Ecosystems and the Nitrogen Cycle

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  • Ecosystem - All the organisms living in a particular area and the abiotic conditions. Ecosystems are dynamic systems - they're changing all the time.
  • Habitat - The place where an organism lives.
  • Population - All the organisms of one species in a habitat.
  • Abitoic factors - The non-living features of an ecosystem.
  • Biotic factors - The living features of an ecosystem.
  • Producer - An organism which produces organic molecules using sunlight energy.
  • Consumer - An organism which eats other organisms.
  • Decomposer - An organism that break down dead or undigested organic material.
  • Trophic Level - A stage in a food chain occupied by a particular group of organisms
  • Abundance - The number of individuals of one species in a particular area. Can be measured by the number of organisms or the percentage cover.
  • Distribution - Where a particular species is within the area of investigation.
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Nitrogen Cycle - 1

Nitrogen Fixation

  • Nitrogen gas in the atmosphere is turned into ammonia by bacteria called Rhizobium. The ammonia can then be used by plants.
  • Rhizobium are found inside root nodules of leguminous plants.
  • They form a mutualistic relationship with the plants - they provide the plant with nitrogen compounds and the plant provides them with carbohydrates.


  • Nitrogen compounds from dead organisms are turned into ammonium compounds by decomposers.
  • Animal waste also contains nitrogen compounds. these are also turned into ammonium compounds by decomposers.
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Nitrogen Cycle - 2


  • Ammonium compounds in the soil are changed into nitrogen compounds that can then be used by plants.
  • Nitrifying bacteria - Nitrosomonas - change ammonium compounds into nitrites.
  • Then other nitrifying bateria - Nitrobacter - change nitrites into nitrates.


  • Nitrates in the soil are converted into nitrogen gas by denitrifying bacteria - they use nitrates in the soil to carry out respiration and produce nitrogen gas.
  • This happens under anarobic conditions - waterlogged soil.
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Increasing Energy Transfer

  • Some farming methods increase productivity by increasing the transfer of energy through an ecosystem.
  • Herbicides kill weeds that compete with agricultural crops for energy. Reducing competition means crops recieve more enery, so they grow faster and become larger, increasing productivity.
  • Fungicides kill fungal infections that damage agricultural crops. The crops use more energy for growth and less for fighting infection, so they grow faster and become larger, increasing productivity.
  • Insecticides kill insect pests that eat and damage crops. killing insect pests means less biomass is lost from crops, so they grow to be larger, which means the productivity is greater.
  • Natural Predators introduced to the ecosystem eat the pest species. This means that crops lose less energy and biomass, increasing productivity.
  • Fertilisers are chemicals that provide plants with minerals needed for growth. Crops use up minerals in the soil as they grow, so growth is limited when there aren't enough minerals. Adding fertiliser replaces the lost minerals, so more energy from the ecosystem can be used to grow, increasing the efficiency of energy conversion.
  • Rearing livestock intensively involves controlling the conditions they live in, so more energy is used for growth and less is used for other activities. The efficiency of energy conversion is increased so more biomass is produced and productivity is increased.
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  • Succession is the process by which an ecosystem changed over time. The biotic conditions change as the abiotic conditions change.
  • Primary Succession - This happens on land that's been newly formed of exposed. There's no organic material or soil to start with.
  • Secondary Succession - Thia happens on land that's been cleared of all plants, but where the soil remains.
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Primary Succession

  • Primary succession starts when species colonise a new land surface. Seeds and spores are blown in by the wind and begin to grow. The first species to colonise a new area of land are called the pioneer species - this is the first seral stage.
  • The abiotic conditions are hostile. Only pioneer species grow because they're specialised to cope with the harsh conditions.
  • The pioneer species change the abiotic conditions - they die and microorganisms decompose the dead organis material, humus. This forms a basic soil.
  • This makes conditions less hostile. New organisms are able to move in and grow. These then die and are decomposed, adding more organis material, making the soil richer and deeper in minerals.
  • Larger plants like shrubs can start to grow in the deeper soil, which retains even more water. As more plants move in, they create more habitats, so more animals move in.
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Secondary Succession

  • Succession starts at a later seral stage - the pioneer species in secondary succession are larger plants and shrubs.
  • At each stage, different plants and animals that are better adapted for the improved conditions move in, out-compete the plants and animal that are already there, and become the dominant species in the ecosystem.
  • As succession goes on, the ecosystem becomes more complex. New species move in alongside existing species, which means the species diversity increases.
  • The amount of biomass also increases because plants at later stages are larger and more dense.
  • The final seral stage is called the climax community - the ecosystem is supporting the largest and most complex community of plants and animals it can. It is in a steady state.
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Population Size - Abiotic Factors

  • Population size is the total number of organisms of one species in a habitat.
  • The population size of any species varies because of abiotic factors - amount of light, water, or space available, and the temperature and chemical composition of their surroundings.
  • When abiotic conditions are ideal for a species, organisms grow fast and reproduce successfully.
  • When abiotic conditions aren't ideal for a species, organisms can't grow as fast or reproduce as successfully.
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Interspecific Competition - Biotic

  • Interspecific competition is when organisms of different species compete with each other for the same resources.
  • Interspecific competition between two species can mean that the resources available to both populations are reduced. Both populations will be limited.
  • Interspecific competition also affects the distribution of species. If two species are competing but one is better adapted to it's surroundings than the other, the less well adapted species is likely to be outcompeted - it won't be able to exist alongside the better adapted species.
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Intraspecific Competition - Biotic

  • Intraspecific competition is when organisms of the same species compete with each other for the same resources.
  • The population of the species increases when resources are plentiful. As the population increases, there'll be more organisms competing for the same amount of space and food.
  • Eventually, resources such as space and food become limiting factors - there isn't enough for all the organisms. The population then begons to decline.
  • A smaller population then means that there's less competition for space and food, whcih is better for growth and reproduction, so the population starts to grow again.
  • The maximum stable population size of a species that an ecosystem can support is called that carrying capacity.
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Predation - Biotic

  • Predation is where an organism kills and eats another organism.
  • the population sizes of the predator and prey are interlinked - as the population of one changes, it causes the other population to change.
  • As the prey population increases, theres more food for the predators, so the predator population grows.
  • As the predator population increases, more prey are eaten so the prey population starts to fall.
  • This means that there's less food for the predators so their population decreases.
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Conservation of Ecosystems

  • Conservation is the protection and management of ecosystems so that the natural resources can be used without them running out. This means that natural resources will still be available for future generations.
  • It's a dynamic process - conservation methods need to be adapted to the constant changes that occur within ecosystems.
  • Conservation involves the management of ecosystems - controlling how resources are used and replaced.
  • Conservation can also involve reclamation - restoring ecosystems that have been damaged or destroyed so they can be used again.
  • Preservation is the protection of ecosystems so they're kept exactly as they are. Nothing is removed from a preserved ecosystem, and they're only used for activities that don't damage them.
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Reasons for Conservation


Ecosystems provide resources for lots of things that humans need. These resources are economically important because they're traded on a local and global scale. If the ecosystems aren't conserved, the resources that we use now will be lost, so there will be less trade in the future.


Many ecosystems bring joy to people because they're attractive to look at and people use them for activites. The species and habitats in the ecosystems may be lost if they aren't conserved, so future generations won't be able to use and enjoy them.


Some people believe we should conserve ecsystems simply because it's the right thing to do - organisms shouldn't become extinct as a result of human activity. some people think that we have a moral responsibility to conserve ecosystems for future generations, so they can enjoy and use them to.

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Sustainable Resources

  • Ecosystems can be managed to provide resources in a way that's sustainable.
  • This means enough resources are taken to meet the needs of people today, without reducing the ability for people in the future to meet their own needs.
  • Temperate woodland can be managed in a sustainable way - for every tree that's cut down for timber, another is replanted in its place. The woodland should never become depleted.
    • Trees are cleared in strips or patches - woodland grows back more quickly in smaller areas between bits of existing woodland than it does in open areas.
    • The cleared strips or patches aren't too large or exposed - lots of soil erosion can occur on large areas of bare ground. If the soil is eroded, newly planted trees won't be able to grow.
    • Timber can be harvested by coppicing - cutting down trees in a way that means they are able to grow back. This means new trees don't have to be planted.
    • Only native species are planted - they grow most successfully because they're adapted to the climate.
    • Planted trees are attached by posts to give support, and are grown in plastic tubes to stop them being eaten by grazing animals - this makes it more likely that the trees will survive to mature adults.
    • Trees aren't planted too close together - this means that trees aren't competing with each other for space or resources, so they're more likely to survive.
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Galapagos Islands

  • Explorers and sailors that visited the Galapagos Islands in the 19th century directly affected the populations of some animals by eating them. A giant tortoise found on Floreana Island was hunted to extinction for food.
  • Non-native animals introduced to the islands eat the native species. This causes a decrease in the populations of native species. Non-native dogs, cats, and black rats eat young giant tortoises and Galapagos land iguanas. Pigs also destroy the nests of the iguanas and eat their eggs. Goats have eaten much of the plant life on the islands.
  • Non-native plants have also been introduced to the islands. These compete with native plant species, causing a decrease in their populations. Quinine trees are taller than some native plants. They block out the light to native plants, which then struggle to survive.
  • Fishing has caused a decrease in the populations of some of the sea life around the Galapagos Islands. The populations of sea cucumbers and hammerhead sharks have been reduced because of overfishing. Galapagos gree turtle number have also been reduced by overfishing, and they're also killed accidentally when caught in fishing nets. They're now an endangered species.
  • A recent increase in tourism has lead to an increase in development in the islands. This causes damage to the ecosystems as more land is cleared and pollution is increased.
  • The population on the islands has also increased due to the increased opportunities from tourism. This could lead to further development and so more damage to the ecosystems.
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