Roots of the Sea

What mangroves give the world and why we can’t afford to lose them

Cheryl Carlin

Last year, a group of scientists predicted that mangrove ecosystems would disappear within the next 100 years. Ilka “Candy” Feller, a mangrove ecologist from the Smithsonian Environmental Research Center, talks about what makes these ecosystems special.

What is a mangrove?
Typically we think of mangroves as trees. But there’s a fern that’s a mangrove, and in the Indo-Pacific there’s even an herbaceous plant that’s considered a mangrove. A plant is a mangrove because it lives in a mangrove environment. It’s kind of a circular logic.

So, what is a mangrove environment?
It’s this salty, tropical, intertidal place. If you took a mangrove and planted it in a freshwater system, it will grow there if you keep all of its competitors away. But that’s not its natural environment. They’re halophytes (they thrive in salt water). There are some mangroves that can live in places that have salinity up to 90 parts per thousand, which is almost three times as salty as the sea.

There has been debate among scientists over the years about whether or not mangroves build land. What do we know now?
This was a hypothesis that got put out back in the 1940s about mangroves building land. Then it got debunked, and there was a lot of discussion about mangroves not building land. But in the systems where we work in Belize (where the Smithsonian has a marine field station), clearly the mangroves are building the substrate. In the lagoon between the reef and the mainland, there are thousands of mangrove islands. Scientists have found that these mangroves are built on top of ten or twelve meters of solid peat. And that peat is made of the fine roots of the red mangrove. They’ve cored down and found that this stuff is accumulating at about a meter per thousand years. So you can age the island by the depth of the peat. It’s pretty amazing. Those islands would not exist were it not for the mangroves.

What happens if the mangroves are cut down?
This is a big concern right now because in these places, which are tropical paradises, people see these mangroves as occupying land where they could put a resort. But then there’s no more building peat, no more addition of roots into the substrate. It doesn’t take long before that area is just going to sink, sink, sink and it’s gone. The island is gone. It is an uphill battle, though, getting people to believe that. But people are cutting down the mangroves at a steady pace—and half are already gone. This is despite what we know—that these systems are critical to maintain healthy fisheries in these coastal waters, and they’re critical for protecting the coral reefs from sediment and the freshwater that comes from the mainland. They protect the land itself, and human development, from storm activity. This has been proven over and over again.

You have been investigating another human impact on mangroves—how increased nutrients from farming and other human activities affect the mangrove ecosystem. Why does it matter if there is more nitrogen or phosphorus? Doesn’t this help the plants grow?
In Belize, where I work, the mangroves, like the reefs there, are living in very low nutrient conditions. The mangroves are able to survive in those low nutrient environments, but if you give them more nutrients, they take them up and they grow more. It changes their growth rate, but it also changes the way they process those nutrients internally.

Why should that matter?
It’s important because the leaves of these trees fall to the ground and that becomes the detrital base for the food web. All these little creepy-crawly things—little invertebrates, fungi, bacteria—colonize those leaves, and that gets fed on by all these tiny little detritivores, and then something bigger feeds on those. So it just continues on through the food web, and eventually finds its way out into the surrounding ecosystem.

So what happens next?
This is what we’re working on now. I have fertilization experiments where I take individual trees and give them tiny little doses of either nitrogen or phosphorus. I use these controlled experiments to answer the questions.

You’ve discovered plenty of insects living in mangroves, and even had a couple named after you.
That was a thrill. I’m not an entomologist, so I don’t have the expertise for describing species. But I like to find things. You know, if you go to Florida, to the Everglades, people automatically think that a mangrove is going to have a lot of bugs but that they’re just going to be a few things that bite you—mosquitoes, sand flies, that sort of thing. Well, there are lots of things in there, but it takes some detective work to get them. You don’t find them until you start looking inside the plant. They’re not going to be just walking around on the leaves. Out there in the salty water, even though you get rainy seasons, it’s still a marine environment. So for organisms that need freshwater, the place they’re going to find it is inside the plant. I found that the fauna is characterized by specialists, like miners and borers that live endophytically, or inside the plant tissue. They feed on the plant, but they don’t kill it. They’re just part of the system.

You seem to enjoy your job.
It’s very exciting, to have this question that’s been there forever and you’ve been able to figure it out. It’s very gratifying to discover new things. It’s also fun to travel, to go to these places. It’s tropical, beautiful. The swamp is very exciting. I know it’s not everyone’s cup of tea, but I love it.

Are there any downsides to what you do?
It’s hard to be confronted with all of the environmental degradation going on in the mangrove system—something that I love and know how important it is. Constantly seeing it destroyed is so very difficult. I sometimes don’t want to go to places because I know what I’m going to see there. It’s very hard.