Human stressors limit the natural expansion of tropical corals

Rising temperatures and disease outbreaks are significantly impacting coral reefs around the tropics. With higher latitude marine environments potentially acting as sanctuaries for vulnerable coral species, understanding the mechanisms of coral population expansion and sustainability is crucial. Yet, the limited scope of contemporary observations constrains our grasp on this process.

Looking at the past to predict the future 

What can the deep past tell us about the future of coral reef communities? A recent study led by Florida Atlantic University (FAU) has attempted to answer this question by offering geological perspectives on coral range expansions. 

The investigation involved reconstructing the composition of a Late Holocene-aged subfossil coral death assemblage found in an unexpected location off Southeast Florida and comparing it with today’s reef compositions.

Historical northward migration of tropical coral communities

Situated along one of the most urbanized coastlines in the United States, the Late Holocene coral death assemblage known as “Pompano Ridge” documents a historical northward migration of tropical coral communities induced by regional climate warming over 2,000 years ago. This raises the question: Could such a migration occur again due to current climate change? 

Modern environmental stressors

The findings point to significant differences in coral composition between the Late Holocene and modern assemblages. These differences suggest that the modern environmental stressors absent during the Late Holocene are likely to restrict the capacity of modern higher latitude environments in Florida to serve as climate refuges for at-risk, temperature-sensitive corals.

“Today’s environmental conditions and ecology have substantially deviated from long-term Holocene baselines that occurred over millennial timescales,” noted study co-author Anton E. Oleinik, an associate professor of geology at FAU. 

“Based on the findings from our study, we are not overly optimistic that Florida’s subtropical reefs will be able to support range expansions of reef-building coral species reminiscent of the Late Holocene any time soon.”

Late Holocene coral assemblages

The researchers discovered that Late Holocene coral assemblages were primarily dominated by now critically endangered Acropora species (stony coral) between 1,800 and 3,500 years ago, mirroring the healthy zonation patterns characterizing pre-1970s Caribbean reefs.

“What’s really remarkable is that we didn’t find these subfossil corals in the middle of the Caribbean like Belize or Bonaire. We found them right here in South Florida well beyond their present-day core range. What existed here thousands of years ago is similar to what we would have seen in Jamaica in the 1950s and 1960s,” said study lead author Alexander B. Modys, a geoscientist at FAU.

In contrast, the modern reefs off Southeast Florida are now increasingly dominated by stress-tolerant species like Porites astreoides (mustard hill coral) and Siderastrea siderea (round starlet coral), due in part to their resilience to thermal stress and recent disease outbreaks.

Natural expansion of tropical corals

The scientists identified significant changes in coral composition over time, suggesting that the natural expansion and sustainability of tropical corals, observed in the past, are unlikely to occur today without human intervention. 

“The rapid decline of southern source populations and the added anthropogenic stressors that weren’t present during the Holocene are likely inhibiting the natural expansion of tropical corals we’d expect to see with climate warming. They aren’t going to get there on their own so more aggressive conservation strategies like assisted migration may be needed,” Modys explained.

Identifying potential climate refuges for corals

The examination of the Late Holocene record from Pompano Ridge lays the groundwork for identifying potential climate refuges for corals and for developing restoration and management strategies that replicate the successful ecological attributes of historical coral communities. 

However, as Oleinik concluded; “It is important to emphasize that the long-term sustainability of these restoration activities will ultimately depend on the rate and magnitude of present-day climate warming. If climate warming continues at its present rate, it will become too warm even at historically cooler, higher latitude locations like Southeast Florida, and sadly, these restoration programs won’t be enough.”

The study is published in the journal Nature Communications Earth & Environment.

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