Monday, 1 August 2011

Coastal Habitats

Common Sandpiper on mangrove creek
So, in what's probably going to be the last coastal blog until I get back to the beach again, I thought I'd give a first introduction to some of the interesting coastal habitats. (Note the logical progression - off stuff in the sea, interesting things on the shore, and now interesting things on the land but near the sea. Anyone would think I had it planned!) So, let's start by thinking about what's down by most beaches. And in East Africa you don't have to spend long before realising it's basically a mixture of coconut plantations and sisal estates, with a bit of cashew thrown in for good measure. So you won't be surprised to learn that the northern coastal regions have some of the highest population densities in Tanzania (here), though to be fair most of the plantations were originally started by colonial Germans and Brits some time ago. So the remaining native habitat fragments are in a rather bad way, and can be hard to find. They are worth seeking out, though, as ther are some special things living in them.

For the purpose of this post, I think I'm going to focus on the two of the most interesting coastal habitats: coastal forest and mangroves. Coastal forest has obvious functional similarities to forests elsewhere in Africa, though the species composition can be pretty different. Mangroves, on the other hand, have no inland analogues, so let's start there.
Mangroves still do grow on open coasts here north of Pangani

If you've not seen them, mangroves are starkly different to any other habitat. They're mainly tropical in distribution (there are some in the Australian temperate zone, and a few on the East Coast of the US, but these are the exception that prove the rule), and once upon a time would have occured on any tidal zone (ie, on land covered by the high tide, but exposed at low tide) with a minimal degree of shelter from the full force of the sea - they used to cover over 3/4 of tropical tidal areas. At one time, that included even rather isolated islands like Maziwe, but now you'll rarely find them on the beach front themselves, but mostly limited to tidal creeks. And you'll know when you find one, because you'll be walking through the coconut plantation and suddenly hit a dense, dark and glossy thicket growing like a wall out of the bar mud or sand (sometimes even rock) substrate.
Mangrove edge in cocnut plantation, Ushongo Beach July 2011

If you can make your way through the mangroves, you'll see some interesting sights. Down on the ground the most obvious things are the arial roots - sometimes poking up like fingers, other species rely on multiple branches from the stem. These have a wonderful scientific name - pneumatophores - and their purpose is simply to allow the plants absorb air. But they also form a unique and busy microhabitat of their own - a mini-forest that traps passing mud and can harbour much richer muds than surrounding areas as well as providing lots of nice little places to hide from predators. Consequently, the root systems of mangroves are home to lots of life - there's always crabs to see, and in areas nearer to low tide limit there are often air breathing fish called mudskippers, which are kind of neat and perhaps remind us of our distant ancestry... What's more, these root systems are often extremely important nurseries for the young of reef fish important in many local fisheries.
Aren't those roots odd! Mangrove pneumatophores, Ushongo Beach

These pneumatophores aren't the only interesting adaptations of mangroves though - not only do the have to survive in water-logged soil, but it's very salty too (and in areas where shallow seawater sits around in the hot sun the concentrations of salt can be extremely high) and most plants can't tolerate salt. Mangroves have a number of adaptations to help, of which the most obvious is the glossy, thick leaf itself which reduces water-loss. But more fundamental are adaptations to the root that we can't see that simply filter the salt out of the water - by the time water arrives in the root, 82 - 97% of the salt has been filtered out. Given how difficult we find it to extract frest from salt water mechanically in desalinisation plants, that is a remarkable adaptation! And then what salt does get into the plant can be collected and excreted through the leaves, giving them a silvery appearance at times.
These roots also  absorb air in another mangrove species, Ushongo Beach

In such harsh environments, it's going to be particularly tricky for seedlings to thrive, so mangrove species have another neat adaptation up their leaves - they're (mostly) viviparous. What? Live-bearing plants?! Yes indeed, most mangrove species don't immediately drop their fruits, but retain them on the plant whilst the seedlings develop to a stage where they can photsynthetise themselves - some seedlings growing entirely inside thr fruit, others growing through and out. Then, when they're ready, the plant drops them either like a dart into the mud below them, or into water to float the seas until washed up in some suitable substrate, when the young plant is ready to go. Which explains why many mangrove species have extremely large distributions, dispersing on cross-oceanic currents.
Mangrove fruits germinating on the plant and ready to fall.

So, that's the structure of the mangrove, and it's obvious that this sort of harsh environment is going to be home to species of plants that are survivors - all their obvious competitors are eliminated by the environment itself. And just as the roots are home to lots of little beasties, the trees themselves are home to a pretty good selection fo birds and animals too. The really special bird to look out for in this area is Mangrove Kingfisher, but the mangroves can often be full of mixed species flocks of starlings (mainly Black-breasted), barbets and greenbuls, etc. Some great birding to be had if you can see into the thickets!

So, all in all, mangroves are wierd but great coastal habitats. And it's a shame there aren't more of them, because they are fantastically useful to us too, not least through their very impressive abilities to mitigate against the impacts of tsunamis.

Hmmm. Well, seems like this is probably a long enough post for now anyway, so coastal forests can wait for another time. Ecologically, they're rather similar to other forest types anyway, even if the species they hold are sometimes very different!

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