Ecosystems - People change communities

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Ecosystems and the stabilty of populations

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Ecosystems and the stabilty of populations

A population is all the freely interbreeding individuals of the same species occupying the same place at the same time. These populations depend on other populations of plants and animals for their survival. The name given to all the populations of different organisms living and interacting in a particular place and a particular time is a community. In an oak woodland, the oak trees are part of its plant community and the woodlice belong to the woodland's animal community. Dynamic (changing) feeding relationships exist within a community. For example, caterpillars can only feed on the leaves of the oak trees at the time of year when leaves are present. The interaction of the community (which is made up of living things called the biotic component) with the physical environment (which consists if the non-living, abiotic component, such as the soil of the climate) in a specific area called an ecosystem. Ecosystems vary in size from very small, such as a pond or a wall, to much larger examples such as woodlands, moorlands or lakes. An ecosystem is more or less self-contained in the terms of energy flow through it and the cycling of essential nutrients such as nitrogen and carbon within it. If these processes are disrupted, the community is affected and the ecosystem becomes unstable.

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Ecosystems and the stabilty of populations (Cont.)

The place where an organism lives is called its habitat. An oak woodland contains many different kinds of habitat and this is why oak woodlands have great biodiversity. Populations of many different species live there, each having the own ecological role within the woodland community. The ecological role of a species within its community is called its ecological niche and consists of all the environment conditions and resources needed for the survival of a species within an ecosystem. If a new species is introduced to an ecosystem, its niche may overlap with that of an existing species and thus can affect the stability of the ecosystem and all the populations of living things within it.

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Population growth

Populations do not stay exactly the same size all the time. The size of a population depends upon its birth rate, its death rate and migration into and out of the population. These are in turn affected by food supply, ability to reproduce, predation and disease. In a stable ecosystem where these factors are balanced, the size of a populations of plants or animals  remains fairly constant. Limiting factors keep the population at the maximum size that can be sustained by the ecosystem. This population size is called carrying capacity and the limiting factors that determine its size are collectively called environmental resistance. The growth of population has three clear phases. In the lag phase, growth is slow because the size of the population is so small that even a doubling of the population does not produce a great increase in population numbers. In the exponential (log) phase, the growth rate of the population is at a maximum because food, water, shelter potential mates and other factors essential for survival and reproduction are abundant. In the third phase, called the stationary phase, the population size remains fairly constant, held at the carrying capacity by limiting factors such as food supply. There are some small fluctuations above and below the carrying capacity due to variations in limiting factors but overall, the size of the population remains fairly constant.

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Population growth (Cont.)

There are two types of limiting factor:

  • Density-dependent factors have effects that depend on population density. They have a greater effect when the population is large and individuals are living close together. For example, it is easier for a disease to spread from one  organism to another when the organisms are living close together. Density-dependent factors are usually biotic ones such as food supply, predation and disease. the larger the population, the greater the effect
  • Density-independent factors are abiotic factors such as temperature, rainfall, light intensity and fire. They have similar effects regardless of the population size/density.
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Population growth (Cont.)


Denisty-dependent limiting factors result in competition between individuals for limited resources such as water, food and territories. If the competition is between different members of different species, it is called interspecific competition. Competition for limited resources between members of the same species is called intraspecific  competition. Intraspecific competition is the driving force behind natural selection. Individuals with phenotypes that give them a competitive advantage are most likely to win the competition and survive, reproduce and pass on their alleles to their offspring.

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Winners and Losers

Some of Britain's wild plants and animals have lived together for thousands of years. During  this time, stable relationships have been established between the populations of these species in their communities. However, many of the species that live in Britain today have been introduced by humans. Some species, like the harlequin ladybird, are recent, accidental introductions. Other accidental arrivals are more ancient such as the field poppy, whose seeds probably arrived in Britain with grain traded with Mediterranean countries during the Neolithic period (about 7000 years ago).

Some species have been introduced deliberately, and many of these are domesticated species used as food sources, such as cattle, sheep, goats, wheat and potatoes. Rabbits were introduced to Britain following the Roman invasion about 200 years ago as a source of food, but now live wild throughout Britain. Introduced species have found ecological niches in many different types of ecosystem, changing the species' interactions and affecting the size and distribution of many populations of native species.

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Winners and Losers (Cont.)

Reds versus greys:

Red squirrels are native to the British Isles. Grey squirrels were introduced to Britain from North America at the end of the 19th century. They were introduced to Britain originally keep in cages in private collections, but following their release into the wild they have become so successful that they now threaten the survival pf Britain's red squirrel populations. The grey squirrel is much larger than the red. Both types of squirrel are adapted to feed in mixed woodland but the larger grey squirrels can out on a lot more body fat than the red squirrels. This gives a better chance of surviving, especially in the cold winter months. As a result, grey squirrels can displace a red squirrel population within 15 years.

The grey squirrel is now widespread in England and Wales, and has established populations in parks and gardens as well as large woods and forests. Meanwhile, the only habitat where the native red squirrels seem better adapted for survival than the grey is in large areas of coniferous woodland such as those found in Scotland, Northumberland and Cumbria. Here, the small seeds of the conifers provide enough energy for the red squirrels, but not for the larger, grey squirrels. Scientists hope that the remaining populations of red squirrels in mainland Britain will survive in red squirrel reserves. These have been created in coniferous forests in places like Formby  on Merseyside, where the grey squirrel find it difficult to take over. In an effort to prevent grey squirrels  from becoming established in red squirrel reserves, the Forestry Commission is developing a new squirrel feeder designed to poison grey but not red squirrels. If the government gives permission for the feeder to be used, it will be a major step forward in controlling the size of grey squirrel populations.

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Winners and Losers (Cont.)

Japanese Knotweed:

Japanese Knotweed is a plant that was introduced to European parks and gardens from Japan, Taiwan and Chine in the early 19th century. Like many introduced species, it has escaped from its garden habitat and has become a real pest throughout Britain. In its native Asian habitats, natural pests and diseases usually limit the size of Japanese Knotweed populations. In Britain, Japanese Knotweed doesn't appear to have any natural enemies ad once it has established itself in a habitat, it is very difficult to get rid of it. A piece of root as small as 0.8 grams can quickly grow to form a new a new plant which rapidly out-competes other plants for light, water and nutrients. Japanese Knotweed doesn't just threaten native wildlife. Buildings and hard surfaces such as flood defences can be damaged by the plant, and drainage channels can become clogged. Estimates of how much it would cost to try to eradicate Japanese Knotweed from Britain are excess of £1.5 million.

Japanese Knotweed poses such a threat to the survival of native species that it is an offence to plant this species or cause it to grow in the wild. Anyone convicted of such an act may face a fine of £5000 and/or 6 months' imprisonment.

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Winners and Losers (Cont.)


Domestic cats are thought to be descended from wild cats that were probably attracted to the earliest agricultural settlements in the Middle East. Here, they would hunt the rodents that infested the grain stores, and because of this they would have been highly valued by the first farmers. That cat's ability to track down and kill rodent pests makes it a well-respected human companion to this day.

The cat Felis catus is Britain's most abundant carnivore, and because there are so many of them, domestic and feral (reverted back to wild from domestic) cats are a major predator of wild animals in Great Britain. In 1997, in an attempt to find out the impact of the domestic cat on Britain's wildlife, scientists carried out a survey

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Winners and Losers (Cont.)


Pigeons were domesticated by humans thousands of years ago as a source of food and fertiliser. Feral pigeons can now be found in cities worldwide. Their ancestors are the rock doves that inhabit rocky cliffs. Feral pigeons occupy the same niche in the urban environment. They perch and nest in ledges, roofs and windowsills and are often found in flocks, feeding by day and roasting by night. Their natural diet is grain and seeds, but they also scavenge food discarded on urban streets. when food is readily available, feral pigeons can breed up to six times a year, allowing rapid population growth. Sometimes, large numbers of pigeons are attracted into an area because people enjoy feeding them. They will also visit bird tables that offer grain and seed. However, large numbers of pigeons living in an urban environment can cause problems. In wet weather, the accumulation of their slippery droppings on pavement can pose hazards to pedestrians. Their dropping are also acidic and can corrode brick and stone work. Gutters and drains blocked by droppings, nest material and dead birds can overflow, causing water damage to buildings, and dead pigeons in covered water tanks can contaminate the water supply.

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Winners and Losers (Cont.)


Foxes first moved into our cities from the countryside in the 1930s. Large areas of suburban housing were build in the period leading up to World War II. The relatively large gardens of theses well-spaced houses provided an ideal habitat for foxes and they quickly increased in numbers. From these new suburbs, foxes then colonised other urban areas. Many cities now have urban foxes. For most towns and cities the fox population reached its carrying capacity many years ago and population is stable,with no significant increases or decreases. There are only a few cities where fox numbers are still increasing and theses are ones that have only recently been colonised. Urban foxes have varied a varied diet that includes earthworms, insects, fruit and vegetables and wide variety of both domestic and wild birds and mammals. Most of the birds they eat are feral pigeons and small garden birds, and the most frequently  eaten mammals are generally field voles that are abundant on allotments, railway lies and other grassy areas.

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Winners and Losers (Cont.)


Wherever there are humans, rats will not be far away! Rats live successfully throughout the UK, in both urban and rural environments. Rats are quick to learn and so will exploit any easily accessible food source. It is the brown, or Norway rat Rattus norvegicus, introduced to Britain by shipping during the 18th century, that is the most commonly found in the UK. Brown rats tend to infest the areas around a building  rather than the building itself. They can create extensive burrows systems but will take advantage of artificial tunnels, such as sewers. They cause widespread contamination of their habitat with droppings, urine and hairs. They also carry many diseases and parasites that are potentially harmful to humans and animals. Until the arrival of the brown rat, the black rat Rattus rattus was the UK's only resident rat species. The black rat is now rare in the UK and is confined to a few port towns and a few offshore islands.

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Genetically Modified Organisms

GM crops:

Humans have improved crop plants through selective breeding for many thousands of years. in 1977, this time-consuming process was speeded up  when it was discovered that a bacterium called Agrobacteriu tumefasciens could be used to introduce foreign genes into plant cells, producing genetically modified (GM) crop plants. Using this bacterium and several other techniques for gene transfer, scientists have since produced many different GM crops.

Many GM crops are modified to be resistant to pests, disease or herbicides. These include commercially important crops such as soya, wheat, corn (maize), oilseed ****, cotton and sugar beet. One of the most commonly inserted genes is the bacterial gene, Bt. It allows the crops plants to produce a toxin that kills insect pests but is harmless to humans. Other crop plants have been engineered to increase shelf life, improve flavour, increase hardiness, increase nutrient content or to be free from certain chemicals that may cause allergies.

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Genetically Modified Organisms (Cont.)

Supporters of GM technology argue that genetically engineered crops will provide solutions to a number of global issues. They could help to protect the environment by minimising pesticide use, they could improve nutrition, and by flourishing in conditions where crops would fail or would have reduced yields, they could help to alleviate world hunger.

Critics of Gm technology fear that herbicide-resistant GM crops become 'superweeds', or that they could accidentally breed with wild plants or other crops, genetically polluting the environment. Large numbers of field trials, carried out by the UK Government and others, reveal that gene transfer does occur. Many scientists agree that the widespread insertion of insecticide genes into crop plants, such as the Bt gene, will also increase the rate of evolution of insecticide-resistant pests.

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Genetically Modified Organisms (Cont.)

GM crops and health:

There are also fear that GM crops could have adverse health effects. If GM crops are eaten by humans, there are worries that the antibiotic resistance marker genes that they contain may be taken up by the bacteria living in the human gut. This could produce populations of human gut bacteria that are resistant to certain antibiotics.

GM crops have been engineered to produce drugs and vaccines. This process is called pharming. Supporters of the technology say the use of these plants will allow the cheao production of new medicines. Critics worry that these plant may cross-breed with varieties of food crops and this could lead to contamination of food supplies and possible dangers to health.

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Genetically Modified Organisms (Cont.)

GMO regulations:

Currently all releases of genetically modified organisms (GMOs) into the environment  are strictly regulated and require consent. Advice on whether a release should be given consent is provided by the Advisory Committee on Release to the Environment (ACRE). When making an application, an Environmental Impact Assessment (EIA) must be submitted. An EIA is a detailed report on the environment impact the project would have. The purpose of the assessment is to make sure the impact on the environment is considered fully. In the European Union, an EIA must cover several areas, including a description of the project, description of impact and details of any weaknesses in knowledge. In the UK there are also regulations on GM food is labelled. If any genetically modified ingredients are used in food it must be stated on the packaging.

Public reaction int he UK, couple with the results from farm-scale evaluations, means it is unlikely that GM crops will be grown in the UK in the next few years. Elsewhere it is a different story. Farmers planted 81 million hectares of GM crops worldwide in 2004, up from 67.7 million hectares in 2003.

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Genetically Modified Organisms (Cont.)

Genetically modified animals:

The most common GMOs are crop plants, but the technology has been applied to all forms of life. Genetically modified animals with particular genes added or 'knocked out' are very important in medical research. Any animal whose genetic composition has been altered by the addition of foreign DNA is said to be transgenic. Transgenic mice can be designed to develop conditions such as Alzheimer's disease by incorporating know human disease genes into their DNA. 'Knock out' mice have certain genes 'knocked out' or disabled. For example, p53 knockout mice have their p53 tumour-suppressing gene disabled. Transgenic and 'knockout' mice allow scientists to learn more about human diseases. There are potentially many uses for genetically modified animals  but there are many ethical concerns, including the possible hazards associated with their release into the environment.

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