THE CONSERVATION COLUMN: Mangroves - Magic in the Mud
Prof. Mark Huxham is Professor of Teaching and Research in Environmental Biology at Edinburgh Napier University, and can often be found on Kenya’s coast leading Earthwatch research expeditions. In this month’s CONSERVATION COLUMN, Mark makes a case why the modest mangrove deserves a little more of our attention…
Some habitats are easier to promote and protect than others. Snorkelling on a coral reef for the first time is an intensely emotional – even spiritual – experience; the vibrancy and wonder of the life there is argument enough for reef conservation. But admirers of mangroves have always had a harder job. Mangroves mean mud, mosquitoes and murky water; they’re home to crocodiles, biting crabs and bad smells. The novelist Joseph Conrad uses their mysteriously twisted shapes in Heart of Darkness to exemplify ‘an extremity of despair’.
Even if you don't actively disparage mangroves you may not have given them much thought as you head off for the clear blue water. I’m hoping to persuade you to look again; mangroves are natural marvels which form an essential part of most healthy tropical coasts. Our seas, and our planet, would be much diminished without them. ‘When we try to pick anything out by itself, we find it hitched to everything else in the Universe’ is how John Muir saw ecology. Mangroves provide the best examples I know of ‘hitching’ between different systems, and understanding how the knots are tied is a joy and a challenge for mangrove ecologists. Let me describe just two of the ways our team of Earthwatch-supported scientists are exploring in Kenya.
An Earthwatch helper hard at work
A crèche for fish
It is tempting to apply the glamour of safari predators to some of the big fish on the reefs. Do the insouciant beauty and sleek muscle-flash of attack make barracudas the cheetahs of the sea? Maybe, but the analogy does not teach us much about their life-cycle. If it did, cheetah cubs would perhaps live underground in tropical forests, in swarming and miniature multitudes where they were grazed on by deer. In the sea your position in the food chain generally depends on your size and your age and those at the top were once near the bottom. So big barracuda eat the fish that eat small barracuda. This sized-based predation means that the ability to hide from predators until you are large enough to become one is a crucial adaptive trait.
Mangroves appear to support high numbers of juvenile fish – so perhaps they are using them as refuges, hiding among the roots until large enough to chance the open waters? The idea makes sense, but finding the evidence involves showing first that there really are lots of fish in the forests and second that those fish are the same ones that live as adults offshore. Neither task is easy. Fishing inside a mangrove is like chasing moles with a rake; the whole point is that catching fish there is difficult. But an array of hard-won data now shows that the forests often support high densities of fish and shrimps. And we can use the naturally occurring isotopes of carbon and oxygen as stamps in the passports of larger fish, revealing their past travels. Because the inner-ear bones (otoliths) of fish grow in radiating circles, like the stem of a tree, inspecting otoliths from large adult fish allows the removal of chemical tracers deposited in their juvenile lives; for barracudas, snappers and others these tracers lead back to the mangroves.
So fish provide one hitch between mangroves, sea-grasses and reefs. Another link is with the planet as a whole, or more precisely with its present and future climate. The mud that is so characteristic of mangroves, and which has been deposited over centuries from the sea, the rivers and the trees themselves, hides a secret. It contains megatonnes of carbon, stored safely in anaerobic inertia and locked away from the atmosphere where it would give a significant push on the pedal of our accelerating climate change problems. Scientists have long known that forests are a vital part of the global carbon cycle but it is only in the last few years that we have realised the disproportionate contribution of coastal vegetation, particularly mangroves, in storing carbon. Although mangroves constitute only 0.7% of tropical forests they are responsible for more than 30% of the carbon that is buried in these ecosystems. Destroying mangroves not only removes habitats for fish but is like removing the cap on an oil well, leaving the fossil carbon stored there to flush into the atmosphere. In experimental work we are doing in Kenya we have watched while the level of the forest floor has sunk in response to tree removal; it’s like watching a very slow fire.
Some of the work is painstaking
So mangroves keep coral reefs healthy and help stabilise the climate, as well as giving wood and medicines to local people and protecting the coastlines from erosion and storms. Their economic value is enormous (although this doesn’t stop people making a fast buck from cutting them down). But they are more than just resources. They are trees evolved to grow in salty, anaerobic mud, buffeted by waves and inundated by the tide. This ranks with polar bears overwintering in their snow holes and arctic terns threading the oceans from pole to pole in its sheer adaptive wonder. And to sit quietly in the forests at dusk as the tide laps in, listening to the snapping shrimp and the calls of mangrove kingfishers, watching the crabs seal their burrows and the lights of the fireflies blinking in the branches… to do that is to see mangroves as beautiful as well as useful.
So enjoy the reefs and the deeps, but don’t forget these unique marine forests on your way there and please join the efforts to understand and to protect them.
Images courtesy of Kate Holt/Shoot the Earth