There are places in the world where water slows down, spreads out and covers the land
in shallows.
In these calm, undisturbed conditions, plants have evolved in many special ways to thrive.
From saltwater mangroves along the tropical coasts, to the bogs and peatlands of the subarctic,
these flooded lands accompany every biome and climate zone on earth.
Their ecosystems are among the richest and most complex to be found anywhere.
I hope you brought your waders.
You’ll need them, as we enter the world’s wetlands.
Wetlands are regions of the planet that have sluggish or standing water at relatively shallow
depths, that allow the growth of plant material.
They can occur in fresh water, such as river floodplains, saltwater, such as in mangroves,
or in interfaces between the two, such as in river deltas.
They occur in practically every parent biome on Earth, from the equator to the polar regions,
and unlike the biomes we have covered so far in this series, are much more decoupled from
climate, with the presence of shallow, undisturbed water being the dominant influence.
They are highly biodiverse areas in terms of both plant and animal species, and for
this and other reasons to be revealed, are of global importance.
The interface of water above land creates a very complex picture in terms of plant and
soil systems, and so this work should only be seen as a summary guide to provide a beginner’s
overview to the subject, and through imagery, provide some reality to it.
Such complexity reveals itself immediately with the myriad of terms used to describe
such places.
In the English language alone there are almost a dozen – swamp, mire, moor, marsh, muskeg,
mangrove, flat, fen and bog.
Some of these overlap, and most are historical terms that modern ecologists have attempted
to corral into tighter technical definitions.
In general wetlands vary depending upon the salinity and mineral content of the water,
as well as overall temperature ranges.
Starting with the most salinity, we have mangroves and salt marshes both of which occur in coastal
areas where the salinity is more or less that of seawater.
Both occur in areas protected from wave action that would otherwise disturb the settlement
and germination of plant seeds.
Mangroves are found within the tropics and some subtropical areas provided no frosts
occur.
They are characterised by a thick covering of salt-tolerant tree and shrub species, and
some species have roots with stick up out of the mud like spikes, so that the plant
can breathe at low tide.
Examples of extensive mangrove forests are the Sundarbans of the Ganges Delta in India
and Bangladesh, the Niger delta in West Africa and the Orinoco river delta in Venezuela.
Salt marshes are found in sheltered coastal areas outside of the tropics that are free
of ice, namely, temperate regions where the area is regularly flooded with seawater at
high tide.
They are formed from salt-tolerant herbs, grasses and sometimes low shrubs that are
able to bind sediments that flow into the area, and so stabilise the marsh, allowing
it to grow.
As sediments play a key role, they are typically found in river estuaries, but may also occur
on the leeward side of barrier islands and spits of land.
Examples of such marshes are the Camargue at the estuary of the Rhone River in Southern
France, the Mississippi delta and The Wash of Eastern England.
In rivers upstream from salt marshes are found tidal freshwater marshes, where tides further
downstream result in the freshwater of the river backing up and flooding, and so do not
directly see salt water.
These are commonly found in large temperate estuaries that experience significant tides,
such as the St Lawrence of Eastern Canada, and the Severn of Great Britain.
The last category of coastal wetlands that experience seawater or tidal influences are
the Intertidal flats – these are flat areas subjected to tides that are too unstable in
terms of sediments and so remain unvegetated.
Morecambe Bay, in the North-West of England is an example of this.
Inland freshwater wetlands come in many forms, and differ in terms of the source of water,
whether they produce peat, whether they are forested or not, and whether they are flooded
seasonally or year-round.
Freshwater marshes are large non-forested flat areas that are either continuously or
frequently flooded, but which have water flowing through them.
Because this water circulates, acidity does not build up in the decomposing dead plant
material, and so does not form peat – which I’ll touch on more in a moment.
The primary vegetation here is grasses and sedges, along with emergent plants – those
with soft stems that are highly adapted to saturated conditions.
Reeds, also known as bulrushes or cattails are a common sight here too.
The largest freshwater marsh is arguably the world’s most famous wetland – the Everglades
of Southern Florida.
Another notable wetland of this type occurs at the confluence of the Tigris and Euphrates
rivers in Southern Iraq.
The Mesopotamian Marshlands have been home to people for thousands of years, but in the
1980s and 90s, the vast majority of these were drained for political and economic reasons,
and today are only 10% of their former size.
Freshwater forested swamps have similar water dynamics to the marshes just covered, in that
they are large flat areas that are either permanently or frequently flooded with fresh
water.
The difference is that the primary vegetation is trees instead of grasses and sedges.
They occur in tropical and temperate regions.
The largest of these is one of the largest wetlands in the world, and that is the central
part of the Amazon river basin that floods over an enormous area for months during the
wet season.
This flooding accounts for the wider distribution of evergreen forest compared to the smaller
area of tropical rainforest climate – areas that have a notable dry season, with either
Tropical Monsoon or Savannah climates, still have evergreen rainforest instead of the seasonal
forest that sees the loss of leaves in the dry season.
This is because the flooding lasts well into the dry season and provides sufficient water
for the trees to continue growing during this time.
Equally as notable are the numerous and iconic flooded forests of the Eastern United States,
which extend from Maine, down the Eastern Seaboard, and along the Gulf States to Texas.
They are the product of flat plains combined with plenty of rain from the Humid Subtropical
climate.
In the north, Red Maples dominate, while in the south Atlantic White Cedars make up much
of the shallower swamps.
Deeper swamps in the south see Cypress trees dominant.
The largest wetland in Europe is part forested swamp, and part freshwater marsh.
Known as the Pripyat or Pinsk Marshes, it straddles the border of Belarus and Ukraine.
Like most swamps, it is relatively impenetrable, and was historically significant both World
Wars in slowing and dividing invading forces into northern and southern groups.
Related to the freshwater marshes and forested swamps are the Riparian wetlands.
Riparian simply means “of a river bank”, and these wetland types centre around a river
which will regularly burst its banks for part of the year.
It can be argued that the Amazon wetlands are of this type – there is a degree of
overlap in terms of classification.
A clear example of a river dominated wetland, however is that of Mississippi.
Or rather, was, for almost all of the riparian wetlands that extended as far as 50 miles
(80 km) on each side of the river for up to 500 miles along its lower length have disappeared
over the last 200 years.
The reasons are obvious.
Such a long navigable river, reaching from the sea far into the continental north was
always going to be an economic power line.
And so the towns and cities that grew up on its banks needed protection from the regular
damaging floods.
Levees, or protective embankments, sprang along its length, keeping the water in, and
eventually leading to the draining of the wetlands, whose fertile soils were converted
to farmland.
That vast area can still be seen outlined today.
The most notable wetland of this type that still survives is the vast seasonally flooded
basin at the tripoint of Brazil, Bolivia and Paraguay known as the Pantanal.
Chances are you’ve seen this in wildlife documentaries, and for good reason, as it
has one of the richest ecosystems in the world.
It is regarded as the largest tropical wetland area, and is the largest flooded grassland
in the world.
And then there is the Sudd of South Sudan, a vast flooded grassland and inland delta
into which the Nile River, flowing north, slows to a crawl, spreading out across thousands
of square miles.
This area is historically significant as despite their might and sophistication neither Ancient
Egypt nor the Roman Empire were able to penetrate its endless channels in the search for the
source of the Nile.
While we’re on that river, no discussion of global wetlands would be complete without
a mention of the lower reaches and end of that mighty watercourse – the Nile Delta.
Since records began, that vast area has been irrigated and cultivated to feed the large
population of Egypt, so we have no way of knowing for sure what the original form of
that wetland delta once was.
Every year, the deluge from the rainy season in the tropical savannah of the East African
highlands would flow into the Nile, find its way through the Sudd and continue north into
Egypt, where it burst the river’s banks.
The water, spilling out onto the floodplain south of the delta as well as within it, would
provide mineral rich silt that fertilised the land and allowing for bountiful harvests
year after year.
This is perhaps the most notable example of a riparian system influencing an entire civilisation.
The yearly floods ended with the construction of the Aswan Dam in the 1960s, and so, like
the Mississippi, this river was tamed by humans, and the last vestige of natural wetlands came
to an end.
As we come to the last two types of wetland, bogs and fens, it’s time to talk about peat.
Many wetland areas have the right conditions for the formation of this acidic concentrate
of dead plant material, which is, in fact, the first stage in the formation of coal.
It is formed when dead plant material, most of which is moss, falls to the bottom and
becomes compressed as further layers accumulate on top.
As the water obstructs the flow of oxygen and as acids form to produce an almost vinegary
level of pH, normal bacterial processes are inhibited and so decomposition is very slow.
As a result most of the carbon in the original plant is preserved instead of being lost as
CO2, as occurs in more regular rates of decomposition.
Consequently, peatlands act as significant carbon sinks, and it is believed that as much
as 30% of all carbon in the biosphere is trapped in peat.
So they play an important part in the overall carbon cycle, and if these are disrupted,
such as in the case of drought, large amounts of CO2 can be released.
This occurred recently in Indonesia, when large areas of peatland there caught fire
during a drought, or were deliberately ignited by farmers, to make way for farmland.
That episode resulted in the release of CO2 equivalent to the entire output of the USA
in one year.
Bogs and fens are both peat-producing, with bogs being the most productive in this regard.
They differ from each other in that bogs get their water directly from rain or snowmelt,
and so the water has low mineral content, which means little neutralisation of acids
in decomposition.
Fens, by contrast get their water additionally from streams or groundwater, and so the water
has higher mineral content and a greater ability to neutralise acids, leading to slower peat
formation.
Peatlands can occur anywhere in the world where there is decaying plant matter that
is left in stagnant water.
The largest peatland of the tropics is believed to be the Cuvette Central of Congo, found
on either side of the vast Congo river as it arcs southwest toward the capital of Kinshasa.
It is one of the least researched and poorly understood of all the world’s wetlands.
In the temperate regions, one of the most well-known peatland areas is that of the western
part of Ireland, where heavy rain combined with post-glacial depressions and forest clearance
millennia ago have led to the production of extensive bogs that make up about one-sixth
of the whole island.
In addition to typical bogs in lower elevations, the forest clearances in the uplands led to
the colonisation of these areas with heather, turning the soil more acid, resulting in slower
plant decomposition and the accumulation of peat.
With Ireland being the most deforested country in Europe, firewood was scarce and so such
peat was a vital source of fuel for home heating for centuries.
But the largest stretches of peatland occur in the subarctic, namely Alaska, Canada, Northern
Scandinavia and Siberia.
The subarctic has so many of these wetlands for a number of reasons.
Being glaciated in previous ice ages, the land is relatively flat, and till, that is
rocky material carried by the ice, would be deposited unevenly, forming depressions that
could then gather water after the ice age.
In the cool subarctic summers, low evaporation rates means any ice melt or rain stays around
longer.
And lastly, poor drainage brought on by permafrost, a layer of soil that is frozen either year
round or for part of the year, acts as a barrier to drainage, and so water builds up above
and often flooding the surface.
The largest wetland in North America is the Hudson Bay Lowland, a mix of peat bogs, fens
and saltwater marshes fringing the whole southern edge of Hudson Bay.
The largest wetland region of any kind in the world is probably the West Siberian Lowland,
that encompasses an area of about half the size of the United States.
It runs east from the Ural Mountains over a vast and extremely flat plain to the Yenisey
River about 1,500km to the East, and from the Arctic Ocean to about 1,200km south.
The area is covered in an endless patchwork of bogs and fens interspersed with soggy boreal
forest, or taiga, and outside of the winter, was essentially impenetrable until the mid
20th Century.
One last region of wetlands worth mentioning is the Prairie Pothole Region that runs from
Alberta down to Iowa.
As its name suggest, this region of the North American Prairie is covered in thousands of
pothole depressions formed by uneven deposition of glacial till during the last ice age.
These potholes fill with water in the spring, and many evaporate by summer, but some are
permanent.
As they don’t quite fit the description of any of the above types, this final point
goes to demonstrate that wetland habitats are as complex and varied as earth is in terms
of topography, hydrology and biology.
Once viewed as wastelands that could be drained to produce valuable fertile farmland, wetlands
outside of the arctic have shrunk dramatically over the last few centuries, and those still
surviving have often been affected by runoff of fertiliser and pollutants which, in a slow-moving
or stagnant water system can disrupt the ecosystem significantly.
More recently this trend has continued in the tropics.
But in developed nations, public awareness of the importance of their rich ecosystems
has resulted in a shift of attitude over the last 50 years, with the result that most are
now protected from drainage.
Between 10-20% of all bird species globally are dependent upon wetlands, making them the
most dependent of all the animal kingdoms.
By comparison, only around 2% of mammals are wetland dependent.
And so reduction in wetland areas puts a stress in particular on bird populations.
The Ramsar Convention, originally signed in 1971, but which meets every three years, has
created a list of over 2000 wetlands globally that are of importance and signatory countries
work to ensure these remain protected.
Lastly, as mentioned, many wetlands act as carbon sinks through the capture of CO2 into
plant matter which, when dead, is locked into the peat.
If these peatlands are disturbed, then this carbon is released once more into the atmosphere,
and may contribute further to climate change.
It is easy to forgive humans for wanting to turn these unproductive areas into land that
can work for them by draining them to produce farmland or for urban development.
But like any species that ultimately wants to survive, its important that we find the
right balance between nature and development.
Let us hope that our realisation of the importance of these wetland areas has not come too late.
The world would be a poorer place without them.
And that’s the wetlands.
I hope you enjoyed this introduction to this fascinating and complex set of biomes.
It is a big subject, and I’m sure that some of you will want to mention particular wetlands
of significance that I did not cover.
Feel free to do so in the comments.
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Thanks for watching, and I’ll see you in the next video.
Until then, stay dry!