For the past two days I’ve been visiting mangrove experts and restoration sites. At Weedon Island in Tampa Bay I met Tom Smith of the US Geological Survey. He’s vitally interested in the question of how mangroves will cope with rising sea levels. On the one hand, their sediment-trapping ability means they can hoist themselves up by their bootstraps, keeping their heads above water by building up the soil around their roots. On the other hand, there are limits to how rapidly they can accumulate sediment.

“If the rise is 1–2 mm a year they can keep up just fine. If sea level goes up 2.5–3 mm a year, they probably can’t,” Tom said. The average sea level rise recorded at Key West for the past century has been 2.2 mm/year—just within the threshold of what peat-building mangroves can accommodate. Is the rise increasing? Not recently, Smith says: “In the last 20 years sea level in the Gulf of Mexico hasn’t risen a lick.”

Southern Florida, being so flat, is especially vulnerable to rising seas. Much of the land has a gradient of 1 in 100,000. Each centimetre of tidal elevation pushes water 10 km inland. Would planting thousands of mangroves keep the waves at bay? Not necessarily, says Tom. Even a small escarpment would prove an insurmountable hurdle to mangroves moving inland. And in many places human infrastructure inshore of the mangrove fringe would arrest the mangrove retreat.

Another of Tom’s research interests has to do with the “ecological goods and services” of a mangrove forest. The relatively new field of ecological economics attempts to put a value on natural habitats such as mangroves based on the contribution they make to the biosphere—in oxygen production, carbon storage, fish nursery functions, substrate for shellfish, roosts for birds, and so on.

“One of the questions we’re asking is how big a patch of mangroves do you need for these services to be noticeable,” he said. “How much of a contribution do you get from a 10-metre-wide mangrove stand compared with, say, a 100-metre-wide stand? How much support for fisheries do you get? Is there a linear relationship between stand size and ecological goods and services? We don’t know the answers yet.”

The same questions interest Robin Lewis, a mangrove restoration specialist who showed me two of his project sites near For Lauderdale, north of Miami. Robin trained as a fish biologist before turning his attention to mangroves in the mid-1970s.

One of the sites we visit, West Lake Park, used to be tomato fields in the early 1900s. An ambitious restoration project was commenced in 1986, involving restoring saltwater flow through the site, removing invasive casuarina trees and reshaping the land to mimic the natural topography of a mangrove wetland. Now boardwalks weave through lush stands of mangroves, all of which have self-seeded (“volunteers,” in restoration lingo).

It takes about 30 years to produce trees of the stature and density of natural stands, Robin says, but the goal for ecological restoration (as opposed to simple reforestation) is not just a nice spread of trees but “functional equivalency” between the restored site and a natural site. That means that all the ecological components are present and accounted for. For mangroves, that can be a lot of organisms. Researchers have found that a typical cubic metre of mangrove mud contains between 20,000 and 40,000 visible organisms—critters bigger than 0.5mm.

We stop to watch the courtship rituals of small fish called mollies, which are circling in the muddy water under the mangrove canopy. The males have a bright blue patch on their tails, which they are flamboyantly displaying to the females. Robin says it takes as little as five years for fish populations in restored mangroves to match those of undisturbed ecosystems. Looking at the swirling mollies, I remark, “If you build it, they will come.”