June 20th, 2009

The nutrient economy

By KENNEDY WARNE

It takes an hour by speedboat from Isla Colón, where the Smithsonian research station is located, to reach Isla Popa, where Candy Feller has one of her experimental sites.

Anne Chamberlain (left) and Emily Dangremont tickle the mangrove roots for crabs on Isla Popa.

KENNEDY WARNE
Anne Chamberlain (left) and Emily Dangremont tickle the mangrove roots for crabs on Isla Popa.


Candy’s main project in Bocas is to look at how nutrients affect mangrove growth. “Nutrients are the currency of food webs,” she says. She studies how that currency moves through a mangrove ecosystem in the same way that economists study how money moves through an economy. The three major elements she’s interested in (the dollars and cents) are carbon, nitrogen and phosphorus. She compares the ratios of each element in the mangrove trees in her study plots, and also in the herbivores that eat mangroves. Those consumers include everything from tree-climbing crabs to twig-boring miners. Candy has found more than 100 species which feed on mangrove twigs alone.

Some of the associations are intricate and fascinating. For example, the exit hole of one borer may be the entrance way of the next species in the chain. One of Candy’s favourites is a nemertine worm which takes up residence in an empty beetle gallery. The worm traps its prey by everting its insides. Charming!

Candy Feller in the dwarf mangrove forest.

KENNEDY WARNE
Candy Feller in the dwarf mangrove forest.


So thoroughly do creatures use the mangrove habitat that a seedling with its leaves in the air and its roots in the water will have terrestrial borers burrowing down from the top and marine borers burrowing up from the bottom.

As well as measuring the nutrients in an undisturbed system, Candy has added nutrients in the form of fertilisers to see how the mangroves and their associated flora and fauna respond. This is an important line of research because nutrient over-enrichment is a growing problem for marine ecosystems around the world. As fertilisers run off the land and into the sea, the delicate nutrient balance of mangrove and other communities (seagrass beds, coral reefs) can be thrown into disarray.

As we step off the boat there’s a surprise: a golden orb web spider has spun a metre-diameter web at head height just to one side of our access point. I hope I don’t forget its location. The spider is close to 15 cm from leg tip to leg tip, and I don’t fancy the thought of it falling down the back of my shirt. (The spider gets its name from the fact that the silk of its web is a golden colour, not white.)

Golden orb web spider near the boat.

KENNEDY WARNE
Golden orb web spider near the boat.


At Isla Popa there is a marked difference in height between the mangroves on the shore and those inland. The shore mangroves are five or six metres tall, while those inland are a bonsai forest of dwarf trees less than a metre high. Candy has found that the shore mangroves are limited by nitrogen while the dwarf mangroves are limited by phosphorus. Adding phosphorus to the dwarf trees results in dramatic growth increases, but adding nitrogen to the shore trees has much less impact.

I head into the dwarf forest with Candy while Emily Dangremont, a graduate student from Berkeley, and Anne Chamberlain, one of Candy’s associates at the Smithsonian, work on the shoreline mangroves. Our tasks include counting seedlings (living and dead) and collecting leaves, seedlings, winkles and two sizes of crab from around the fertilised trees. These will all be physically and chemically analysed back at the lab.

Leaf scars provide an estimate of the age of the dwarf mangroves—as much as 300 years old.

KENNEDY WARNE
Leaf scars provide an estimate of the age of the dwarf mangroves—as much as 300 years old.


The dwarf mangroves (which Candy refers to affectionately as “Charlie Brown trees”) are a revelation. Candy points to the leaf scars on the branches. Each time a leaf falls, a permanent record of its existence is left on the branch. Between two and three leaves fall off per year, so the number of leaf scars gives an estimate of the age of the tree. (You can’t use the normal method of counting rings because mangroves don’t lay down rings annually, but on a more irregular basis.) Though these dwarf trees are smaller than a metre in height, it turns out they are several hundred years old. They grow in a layer of peat that is close to two metres thick, which has formed from the fine roots of generation upon generation of mangroves.

In Bimini, similar dwarf mangrove forests were denigrated as “stunted mangroves” by the Bimini Bay Resort development company, which justified its plan to destroy such stands on the grounds that they were biological rejects—losers in the game of life. Seeing these ancient forests in Bocas, collecting the crabs that nestled in their leaf bases and the molluscs that clung to their branches, underscored the crassness of the developer’s rhetoric. Who can say how much life owes its existence to these trees? And who has the right to deny it?

Further inland still, where the mangroves mudflats meet the terrestrial soils of the jungle, the mangroves are much higher—10 metres or more—and festooned with epiphytes. Bromeliads, orchids and arum lilies perch and twine on trunks that are felted with moss, liverwort and filmy fern. Candy thinks of the epiphytic community as a separate freshwater ecosystem suspended over the saltwater mangrove one. It is sustained by the high humidity and rainfall of Panama. (Bocas receives up to five metres of rainfall annually, spread throughout the year—but, luckily for us, not on the days we are in the field.) The epiphytes provide yet another level of complexity in an already complex system.

Epiphytes on the mangroves are a feature of Panama.

KENNEDY WARNE
Epiphytes on the mangroves are a feature of Panama.


I walk back to the shore mangroves to help Emily and Anne catch crabs. The most effective method is to “tickle” the algae-covered roots at the water line with your fingers, hoping to flush the crabs from their hiding places and cause them to climb up the tree. If you’re quick enough, you can grab the crab before it realises its danger and jumps into the water.

Aratus, the tree-climbing crab we’re going after, has an infuriating knack of staying on the opposite side of the branch to the side its pursuer is on. This isn’t such a problem if the branch is narrow, but on a thick trunk I sometimes circled round and round (with occasional feints in the opposite direction) and only glimpsed bits of disappearing leg or pincer as the crab kept deftly out of sight.

It’s great to be in the company of scientists again, sharing a quest for knowledge. Je ne regrette rien, but at times like this the scientific career path I set aside for journalism has the allure of the road not taken.

Tags: , , ,

Comments are closed.