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%).  

Source: Living Planet Report, WWF.

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.