In theory, we know we could maintain biodiversity within an MPA if we maintained a balance between the rate of species loss and the immigration rate of replacement species.

• If mass bleaching or some other event causes loss of corals on both the target and up-current source reefs, there will be no way for these species to be replaced and the MPA will lose biodiversity.

Reefs normally recover from such natural stresses as tropical storms.

• Human activities increase the burden of stress and may prevent normal recovery by increasing the extinction rate or decreasing the immigration and recovery rate.

Coral bleaching has increased the stakes—it challenges us to take immediate action based on our best information, and to refine our management focus as the science and experience develop.

• Following the lead from fisheries, we need to err on the side of caution and create larger more viable MPAs to safeguard our global coral reef biodiversity and resources.

MPAs need to be large enough to be self-replenishing and sustainable in the face of actual and foreseeable threats.