Losing Home #2: Reef Migration

This is article is part of a series exploring the ramifications of climate change on the physical habitat of coral reefs, and the consequences for the various dependent communities.

Written by Evan Quinter

Edited by Matthew Tietbohl

In the first Losing Home article, we discussed the health of reef-building stony coral in an Anthropocene ocean. Coral must survive in an ocean that becomes hotter, more storm-weathered, and more acidic each day, and these are just some of their natural stressors. Beyond the deafening loss of hard coral life, the physical erosion of coral’s skeletal structure weakens the entire ecosystem. The loss of habitat jeopardizes the survival of marine and coastal organisms dependent upon this structure for key habitat or nutrition. With the nails being driven into this calcareous creatures’ coffin, corals need to adapt, move, or wait for us to fix our mistakes; they went for the most likely option.

Figure 1: Demonstrates the poleward range shifts for LIG reef corals compared to recent populations, with the solid black line representing the medians. Modified from Kiessling, W. et al, 2012 

Coral migrate in incidents of warming environments, with the most recent example coming from the Last Interglacial (LIG) about 125,000 years ago. Even when accounting for undersampling and potential bias in ancient populations, Figure 1 depicts a significant poleward range shift in LIG when compared to recent reef coral, suggesting rising temperatures may have forced LIG reef coral away from the tropics. This poleward migration continues today, as temperate corals off the coast of Japan have moved northward over the past 80 years. The pace of movement, a speedy 14km per year, is indictive of a rapid shift in tropical ecosystems pushing favorable conditions away from the equator.

As the base of reef ecosystems embarks on their equatorial emigration, other benthic and pelagic reef marine organisms follow in corals wake. Over the past 60 years, marine species richness has declined along on the equator and slightly increased in temperate regions. By analyzing distribution data for 48,661 species, the team of a recent PNAS article found marine species were fleeing from the equator due to temperature spikes above 20 ℃. The clearest dips in species richness resulted from heat susceptible species like bathydemersal fish and certain benthic organisms, emphasizing the pivotal damage of global warming. 

Figure 2: Species richness over the past 60 years grouped into 3 different time periods.Equatorial species richness continues to decline as marine organisms move into temperate regions and away from changing equatorial environmental conditions. Modified from Chaudhary, C. et al., 2021

The authors synthesize their findings to the grave point that, “…the ocean around the equator is already too hot for some species to survive…”. A mass exodus from the equator could ripple across other latitudes, as marine organisms continue to shift closer and crunch into cooler habitats. Through resource competition and invasive speciation, marine environments around the world will feel the strain of refuge reef organisms moving to cooler waters. Global warming is continuing to erode the health and stability of reef corals, consequently displacing tropical reef organisms away from their natural habitats and disrupting the global distribution of marine life.

If you’d like to read an interview with the recent studies author, read below:

Marine life is fleeing the equator to cooler waters. History tells us this could trigger a mass extinction event (theconversation.com)

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