The March of Mangroves What the expansion of mangroves holds for Florida’s future.

By Lianne D'Arcy, Education & Communications Intern / UF Thompson Earth Systems Institute

Florida’s living indicators of climate change. Protectors of the coast. Walking trees.

Straddling the land and sea, mangroves have long been a fixture of Florida’s coastline. All 469,000 acres of red, black, white and buttonwood mangrove forests are considered some of Florida’s true native species.

Within the state, mangroves were once distributed exclusively throughout Florida’s southern region, providing vital services to their marine and estuarine environments. A mangrove’s roots prevent erosion by binding soils, provide critical habitat for other plants and animals, filter pollutants and store carbon, a process that helps mitigate climate change by reducing the amount of carbon dioxide in the atmosphere.

Today, climate change and coastal development have altered the mangrove’s traditional geographic span, extending its range farther north and more inland. Red and white mangroves can now be found as far north as Cedar Key, an island off the coast of Florida’s Big Bend, about two hours north of Tampa. Black mangroves can even be found northward of Jacksonville. Between 1984 and 2011, mangroves have doubled their range of expansion along Florida’s northeast coast. A mangrove's southern territory is now expanding farther into the Everglades, where intruding saltwater provides room for them to grow.

While this march of mangroves opens the door to new hope for Florida’s future, their expansion simultaneously poses a threat to many current ways of life.

Why are mangroves moving?

“As mangroves walked, no one knew that those feet would eventually be moving so fast. Now, because of climate change, they’re running.” -Xavier Cortada

Four major factors limit the range of mangroves: temperature, salt water, tidal range and soil type. Climate change’s rising seas, changing ocean currents, storminess and increasing temperature levels all impact the delicate balance of mangroves' coastal ecosystems, forcing them to change and adapt. Like most Floridians, mangroves are not a fan of cold weather, but the rise of global temperatures has allowed them to expand their territory.

According to the U.S. Fish and Wildlife Service, though the intrinsic connection of all these factors can easily impact mangrove distribution in Florida, the tree possesses many traits that help it adapt. For example, some mangrove species have aerial roots that permit the plants to absorb vital nutrients from the air when needed; their viviparous nature means that their seeds begin to germinate while still attached to the plant, allowing their propagules to be dispersed and spread far from the parent plant through water; and their ability to secret salt enables them to endure various and fluctuating levels of water salinity, or saltiness.

Red Mangrove Propagules by Katja Schulz (CC BY 2.0). These "seeds" sprout while still attached to the parent plant, and eventually disperse through water before rooting in the shallows.

The pressure put on mangroves by climate change compounded with their resilient nature is why this species is able to move throughout Florida.

Xavier Cortada, University of Miami art professor of practice and founder of the mangrove-restoration, eco-art Plant(T) Project, encourages Miami residents to plant mangrove seedlings in their yard. The project aims to foster community building and facilitate conversations about climate change between neighbors.

“As mangroves walked, no one knew that those feet would eventually be moving so fast. Now, because of climate change, they’re running,” Cortada said.

Making the most of their march

As ‘protectors of the coast,’ mangroves serve as a natural defense system against storm surges, sea level rise and coastal erosion.

During hurricane Irma in 2017, Florida’s mangrove forests prevented over $1.5 billion in direct storm damages to properties, which represents a 25% savings in counties with mangroves, compared to those without. About 626,000 people living behind mangroves saw reduced storm damage from Irma due to the natural barrier the trees created. As rising seas continue to threaten Florida’s coast, the safety provided by increased mangrove cover can be instrumental in protecting the state’s seaboard.

Mangroves also trap and cycle various organic materials, chemicals and elements in their midst, improving the quality of the water flowing around them. This includes nutrients like nitrogen, phosphorus and other common pollutants. Their roots also serve as an attachment surface for marine organisms that provide the same cycling and filtration services. Mangrove roots and the organisms attached to them filter groundwater and stormwater runoff, helping prevent harmful pesticides and herbicides from further spreading into the ecosystem.

As a keystone species of coastal environments, mangroves play a critical role in wildlife diversity. Their complicated root system provides more than 1,300 species of animals room to breed, nest, forage and shelter. Mangrove canopies also provide critical habitat for species of songbirds that occur exclusively in this habitat type, such as the mangrove cuckoo or the Florida prairie warbler. Some of Florida’s most endangered species, such as the Key deer or Florida panther, also rely on mangrove forests and coastal hammocks during certain stages of their life cycle.

Florida panther by Connie Bransilver from the United States Fish and Wildlife Service (CC BY 2.0)

Exploring potential negative impacts

Though the expansion of mangrove canopies provides many potential benefits to both humans and Florida wildlife, their march opens the door to an immense amount of uncertainty.

As mangrove forests encroach on established salt marshes, those regions will witness a substantial change in ecosystem structure. Though the specific impacts are largely unknown, and will vary depending on time and location, studies reveal that the movement of mangroves into such spaces will decrease the habitat available for flora and fauna that require an open vegetation structure, and thus affect the species that rely on these habitats, like Florida’s wintering warblers or foraging Seminole bat. This change will also create shifts in human activities such as birdwatching, hunting or fishing.

Projected sea level rise, at any rate, will also prompt saltwater intrusion into the freshwater habitats of already-shrinking Florida’s Everglades, further permitting inland mangrove expansion. This inland encroachment may compromise the land and water resources within these habitats, putting existing freshwater animal and plant populations into jeopardy, such as the rainwater killifish or Florida’s famous sawgrass. Another study found that if sea levels continue to rise at the current rate — currently about one inch every three years, with an upward trend in acceleration — a much more aggressive encroachment of mangroves into the Florida Everglades can be expected.

These changes to the Everglades will also be felt by the Miccosukee Indians, who live there and rely on the freshwater habitat for traditional and cultural practices. Saltwater intrusion and the ensuing mangrove encroachment will push out native species, alter traditional hunting grounds and flood sacred tribal areas.

Scientists at the Florida International University have even called the inland expansion of mangroves a ‘death march,’ as they are likely to be submerged by water in 30 years alongside the very habitat they’ve encroached upon.

“They’re a really resilient species,” Cortada said. “I used to say the only thing they can’t outcompete was a bulldozer. But nowadays, I say they can’t outcompete rising seas."

How humans harm mangroves

These changes in coastal ecosystems and mangrove canopies can be primarily attributed to human impacts. In addition to the accompanying pitfalls of climate change, humans have been altering mangrove habitat for years. In the last decade alone, at least 35% of the world’s mangroves have been destroyed. From large-scale coastal developments to a single homeowner trimming back branches, mangrove forests have been forced to adapt to changing situations.

Though these ‘roots of the sea’ are currently protected by law to prevent future losses, the impacts of human interference and climate change will continue to alter their future range of expansion.

Forecasting Florida’s Future

Scientists do not currently know whether the migration of mangroves will ultimately benefit or harm Florida’s complex environment.

Julie Walker, a Ph.D. student in interdisciplinary ecology at the University of Florida, studies the environmental impacts of northward mangrove expansion into temperate saltmarsh.

“For better or for worse, mangroves are going to be moving, and protecting wetlands of all sorts will be important saving us from the threats we’re being faced with in the light of a changing climate,” Walker said.

Though much is still unknown, if global temperatures remain on the rise and climate change continues to threaten ecosystems everywhere, one thing is certain: Mangroves will remain on the move.

This story is part of the UF Thompson Earth Systems Institute's student-produced Earth to Florida newsletter that curates the state’s environmental news and explains what’s going on, why it matters and what we can do about it.

The University of Florida Thompson Earth Systems Institute is advancing communication and education of Earth systems science in a way that inspires Floridians to be effective stewards of our planet.

Created By
Lianne D'Arcy


Created with images by Maxwell Ridgeway - "The Mangrove" • Charles Jackson - "Key Largo, Florida" • Sterling Lanier - "woodstorks everglades" • Mark - "Northbound, 7 am." • Brandon Paul - "untitled image"