A new study from the University of South Florida illustrates how climate-driven changes can have a cascading effect across ecosystems. The experts have discovered that as Florida’s climate becomes more tropical, oyster reefs are steadily being replaced by mangrove islands.
Study lead author Stephen Hesterberg was conducting fieldwork in Tampa Bay when he noticed that mangroves were overtaking most oyster reefs. This widespread transition poses a major threat to species which are dependent on oyster reef habitats, such as the American oystercatcher.
In collaboration with doctoral student Kendal Jackson and Professor Susan Bell, Hesterberg set out to investigate the extent and the cause of this habitat conversion. The researchers determined that warmer temperatures and fewer winter freezes had allowed mangrove islands to start taking over as the dominant salt marsh vegetation.
“Rapid global change is now a constant, but the extent to which ecosystems will change and what exactly the future will look like in a warmer world is still unclear,” said Hesterberg. “Our research gives a glimpse of what our subtropical estuaries might look like as they become increasingly ‘tropical’ with climate change.”
The analysis of aerial images – captured from 1938 to 2020 – revealed that 83 percent of the monitored oyster reefs in Tampa Bay had fully converted to mangrove islands. The experts also found that the rate of conversion accelerated throughout the 20th century.
According to Professor Bell, this study provides a unique opportunity to examine changes in adjacent coastal ecosystems and generate predictions of future oyster reef conversions. “As we change our climate, we see evidence of tropicalization – areas that once had temperate types of organisms and environments are becoming more tropical in nature.”
While the transition to mangrove islands is already well-advanced in the Tampa Bay estuary and further south, Professor Bell said Florida ecosystem managers in northern coastal settings will face tropicalization within decades.
“The outcome from this study poses an interesting predicament for coastal managers, as both oyster reefs and mangrove habitats are considered important foundation species in estuaries,” said Professor Bell.
Oyster reefs provide coastal protection from flooding, improve water quality, and reduce shoreline erosion. As reefs transition to mangroves, these ecosystem services will be greatly limited or lost altogether.
It will become increasingly difficult to maintain oyster reefs in Florida’s shifting climate, but human intervention could make it possible. In his new role as executive director of the nonprofit Gulf Shellfish Institute, Hesterberg will continue to investigate ways to restore oyster reefs to potentially avoid mangrove conversion.
The study is published in the journal Proceedings of the National Academy of Sciences.