A fresh water mass extinction: is it on the horizon?
In this blog I have spoken a lot about how
fresh water issues such as scarcity and contamination have affected humans, but
with the recent release of WWF’s Living Planet Report it seems like a
good time to discuss the effect these problems have on fauna and freshwater
ecosystems.
Despite covering less than
1% of the Earth’s surface, freshwater contains a disproportionate amount of
species, almost 6% in fact (at least 100,000), including around a third of all
vertebrates. However there is no doubt that they are under threat, largely, if
not entirely, down to human activity. Strayer
and Dudgeon, 2010 state habitat degradation, pollution, flow
regulation and water extraction, fisheries over exploitation and alien species
introduction as the primary causes of this, and there is an increasingly
overwhelming case for adding climate change to this list. The Living Planet
Index revealed that global populations of vertebrate species had decreased by
58% between 1970 and 2012 with the decline being much more severe in the
freshwater ecosystem (81% - as shown in the following diagram) compared to terrestrial
(38%) and marine (36%).
The fact that 1970 is the starting point of
this study makes it likely that significant reductions in population of
freshwater species had already occurred. After the Second World War, the
building of dams proliferated peaking at around 5500 large dams being
constructed per year in the 1970s (Jones, 2014) and although the effects of
these are hard to predict, flow interruption can have negative effects on the
ecological integrity of flood plain rivers due to changes to patterns of
flooding and degradation of downstream channels (Ward and Stanford, 1995) as well as blocking
migratory species, and creating calm bodies of water with different
temperatures to rivers that may favour different species whilst encumbering
others. Dams also block sediment transport which can prevent vital nutrients
reaching floodplain soils (Holland, 2016). There were also fewer
regulations on industry back then which allowed the likely increased
contamination of waterways and in turn habitat degradation.
Since 1970, dam building has remained a
driver of this diminution as although construction has reached somewhat of a
standstill in Europe and USA, it is still prevalent in developing nations such
as China, Brazil and India and there is now 10,000km³ of freshwater stored in
dam reservoirs, a staggering five times the amount in surface rivers. The
reason for this vast amount of water being needed is of course the increasing
consumption of freshwater by humans that has occurred in line with population
increases (although these increases were also taking place pre-1970). The
following image shows dams being planned and in construction:
Another side effect of this population rise
is the over exploitation of fisheries that has taken place due to an
ever-increasing demand for food. This mainly refers to the unsustainable
harvest of fish from freshwater, but indirect over exploitation can occur as
other species are inadvertently caught in fisheries. Studies have concluded
that inland waterways and ecosystems have been poorly managed, and that fish
stocking has been prioritised over habitat management (Aps, Sharp, and Kutonova, 2004) which in the long term has resulted in declined
numbers.
In terms of how pollution
can affect fresh water ecosystems, it is similar to as mentioned in the
previous blog post on water contamination. Pollutants can include chemicals and pesticides, raw
sewage, petroleum and even thermal discharge. Toxic chemicals, such as PAHs
(polycyclic aromatic hydrocarbons) and PCBs (polychlorinated biphenyl) released
from industry, and pesticides can have a range of life-threatening effects on
aquatic creatures. Depletion of oxygen levels can be triggered by nutrients
from agricultural runoff causing eutrophication, as well as the decomposition
of faecal matter (WWAP, 2006).
In 1970 when data collection for the LPI
started, climate change would not have been considered one of the primary
threats to global populations of wildlife. But as carbon dioxide emissions
continue to increase and global temperatures exceed 1°C above pre-industrial
levels, it can no longer be ignored. Fresh water ecosystems are especially
vulnerable to climate change because the species which inhabit them are largely
unable to move to a different environment as theirs changes. On top of this, fresh water temperature and abundance are both climate dependent – increased global
temperatures can lead to droughts and additional strain being placed on rivers
and wetlands with unsustainable extraction levels in order to irrigate crops. This
can result in these areas drying up with obvious loss of habitat.
So is there is a solution to this worrying
problem of population and species decline?
First of all, it makes sense to protect river
and lake ecosystems which are currently untouched. As for those regions which
have already been affected by human activity, reconciliation ecology is a term
that has been used to ‘encourage biodiversity in human-dominated ecosystems’.
It is a recognition that destruction of habitat takes its toll on species. Although
it generally applies to smaller, novel ecosystems, this concept is important in
changing mind sets towards preservation.
The LPI notes an increase in migratory fish
species since 2006, which it puts down to improving water quality in regions
such as Europe, and fish passes being added to man-made obstructions to allow
migrating fish to move through. If these could be applied globally, especially
in the previously mentioned nations where dam construction is still widespread
and water quality is generally lower, then it could have a huge effect.
Restoration of ecosystems to the condition they were in before humans interacted
with them is largely unrealistic, but dam removal projects are the closest
thing to this. A number of these have taken place in the USA, where outdated
structures are removed often leading to environmental restoration, although due
to the huge demand for freshwater from humans it is impossible to make dam
removal a widespread process.
There are definite steps forward but the
danger is that they are being overwhelmed by the setbacks which could lead to a
mass extinction of freshwater species.