Marine Refugia Past, Present, And Future: Lessons From Ancient Geologic Crises For Modern Marine Ecosystem Conservation

Refugia are one means of species survivorship during a global crisis. As the Earth is facing a major crisis in the marine biosphere, the study of refugia through past extinctions and other global crises is relevant to creating and maintaining effective marine reserves (including marine protected areas and other formally established havens for conservation). A synthesis of previous studies identifies the following properties common to most definitions of a refugium: (1) During a global crisis, a species can persist in a refugium, which can include a range shift, habitat shift, or migration or contraction to an isolated geographic area. Subsets of isolated geographic refugia include life history refugia (areas necessary for breeding), cryptic refugia (small areas, must remain connected for populations to remain viable), and harvest refugia (defined from the modern literature to escape overfishing pressure). (2) In the refugium, the habitat may remain stressed but is sufficiently habitable for the species to maintain sufficient albeit small populations (relative to pre-crisis population size) over many generations. (3) After the crisis ends, the species emerges from the refugium and expands during the recovery interval. Otherwise, the refugium will become a refugial trap in which the species remains a relict population or ultimately becomes extinct.The present understanding of refugia from the geologic past comes from three sources, namely fossil data, phylogeographic reconstructions, and species distribution models, the latter two being more common for studies across the last glacial maximum. The synthesis herein suggests several important factors when considering the future of marine reserves. Because climate change is an ongoing process, the present refugia of marine reserves may not be sufficient for the future survival of marine species. Short-term refugia of some present marine reserves may deteriorate because of further climate change and have to be abandoned for new long-term options as new habitats become available. Cryptic refugia of small reserves must remain connected in terms of species' dispersal and exchange, but must also be flexible, in that cryptic refugia naturally are sometimes ephemeral because of habitat heterogeneity through time. Finally, habitats in marine reserves must be of sufficiently low stress to maintain viable populations, but should frequently be re-evaluated to avoid becoming refugial traps in the future.