Marine Refugia Past, Present, Future: Lessons from Ancient Geologic Crisis For MODERN MARine ecosystem conservation
How did some species survive mass extinction? They found refuges - a place of safety where a large enough population could survive (but not necessarily thrive) through a global, mass extinction-causing, environmental disaster. The high level of global environmental stress experienced by ancient species during and after a mass extinction required one or more of several strategies for finding a place of refuge:
species had to move to new places where there was a lower level environmental stress that was survivable, but not necessarily stress-free (i.e., shift their geographic range or move to a refugium),
species underwent a decrease in geographic range or population as they died out of areas where environmental stress was too high (i.e., range shift or contraction into one or more small refuges), or
species had to adapt to a new habitat - they had to move to a slightly different habitat, like into deeper water, even if it meant that only part of the population could survive this new habitat well enough to reproduce.
Sometimes refuges are needed only for part of an organism’s life span - like breeding areas - to be able to survive environmental stress. Other times, species find themselves in tiny areas of small populations - cryptic refuges - but so long as these populations remain connected to each other for breeding, they can act like one big refuge.
Once a species is in a refuge, it will need to survive for as long as the environmental crisis lasts. An environmental crisis that causes a mass extinction can last long past the end of the extinction itself, so that species in refuges must remain in those refuges long past the end of the mass extinction. The need for a refuge ends only when the environment recovers; then species can expand outside of that refuge, or, if conditions preclude moving from the refuge (for some reason, the species cannot move into their previous habitat, or new environmental conditions are so different that a species is “stuck” in its refuge), species can remain in a refuge as a “relict population” or “living fossil.” Sometimes, refuges fail and the species goes extinct anyway because the refuge has become a trap.
When studying ancient mass extinctions, these are the “take-away” messages that ancient refuges have for the present and future success of marine protected areas:
Present-day marine protected areas are based on present-day conditions; as climate warms and the oceans change, these refuges could fail. Therefore, we must consider places where species will need to move and create those new protected areas ahead of time.
During the worst mass extinction ever faced by life on earth, the global environmental crisis was so bad that refuges were ephemeral: through time, refuges would fail because they deteriorated. However, some species were able to move to newly opened refuges, and thus survive the extremely long global environmental crisis by “hopping” from refuge to refuge. But during shorter, less intense mass extinctions, refuges didn’t face the same failure rate. What this means is that, with the increasing length of time or intensity of a global environmental crisis, refuges must increase in size to cope with the increasing magnitude of the crisis. However, there comes a tipping point where the global environmental crisis is too severe or lasts too long for a single refuge to support a species. Refuges instead will be ephemeral and species will need to move from refuge to refuge to cope with the environmental stress.
We will need to continually monitor marine protected areas to make sure they do not become refugial traps that will fail. Refuges were never stress-free, but they were low enough in stressful conditions that species were able to survive in them.
One final point that is not in the paper but needs to be brought up: this is on geologic time scales, which means that ancient mass extinctions and their aftermath lasted thousands to millions of years. Yes, our marine protected areas will have to be in place and remain successful for a long time during climate change, overharvesting, and pollution, along with all of the associated environmental deterioration. Plus, marine protected areas will have to stay successful and have sufficiently low levels of environmental stress so that species can remain safely inside them until the global environment recovers.
If we were to immediately stop ALL of the crises that are threatening species, it would be hundreds or maybe thousands of years for the ocean environment to recover to levels similar to the ocean of early human history. But as we are unable to immediately stop our climate-changing activities, we haven’t substantially reduced reduced harvesting of seafood and other resources from the oceans, and we are still producing chemicals, plastics, and other pollutants that find their way into the oceans, we will likely need marine protected areas to be present - and large enough - for thousands to even hundreds of thousands of years. A note on “large enough:” we require species to not survive in a refuge, but that refuge has to be large enough and sufficiently low stress for species to support our resource needs, in addition to supporting their own survival.
Reference:
Schneider, C.L., 2018, Marine refugia past, present, future: lessons from ancient geologic Crisis for modern marine ecosystem conservation. In Tyler, C.L., and Schneider, C.L., Marine Conservation Paleobiology. Topics in Geobiology 47, pp. 163-208.