Change in Habitats

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A sward of permanent pasture containing a variety of grass species

A habitat is an area of the environment where an organism lives, feeds and breeds. Some organisms are site specific in their habitats, for example fish need water. Other organisms use different habitats for feeding, breeding etc. i.e. some birds will nest in trees but feed on the ground. 

Predatory organisms depend upon other species for their existence and therefore their habitat would be the same as that of their prey, thus creating a 'knock on' effect through the whole ecosystem.  This section addresses the effect on ecosystems (habitats) of agriculture in general.  Examples of agricultural practices and their effects on specific organisms will be referred to throughout the remainder of the course.

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Agricultural factors effecting hedgerows, trees and woodlands as a habitat

Hedgerows are seen as one of the defining features of English landscape and are important habitats for wildlife, such as birds, small mammals and invertebrates. They are often the oldest remaining feature in the countryside, providing important evidence of its historic development.

Managed (left) and unmanaged (right) hedge

Advantages of hedges:

bulletShelter for lambing/calving and crops
bulletLivestock retention
bulletMay help prevent water run off
bulletPotential yield increases due to change in microclimate
bulletIncrease resistance to soil erosion
bulletReservoir of predators of pests
bulletBreeding site for birds
bulletNatural habitat corridor

Although the countryside has never remained static in form, changes in recent decades have led to concerns at the rate at which hedgerows were disappearing. This was not only because they were deliberately being removed (e.g. to make larger fields) but also because they became derelict (i.e. they ceased to be cut and managed as hedges and grew into lines of bushes and trees). These problems are tackled separately through legislation to control removal and financial incentives to encourage management. More information on hedgerow management can be found at the DEFRA website.

Pause for thought.........List four hedgerow characteristics which you believe are of benefit, firstly to the environment, and then to farming. 

Hedgerow removal

Although arable farming is not the only reason for hedgerow removal, it has, in most recent times been the main cause. Arable farmers have less reason to retain and maintain hedgerows and are more likely to see economic reasons for enlarging fields and widening gateways.  Under the Hedgerows Regulations 1997, it is against the law to remove most countryside hedges without first getting the permission of your local council. Between 1984 and 1993, the length of managed hedgerows in the UK decreased by nearly a third. In the latter part of that period, the decline slowed, with the rate of new planting of new hedgerows exceeding the rate of removal between 1990 and 1993 (see below).       

Reasons for destruction:

bulletLarger fields to facilitate larger farm machinery e.g. an increase in field length from 110 m to 800m reduces the turning time of agricultural machinery by about 80%.
bulletBoundary hedges become redundant as farms are amalgamated or livestock enterprises abolished
bulletFencing as an alternative to control stock
bulletIndoor facilities - unnecessary for hedges to provide shelter for lambing or calving
bulletPossibly harbour pests, disease and weeds

Source: DETR hedgerow survey 1993                         

This decline in hedges observed during the 1980s does appeared to have halted and in some areas may have been reversed as the Countryside Survey 2000 reported no change in hedge length between 1990 and 1998. Results from the 2007 survey can be found at the Countryside Survey website.

Pesticide & fertilizer drift

This can be a particular problem on intensively managed arable land. Herbicides were often applied to 'hedge bottoms' to eliminate arable weeds such as Cleavers (Galium aparine) or Baron Brome (Anisantha sterilis). This proved to be a very damaging practice for the flora and fauna of hedgerows, and there is research evidence to show that it is an ineffective strategy for weed control as herbicides may not kill all of the plants they come into contact with. Accidental spray drift into hedge bottoms is very common, particularly in arable fields that are cultivated right up to the field boundary. The removal of hedge base flora has been shown to reduce insect numbers, having an indirect effect on predatory invertebrates, bird, mammal and where relevant, reptile and amphibian populations. The same factors apply to the use of insecticides, many of which are 'broad spectrum' and consequently lethal to a diverse range of invertebrates.  Small mammals such as shrews, and bird species such as tits and partridges depend on field margin insects to rear their young.  In some studies, fewer bird species have been recorded in hedgerows adjacent to arable fields compared to those bordering grassland, this has often been attributed to insecticide drift, but could be attributed, to some extent, to the larger field sizes, and greater frequency of management operations on arable land.

Fertilizer drift into the hedge base will favour certain plant species at the expense of others. The species that thrive are generally of little conservation value, such as stinging nettle (Urtica dioica), creeping thistle (Cirsium arvense), barren brome (Anisantha sterilis) or cleavers (Galium aparine).

The hedge bottom on the right has probably been untreated for a longer period of time than the one on the left

Pause for thought...........Nettles, Thistles, Barren Brome and Cleavers have little conservation value, and are serious agricultural pests. Does this mean we should not be concerned if they are completely eradicated or do they still have a vital role in the ecosystem?

Close cultivation

Cultivating land right up to the hedge base can be detrimental to the flora and fauna. It:

bulletReduces the width of, or totally removes the 'grassy strip' between the hedge bottom and the crop, therefore restricting habitat by plant removal
bulletIncreases the likelihood of pesticides and fertilizers entering the hedge itself
bulletCan also directly damage roots of hedgerow shrubs and trees.

Intensive grazing

Increased stocking densities can lead to hedgerow damage. Strip grazed dairy cattle if tightly restricted, will 'graze out' hedge bottoms. Sheep are particularly damaging to hedges, especially when lambs are present. Lambs like to lie and play under hedges. Sheep graze the hedge bottom flora very intensively and often browse the lower side shoots of woody species in hedgerows, this, alongside the trampling effect, causes considerable damage often resulting in 'gappy or open hedges'. These are a common site in areas of lowland sheep production, the only foliage being present is above the level of the sheep's heads. These open hedges with poor ground flora lose their 'connectivity', are poor habitats and do not provide enough cover or food to be attractive corridors for the movement of wildlife. 

Field boundaries or wildlife corridors, hedgerows remain a vital part of the agroecosystem

Hedgerow Management

Farmers are generally responsible for hedgerow management, so this could possibly be considered as a direct effect of agriculture. Hooper (1992) identified four types of hedgerow based on their management:

1. Unmanaged hedge, up to 4m tall and 4m wide: (1a) either grazed out at the bottom or (1b) considerable growth of bramble in the sides, as a habitat this (1b) is the most desirable, but agriculturally it is undesirable and unmanageable. 

2. Up to 4m tall and 2m wide with only the sides trimmed, with the density of the lower part dependent on the last renovation (coppicing or laying).

3. A or Box shaped hedge up to 2m tall (usually 1.5m tall and 1.5m wide at bottom), well managed.

4. Small hedge only 1m tall by 0.75m wide, commonly found in arable districts. This type is of limited use as a habitat.

Over trimming of hedges is also very widespread, with types 3 and 4 being the most over-trimmed. Annual trimming of blackthorn and hawthorn hedges prevents flowering, as flowers of these species develop on the  previous years shoot growth, flowers and fruits are important food sources for birds and insects.

Field and hedgerow trees

Trees not only look nice but are valuable to both agriculture (windbreaks therefore soil and crop protection, livestock shelter etc) and the environment (habitats etc.)

 

Shoot die back in Ash (Fraxinus excelsior) and Oak (Quercus robur) can be frequently observed in the countryside. Both of these species are common field and hedgerow trees. Elm (ulmus spp.) was also a common hedgerow species prior to the devastation of more or less the entire UK population from Dutch elm disease. The cause of this shoot die back in Oak and Ash has been studied jointly by the Forestry Commission and the University of Aberdeen, who demonstrated a strong association between Ash die-back and arable farming. The main causes being close cultivation (particularly with deep cultivation implements) and soil compaction (Hull & Gibbs, 1991). It is reported that 60-80% of tree root volume is found in the top 20cm of soil, (i.e. the ploughing zone) and that ash is a relatively shallow rooting species. The effect of root damage is greatly exacerbated in dry years.

Conclusions  

Arable farmers may see justification in trimming hedgerows annually to maintain a very low box shape, but this is detrimental to many species of wildlife. Birds prefer less intensively managed hedges. The main reasons for maintaining small tightly trimmed hedges is to minimize yield loss at field margins and a desire for 'tidiness'. Barr et al (1991) reported a significant increase in relict (surviving, left behind, in this case specifically unmanaged) hedgerows or lines of trees and shrubs through lack of management.  

CLICK HERE - for the Woodland Trust pages on hedges.

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Wetlands

Large areas of marshland and wetlands have been drained since the 17th Century notably in the Cambridgeshire Fens and Somerset Levels, the latter of which is now an Environmentally Sensitive Area (ESA). This reclamation and drainage continued through the 18th, 19th and 20th Centuries . Wetland species decline is more severe than any other habitat.

bulletIntroduction of drainage grants, 1940
bulletArea drained under grant aid, 5 440 ha in 1941 to peak of 100 000 ha/year in 1970's
bulletDrainage grants abolished in 1987
bulletOver 750 000 ha drained in 1970's of which 200 000 ha was extensively managed grassland (50% which was rough grazing)
bulletMost drainage to improve or intensify production from existing grasslands
bullet40% of drained land done to introduce arable cropping
bulletResearch shows that drainage does not necessarily improve grassland productivity 

The area on the left could be potentially drained for agricultural use, it would be impractical to drain the area on the right at present

Pause for thought....List 5 non-agricultural benefits related to land drainage?

 Effects of drainage on botanical diversity:

bulletWetlands contain distinct, diverse and often rare plant communities
bulletDitches can contain distinct plant communities at water level:
bulletSubmerged, emergent and floating
bulletand on the banks and edges

Productive grasses:

bulletPasture grasses differ in their tolerance of waterlogging
bulletProductive grasses (Perennial Ryegrass, Cocksfoot etc.) are less suited to wet soil conditions than species of lower productivity  (Bents and Fescues.)
bulletWet soils can have higher rates of nitrate mineralisation
bulletDrainage therefore can require an increase in fertilizer inputs

Floristic consequences:

A study from 1840 to present day of 526 species over three marshland habitats shows:

bulletOver 50% have declined
bullet123 have severely declined
bulletOnly 34 species have increased
bulletOf these only 7 species have increased to any great extent
bullet93 species became extinct in at least one marshland habitat
bullet23 species extinct in two or three of the sites
bulletDrainage is claimed to be responsible for localized extinction of  20 -33% of already declining wetland species

Biodiversity!

Pause for thought.........Can you 3 problems associated with the reinstatement of wetland areas?

 CLICK HERE  for heathlands, uplands and chalk downlands, more habitat information

Ponds and Lakes

Ponds and lakes - leisure and wildlife

Ponds and lakes are attractive country landscape features, and are often of historical importance. As well as being attractive features, they offer  social (picnic sites, pleasure cruises etc) and sporting opportunities (sailing, fishing etc.) and can therefore, if sited on farms, offer potential diversification opportunities.  They are also useful habitats, ponds for example, collectively support two thirds of Britain's freshwater plants and animal species. The chart below illustrates the decline in UK ponds since 1984.  Showing a similar trend to hedges the number of lowland ponds in the UK was found to have increased by 13,000 between 1990 and 1998 in the Countryside Survey 2000.

Source: DETR. Lowland pond survey 1996

 

Ponds and lakes are subject to the same codes of practice and legislation as other water courses. One particular

problem for farm based ponds and lakes (and drainage ditches) is eutrophication.

Eutrophication is defined as:

'The enrichment of waters, by inorganic plant nutrients, which results in the stimulation of an array of symptomatic changes. These include the increased production of algae and/or other aquatic plants, affecting the quality of the water and disturbing the balance of organisms present within it. Such changes may be undesirable and interfere with water uses' (Organisation for Economic Co-operation and Development, 1982 (modified)).

Eutrophication can be a naturally occurring phenomenon but there is concern over the number of rivers, lakes and coastal waters in which it is developing or being aggravated through human inputs.

In England and Wales, eutrophication as a national water quality issue has had an increased profile since the late 1980s, when blue-green algal blooms were a widespread occurrence in standing and slow-flowing fresh waters. 

Eutrophication on a nature reserve with heavy industry on one side and farmland on the other


In lakes, eutrophication can lead to excessive blooms of algae, the most well known being
potentially toxic blue-green species. Communities of suspended or floating microscopic algae called phytoplankton, can reduce water clarity resulting in the reduced growth of rooted plants. This loss of habitat has detrimental knock-on effects for invertebrate and fish communities. Algae can also cause large fluctuations in dissolved oxygen concentrations as they photosynthesise during daylight hours, adding oxygen to the water, but respire at night, consuming oxygen. In bloom conditions this can cause problems in the early morning when low oxygen levels can lead to invertebrate and fish mortalities. Decay of algal blooms and other plants can also deoxygenate the water, killing fish and other wildlife. High concentrations of algae can affect water treatment for public supply by blocking filters and affecting taste and odour. Some 84% of wetland sites in England designated as Sites of Special Scientific Interest (SSSI) are thought to show symptoms of eutrophication.

The symptoms of eutrophication are as diverse in rivers as they are in lakes. Eutrophication can lead to rivers being choked with tolerant species, which tend to displace more sensitive plants of higher conservation value. Excessive growth of rooted plants can cause a reduction in flow capacity of rivers and other drainage channels. If left uncut, such growth can increase the chance of flooding. They also create a nuisance to anglers, particularly growths of Cladophora. In fast flowing salmonid rivers, growth of bottom dwelling (benthic) algae can threaten spawning success by reducing water flow through gravels. Environment agency

Source: Environment Agency


In the Middle Ages sheaves of corn were ceremonially thrown into village ponds and wells to ensure the purity of the water. More recently farmers have found, often by accident, that a
bale of straw in the farm pond keeps it clear of algae. 

According to a paper by Edward Roberts about the Bishop of Winchester's Fishponds in
Hampshire (12-15 Century), the fishponds were drained regularly and 'Occasionally, a crop
of barley was taken from the dried pond bottom...', which we understand was to help
control algae.

Research over fifteen years at the Centre for Aquatic Plant Management, an outstation of
The Institute of Arable Crop Research at Long Ashton, has proved the effectiveness of
barley straw in the control of all forms of algae. 

Pause for thought.........Is some eutrophication of water supplies to be expected and unavoidable or should society be aiming for zero tolerance?

CLICK HERE  for more information on toxic blue green algal blooms 

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Agricultural Threats ] Climate Change & Agriculture ] [ Change in Habitats ] Change in Species ] Basic Conservation Measures ] GMO's ]

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