Habitat Loss

Loss of habitat is a huge problem facing many areas in the UK and across the world. It generally results in loss of biodiversity and can eventually lead to eradication of a species entirely. In rivers especially, habitat loss is an issue because people often try and alter river channels to suit their own needs without thinking about the impacts this may have on wildlife. Changing a river’s natural flow can directly impact habitats by simply making them unsuitable for certain species, or indirectly impact them by making them more susceptible to disturbances or encouraging invasive species that may predate or outcompete the native ones. Over the decades, the River Thame has been dredged, diverted and dammed in our attempts to control it, much of which turned out not to achieve its aims, and all of which affected the wildlife. Here we look at some specific examples of habitat loss.

Urbanisation

The south-east of England is the most crowded part of the country, so there is enormous pressure on all resources, including water. This comes from all human activities including

  • Domestic consumption – drinking and washing.
  • Sewage disposal and treatment.
  • Industrial consumption.
  • Agriculture.
  • Leisure pursuits such as angling and shooting.
  • Domesticated animals, including pets.

Although the River Thame has only one large town (Aylesbury) in its valley, it is still quite densely populated and there are settlements along its length all of which require sewage treatment plants to deal with their waste, and from which treated water flows into the river.

Sedimentation

This cattle drinking spot will mobilise sediment, allowing it to flow in to the main river channel and disrupt habitat.

This cattle drinking spot will mobilise sediment, allowing it to flow in to the main river channel and disrupt habitat.

Sedimentation is when sediments are deposited from the water onto the streambed. It is an important process that is necessary to sustain aquatic ecosystems, but when accelerated it becomes an issue for benthic organisms. They can be buried under surplus sediment becoming unable to escape. These bottom dwelling animals are usually at the base of the food chain so there is a knock-on effect on whole ecosystems. Excess sedimentation is often a problem in rivers near to agricultural land. When natural wetlands or woodland are converted to fields for crops or livestock, there is more soil erosion and the amount of sediment runoff into streams is increased. This is because the soil is disturbed, which leads it to become loose and it can then be carried into channels by rainwater. Construction work close to rivers also exposes sediment from the earth which adds to sedimentation.

Channel straightening

An example of severe channel straightening, along with an artificial bank.

An example of severe channel straightening, along with an artificial bank.

Channels are usually straightened to reduce flood risk in a specific area. Water flows faster in a straightened channel so it speeds past and is less likely to flood a vulnerable area. However, this is a drastic river management technique and has many knock-on consequences. It means that flooding is more likely to occur downstream of where the channel has been straightened – which not only impacts human houses but also wildlife habitat which is not accustomed to flooding. Erosion is also increased downstream with the increased water velocity which can disrupt benthic habitats. When it is straightened, the newly created channel leaves little of the previously established habitat. The meanders that the river once flowed through are disregarded and quickly dry up – any organisms that lived in them are unable to survive.

Culverts

The Chalgrove Brook passes through multiple culverts, this is one in Watlington.

The Chalgrove Brook passes through multiple culverts, this is one in Watlington.

Culverts are usually pipe-like structures which are embedded into surrounding soil that allow water to flow under a passageway such as a road or railway. When poorly designed they can have hugely negative impacts on habitats. Their initial construction is usually highly disruptive as it requires removal of vegetation and sculpting of the environment around the culvert. Culverts can prevent passage of many animals, especially fish, when not properly installed to match the flow of the river. They can become blocked up with sediments, further impeding transport of many species, which can prevent breeding and lead to declines.

Invasive species

Invasive species in rivers can greatly alter the freshwater habitat that native species rely on. Invasive plants especially cause problems by growing rapidly and overtaking channels, blocking light from other organisms and taking a lot of oxygen out of the water. Invasive predators also cause loss of habitat by taking prey from native species and leaving stretches of river bare, homogenous, and unable to support much life.

Water abstraction

Excessive abstraction of natural water sources for irrigation or industrial or domestic use can often mean riverbeds dry up, causing irreversible damage to habitats. Currently there is no abstraction for drinking water purposes in our catchment.

Pollution

Chemical pollution in rivers includes run-off of fertilisers from fields and outputs from sewage treatment works. This changes the composition of the water making it uninhabitable for some species. Fish and native aquatic plants are hugely affected by this so habitats are completely changed. Polluted freshwater habitats are often only suitable for hardy invasive species, so they spread through catchments, outcompeting other species.

Much of the Thame valley is intensively farmed which can result in fertilisers and pesticides running off into the river, or in cattle gouging river banks and dropping dung in the water. Farming practices have had a strong, and usually negative, effect on wildlife as arable fields have been cultivated for maximum efficiency, leaving no habitats for birds and insects. Thankfully, nowadays many farmers along the river are aware of this negative impact and are leaving space for wildlife and making efforts to prevent damaging run-off into the waterways.

That being said, there is good news too: Thames Water have upgraded the Aylesbury sewage plant so it is modern and able to cope with the demands upon it, and a lot of the agricultural land along the river banks is in the form of flood meadow or has a buffer zone of reeds or wildlife strips between the crop and the water. Where farms have Level 1 Environmental Stewardship status, as in the case of Waddesdon Estates, this is managed with particular care. There is a lot of rather messy scrub land along the banks of the Thame which, although it isn’t beautiful, can be a great habitat for birds and mammals

Climate change

Climate change is continuously affecting weather patterns in unpredictable ways. In some areas, more frequent and severe flooding changes river habitats as the increased flow causes more erosion of the bed. Conversely, droughts are also becoming more frequent in other areas – this means channel habitats dry up and can become unable to support such a range of life.

There are many ways in which it is possible to prevent or mitigate habitat loss. Check out our habitat improvement page to learn more.