A multicolored coral reef with blue settlement tiles pinned to it
This coral reef in the Palmyra Atoll National Wildlife Refuge in the Pacific Ocean has settlement tiles pinned to it so that researchers can quantify coral settlement on it. Credit: Nichole Price/Bigelow Laboratory for Ocean Sciences

“These data add to the increasing documentation of the ‘tropicalization’ of temperate systems.”

While millions of human refugees are expected to mass migrate in response to the climate crisis, much is still unknown about how other species, on land and underwater, will respond to changing conditions. A large, international team of researchers recently explored how the worldwide geographical distribution of coral recruitment has been changing over time.

“Despite widespread climate-driven reductions of coral cover on tropical reefs, little attention has been paid to the possibility that changes in the geographic distribution of coral recruitment could facilitate beneficial responses to the changing climate through latitudinal range shifts,” the researchers wrote.

The team’s analysis, published in the Marine Ecology Progress Series, indicates that although global coral recruitment has declined by 82% and plummeted by 85% in the tropics since 1974, recruitment in the subtropics has jumped by 78% over that time period.

“Thus, coral recruitment appears to be moving poleward,” Mark Hay, an experimental marine ecologist at the Georgia Institute of Technology in Atlanta, wrote in an email to Eos. Hay, who wasn’t involved with the report, added, “These data add to the increasing documentation of the ‘tropicalization’ of temperate systems.”

First of Its Kind

The report is “the first of its kind at this scale,” said Nichole Price. She’s a benthic marine ecologist at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine, and the lead author of the study.

“The novelty of the study is in its temporal and spatial scale, the broad extent of the data presented, and in focusing on a critical process (recruitment) that is rarely investigated over such scales,” Hay wrote. “The study covered five continents, nearly four decades (1972–2012), and greater than 1,200 records of coral recruitment,” he added.

Price and her colleagues analyzed data from 92 studies (including 68 published and 24 unpublished) furnishing 1,253 records of coral recruitment.

The researchers examined studies focusing on recruitment to deployed settlement tiles. The team’s main conclusions about coral recruitment trends were based on tiles that had been deployed for at least 3 months. Most of the included tiles were made of terracotta (or another ceramic material), with some composed of polyvinyl chloride, acrylic, or calcium carbonate. Their areas ranged from 0.01 to 1.5 square meters.

The majority of the tiles (96%) were deployed at depths of 20 meters or less. The analysis included only tiles deployed on fringing reefs, on barrier reefs, or in lagoons. “Following a period of immersion, the numbers of coral recruits (typically ≤1 cm diameter) settled on the tiles are counted under magnification, and are usually reported as density (i.e. number of recruits per area or tile) for each deployment,” Price and her colleagues wrote in the study.

“For any large data compilation such as this” covering “global-scale patterns over multiple decades,” researchers often encounter “issues of variance due to patterns being potentially confounded in time, space, methodologies, etc., but this is such a large data set and the patterns [are] clear enough that I find the documentation both convincing and useful,” Hay wrote.

“Additionally, the variance that is inevitable in such studies makes it more likely that real patterns will be  missed rather than false patterns found,” he noted.

Are Corals Finding Climate Refuge?

Though the tropical and worldwide declines still outweigh the boost in subtropical recruitment, these results provide a “glimmer of hope.”

Though the tropical and worldwide declines still outweigh the boost in subtropical recruitment, these results provide a “glimmer of hope,” Price said. The numbers suggest that some corals “may find refuge” in the face of rising temperatures and other oceanic conditions ill suited to their survival, she noted.

Still, the long-term effects of relocation on corals are unknown. Differences in environmental factors in the subtropics, such as light availability and seasonal temperature variations, could impact coral populations, Price said. Researchers also don’t know how interactions with other organisms, especially kelp, will play out.

Corals and seaweed growing in the same location
This coral reef and seaweed are both growing in the waters off the coast of Nagasaki, Japan. Tropical reef-building corals and kelp don’t naturally exist in the same locations, but that’s changing with coral reef migration. Credit: Soyoka Muko/Nagasaki University

Both kelp and reef-building corals behave as ecosystem engineers, constructing three-dimensional structures that serve as homes for other living organisms, Price noted. Only time will tell whether their habitation of the same space will lead to competition or some form of coexistence.

Another possibility? The climate crisis might also drive kelp species to migrate, Price said.

—Rachel Crowell (@writesRCrowell), Science Journalist


Crowell, R. (2019), Tropical corals are migrating away from warming waters, Eos, 100, https://doi.org/10.1029/2019EO131217. Published on 23 August 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
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