10444-10449 </ref>. Research has shown that some corals contain thermally-resistant clades of zooxanthellae. Corals housing clade D symbionts and certain types of thermally-resistant clade c symbionts allow the coral host to not bleach as severely, if at all, as other corals experiencing the same stressor.<ref> name=Berkelmans/&amp;gt; </ref> Scientists remain in debate if thermal resistance in corals is due to a mixing or shifting of symbionts, or thermally resistant vs. thermally-sensitive types of zooxanthellae. Species of coral housing multiple types of zooxanthellae can withstand a 1-1.5° C change in temperature.<ref> Berkelmans, R. and M.J.H. van Oppen, 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. ''Proceedings of the Royal Society of London Series B'', 273: 2305–2312.</ref> However, few species of coral are known to house multiple types of zooxanthellae. Corals are more likely to contain clade D symbionts after multiple coral bleaching events.<ref> Berkelmans, R. and M.J.H. van Oppen, 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. ''Proceedings of the Royal Society of London Series B'', 273: 2305–2312.</ref>

== Reef Recovery<br> ==

Research studies of the Mediterranean species of coral ''Oculina patagonica'' show that the presence of endolithic algae in coral skeletons may be responsible for post-bleaching recovery as an alternative source of energy.<ref> Fine &amp;amp;amp; Yossi, 2002.</ref> During bleaching, the loss of zooxanthellae decreases the amount of light absorbed by coral tissue, which allows more photosynthetically active radiation to penetrate the coral skeleton. Greater amounts of photosynthetically active radiation cause in coral skeletons cause an increase in endolithic algae biomass and increased production of photoassimilates.<ref> Fine &amp;amp;amp; Yossi, 2002. </ref> As the energetic contribution of zooxanthellae to coral tissue decreases during bleaching, the contribution of phototrophic endoliths increases. The translocation of endolith assimilates as an alternative energy source in corals could explain the rapid recovery and survivorship of ''O. patagonica'' after bleaching events.<ref> Fine &amp;amp;amp; Yossi, 2002. </ref>

== Managing coral reefs<br> ==

Currently, scientists are experimenting with transferring, or 'seeding', thermally-resistant zooxanthellae to corals that may host multiple types of zooxanthellae in attempt to help prevent bleaching.<ref> http://coralreef.noaa.gov/conservation/keythreats/ </ref> MPA networks have begun to apply reef resilience management techniques in order to improve the 'immune system' of coral reefs and promote coral health.<ref> http://www.reefresilience.org/Toolkit_Coral/C1_Intro.html </ref> The Nature Conservancy has developed, and is continually refining, a model to help manage and promote reef resilience. Although this model does not guarantee reef resilience, it is a thoughtful and understandable management model to follow. The principles outlined in the model are:

<br> 1) '''Representation and Replication.''' Coral survivorship is ensured by representing and replicating resilient species and habitats in an MPA network. The presence of resilient species in management in MPAs will help protect corals from bleaching events and other natural disturbances.

2) '''Critical Areas.''' Conservation priority areas provide protection to critical marine areas, such as sources of larvae for coral reef regeneration or nursery habitats for fish spawning.

3) '''Connectivity.''' Preserving the connectivity between coral reefs and surrounding habitats provides healthy coral communities and fish habitat.

4) '''Effective Management.''' Resilience based strategies are based on reducing threats to maintain healthy reefs. Measurements of effective management of MPAs allows for adaptive management.<ref> http://www.reefresilience.org/Toolkit_Coral/C1_Intro.html </ref>

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== References ==

<references /> Holling, C.S., 1973. Resilience and stability of ecological systems. ''Annual Review of Ecology and Systematics'', 4: 1-23.<br>

Berkelmans, R. and M.J.H. van Oppen, 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. ''Proceedings of the Royal Society of London Series B'', 273: 2305–2312<br>

Fine, Maoz, Loya, Yossi, 2002. Endolithic algae: An alternative sources of photoassimilates during coral bleaching. Proceedings of the Royal Society, Vol 269, No. 1497: 1205-1210<br>

Sampayo, E.M., Ridgway, T., Bongaerts, P., Hoegh-Guldberg, O., 2008. Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type. PNAS Environmental Sciences, Vol 105, No. 30:10444-10449

http://coralreef.noaa.gov/conservation/keythreats/

http://www.reefresilience.org/Toolkit_Coral/C1_Intro.html

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== External links ==<br>

[http://www.reefresilience.org/ http://en.wikipedia.org/wiki/Resilience]

[http://www.reefresilience.org/ http://www.reefresilience.org/]