Can evolution save coral reefs?

Written By Fedra Herman

Most people know by now that global warming affects many different ecosystems across the globe. A major concern of many people is the dramatic, worldwide decline of coral reefs. Researchers are racing against the clock to find a solution to how we can help ensure the future of coral reefs. Natural evolution is believed to be too slow to allow corals to adapt to global warming. Indeed, when counting on the development of entirely new mutations that may genetically benefit stony corals and help them to survive in warmer temperatures, evolution is simply too slow. But, there exists something as rapid adaptation. Maybe this will be fast enough to allow coral reefs to survive the rising ocean temperatures?

Figure 1. © Image by Marcelo Kato from Pixabay
( https://pixabay.com/photos/fish-coral-reef-sea-underwater-2659613/  )

 Rapid evolution and corals

 Scientists are interested as to what extent coral reefs can save themselves, and rapid adaptation might be part of the salvation needed. There are two reported ways that corals can undergo rapid adaptation. Firstly, the standing genetic variation of a population might already contain genes encoding information for higher temperature tolerance. This means that the current genome may contain the required adaptive alleles for higher temperature tolerance. Even though environmental adaptation is often considered a very slow process, extreme changes in environmental temperature might result in rapid changes, both morphologically and genetically (Campbell-Staton, 2017) . This is caused by a shift in gene expression patterns due to the selection of these pre-existing adaptive alleles. Thereafter, the newly selected alleles can rapidly spread throughout the population. Another option that can result in fast adaptation of a coral species occurs in a metapopulation in which a species is distributed across multiple locally adapted sub-populations. A particular type of coral can live across a gradient of temperatures and thus be already adapted to a variety of temperatures. Through emigration, these adapted corals spread migrants over large distances, bringing new genetic information into less adapted sub-populations. When breeding occurs, adaptive alleles can be redistributed across the sub-population and individual thermally tolerant variants, living in warmer parts of the ocean, can therefore save other less adapted sub-populations by sharing their genetic information. Several recent papers have argued that this second mode of adaptation probably will be of major importance for reef-building corals as they adapt to warming (Bay et al., 2017; Kleypas et al., 2016). However, we need to keep in mind that cool-water corals can adapt to warmer oceans, but only under mild warming scenarios, as explained in the paper written by Bay et al . in 2017. Furthermore, the connectivity between the sub-populations needs to be high, so that larval transport can be allowed (Kleypas et al., 2016).

 The role of migration and selection in coral resilience

 In a recent collaboration between two researchers from the USA (Mikhail V. Matz from the University of Texas at Austin and Benjamin C. Haller from Cornell University) and an Australian scientist (Eric A. Treml from Deakin University), a model was designed to study the effects of global warming on coral reefs across the central Indo-West Pacific region. They performed simulations to model warming in coral metapopulations of 680 reefs over 200 years. They studied four different models: (1) disabling new mutations, (2) disabling migration, (3) disabling selection, and (4) disabling both migration and selection. The researchers studied the simulations of these models under two conditions: a “business-as-usual” condition and a situation with a reduction in greenhouse gas emissions. Like expected, the slower warming situation resulted in the majority of the reefs being unaffected. But to their surprise, even under the rapid “business-as-usual” warming scenario, certain coral reefs located in the Mid- and Southern Great Barrier Reef, New Caledonia, Vietnam, Taiwan, and Japan survived. This could not be concluded for all reefs and most suffered severely under the pressure of the rising temperatures.

 Why did individual reefs survive even under the “business-as-usual” scenario? The scientists reported three factors that can predict the adaptive potential of reefs. The first element is the pre-warming temperature of the reef. Lower current water temperatures make a reef better protected from the rising ocean temperatures in the near future, as seen in the figure below adapted from Matz et al. (2020), (Figures 2 a, c, and e). After 200 years, most of the reefs that survived had a pre-warming temperature of less than 22.5°C. A second important factor is the reef-specific warming rate. Some reefs warm more quickly than others, resulting in a higher coral cover loss. Lastly, the proportion of recruits that come from locations that are at least 0.5°C warmer, or the pr05  , is an essential predictor of the survival of a reef. The ingredients for a resilient reef are a low warming rate, a low pre-warming temperature, and a location downstream, so the reef can accumulate a large number of migrants that provide new genetic information.

 What happened if the scientists simulated the “no migration model”? Pretty much all reefs collapsed after 100 years, and after 200 years no reefs survived, as seen in Figures 2 b, d, and f. Even the most resilient reefs suffered under these circumstances and even more so in the absence of selection. These observations indicate that migration and selection are critical if we want to ensure the survival of coral reefs.

 As a side-note, the authors of the paper wanted to highlight that their developed model isn’t a model of reality. The model does not account for a variety of other factors that influence reef health, e.g. the change in migration rates with warming, the competition with algae, possible disease, ocean acidification, etc. They also focused on branching acroporid species of the genus Acropora, as they provide the most structural complexity required by reefs to support diverse fish and invertebrate assemblages and are a fast-growing species of coral. For corals that grow and mature slower than this genus, adaptation would be unlikely, even when selection and migration are not excluded.

 Should we help corals evolve?

 This research showed that the majority of reefs, especially those that are already warm and do not receive immigrants from warmer places, are highly prone to declines due to global warming. You can ask yourself the question if it is our responsibility to help corals evolve? The authors believe that natural evolution is way more efficient than any lab-based effort, but some intervention might help. Conservationists can help facilitate genetic influx from warmer places, and hereby raise the local pr05, by more recent techniques termed “assisted migration” and “assisted gene flow”. This practice will help reefs that do not naturally receive any immigrants from warmer locations. But the only way to really ensure the survival of coral reefs in the long-term is by reducing our greenhouse gas emissions, so we give corals the time necessary to adapt to their changing environment.

If you are interested in this topic, a seminar about this paper can be watched here.

Figure 2. Spatial structure of the model and variation in coral cover (proportion of occupied carrying capacity) across reefs during warming. Sizes of points on all panels indicate habitat size at each location. (a) Pre-warming temperatures and migration patterns. (b) Prewarming coral cover. (c, d) Coral cover after 100 years of warming under the main settings (c) and under the scenario in which migration was set to zero during warming (d). (e, f) Same as (c) and (d) but after 200 years of warming.

References

Bay, Rachael A, Rose, Noah H, Logan, Cheryl A, & Palumbi, Stephen R. (2017). Genomic models predictsuccessful coral adaptation if future ocean warming rates are reduced. Science Advances, 3 (11), E1701413.

Campbell-Staton, Shane C, Cheviron, Zachary A, Rochette, Nicholas, Catchen, Julian, Losos, Jonathan B, & Edwards, Scott V. (2017). Winter storms drive rapid phenotypic, regulatory, and genomic shifts in the green anole lizard. Science (American Association for the Advancement of Science), 357 (6350), 495-498.

Matz MV, Treml EA, Haller BC. Estimating the potential for coral adaptation to global warming across the Indo-West Pacific. Glob Change Biol. 2020;26:3473–3481. https://doi.org/10.1111/gcb.15060

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