OCR Biology A2 Ecosystems

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Define the term ecosystem
All the living organisms and all the non-living components in a specific habitat, and their interactions
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What is a biotic factor?
Any living component of an ecosystem that affects the life of another organism
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Give an example of a biotic factor
Predation / Food supply
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What is an abiotic factor?
A non-living chemical or physical factor in the environment
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Give an example of an abiotic factor
Soil pH / temperature
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Define the term 'producer'
Photosynthetic organisms which supply chemical energy to all other organisms
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Define the term 'consumer'
Organisms which consume other organisms e.g. rabbits eat plants
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Define the term 'decomposer'
Living things which feed on waste material or dead organisms
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Define the term 'trophic level'
The level at which an organisms feeds in a food chain
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Outline a basic food chain
Producer --> Primary consumer --> Secondary consumer --> Tertiary consumer
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What do the arrows in a food chain show?
The direction of energy transfer
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What do the bars in a pyramid of numbers represent
The number of individuals of each species
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Why do tertiary consumers need to consume more biomass than secondary consumers to get the same amount of energy?
Because respiration in the secondary consumer releases energy from organic molecules, some of which is converted to heat
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What do the bars in a pyramid of biomass represent?
The dry mass of all of the organisms at a particular trophic level
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How is a pyramid of biomass worked out?
By putting the organisms in an oven at 80 degrees C until all the water has been evaporated
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How is a pyramid of energy worked out?
Burning the organisms in a calorimeter and working out how much heat energy os released per gram
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Why is a pyramid of energy rarely used?
It is very destructive to the ecosystem and is time-consuming
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Give two limitations of using pyramids of energy
They only take a snapshot of an ecosystem at one moment in time; population sizes continuously fluctuate, providing a distorted idea of the efficiency of energy transfer
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What is productivity?
The total amount of energy fixed by photosynthesis. The net flux of atmospheric carbon to plants per unit time
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What is the other name for the productivity of plants?
Primary productivity
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What is the 'gross primary productivity' (GPP) ?
The rate at which plants convert light energy to chemical energy
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What is the 'net primary productivity' (NPP) ?
The rate at which carbohydrate accumulates in the tissue of plants of an ecosystem and is measured in dry organic mass. NPP = GPP - respiratory heat loss
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What are the average NPP levels in plants in the wild?
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Give 5 ways in which NPP levels can be increased
5 of: Light banks, warmer temperatures, irrigation, fertilisers, crop rotations, pesticides, herbicides, fungicides
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Why is transfer of energy from producers to consumers inefficient?
Because primary consumers don't make full use of a plant's biomass, much of it is egested or lost as heat
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Why are animals harvested just before adulthood?
Because young animals invest larger proportions of energy into growth than adults, so it minimises loss of energy from the food chain
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Which drugs are animals treated with and why?
Antibiotics as it avoids unnecessary energy loss of energy to pathogens and parasites
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Why are animals bred for food kept in pens where they cannot move around?
So that they do not waste energy that could be used for growth on movement
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Define 'succession'
A directional change in a community of organisms over time
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What is primary succession?
The development of a community from bare ground
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Give an example of primary succession
The island of Surtsey in Iceland
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What is the first stage of primary succession?
Algae and lichens live on the bare rock. This is called a 'pioneer community'
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What is the second stage of primary succession?
Erosion of rock and a build-up of dead organisms produce enough soil for larger plants like mosses and ferns to grow. These succeed the algae and lichens
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What is the third stage of primary succession?
Larger plants succeed the smaller plants continously until a final, stable community is reached called the 'climax community'
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What do climax communities generally consist of in the UK?
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Where can all of the stages of succession be seen at once?
In sand dunes on the beach
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What are the pioneer plants in sand dune succession?
Sea rocket and prickly sandwort
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Which organisms succeed sea rocket/prickly sandwort in sand dune succession?
Sea sandwort and sea couch grass
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Which plants succeed sea sandwort/sea couch grass in sand dune succession?
Sea spurge and marram grass
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Which plants succeed sea spurge/marram grass in sand dune succession?
Hare's-foot clover and bird's-foot trefoil
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Which plants succeed the clover/trefoil in sand dune succession?
Sand fescue and viper's bugloss
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What is meant by the 'distribution' of organisms in a habitat?
If a particular species is present or absent
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What is meant by the 'abundance' of organisms in a habitat?
The number of individuals of each species
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What can be used to increase the accuracy in estimating percentage cover of a species?
A point frame
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How can it be decided where to take samples from in a habitat?
By using random numbers to plot coordinates or by taking samples at regular distances across the habitat
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How can ecologists work out how many quadrat samples they need to take in a habitat?
By creating a graph of cumulative frequency against quadrat number. Where the graph levels off, that is the minimum number of quadrats to use. Ecologists often double this number
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How can you estimate a species' population size throughout a whole habitat?
By using this equation: Population size of species = mean number of individuals of the species in each quadrat divided by the fraction of the total habitat area covered by one quadrat
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What are the two types of transect you can use?
Line transect and belt transect?
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How does a line transect work?
At regular intervals, a note is made of which species are touching the tape
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How does a belt transect work?
At regular intervals, place a quadrat next to the line, moving it along the line after looking at each quadrat
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How do bacteria and fungi involved in decomposition feed?
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Descibe the feeding process of saprotrophs
Enzymes secreted onto dead and waste material. These enzymes digest material into smaller molecules, which are then absorbed into the organism's body. Once absorbed, molecules are stored or respired
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Why can't plants absorb nitrogen directly from the air?
Because nitrogen is so unreactive
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Which 3 circumstances allow the fixation of nitrogen?
The presence of nitrogen-fixing bacteria, through the Haber process, or when lightening strikes
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What do nitrogen-fixing bacteria do?
Live freely in the soil and fix nitrogen gas from air within the soil, using it to manufacture amino acids
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Where else can nitrogen-fixing bacteria live?
In the root nodules of plants of the legume family e.g. beans
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Describe the relationship between nitrogen-fixing bacteria and plants
It is mutualistic, as the bacteria provide the plant with fixed nitrogen whilst receiving carbon compounds like glucose in return
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Why do the conditions within the root nodules remain anaerobic?
Due to proteins such as leghaemoglobin which absorb oxygen
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How is nitrogen fixed?
Bacteria use an enzyme called nitrogen reductase to reduce nitrogen gas to ammonium ions under anaerobic conditions
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When does nitrification occur?
When chemautotrophic bacteria in the soil absorb ammonium ions
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Where do ammonium ions in the soil come from?
They are released by bacteria involved in the putrefaction of proteins found in dead or waste organic matter
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How do chemoautotrophs get their energy?
By oxidising ammonnium ions to nitries or by oxidising nitrites to nitrates
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Which bacteria oxidises ammonium ions to nitrites?
Nitrosomonas bacteria
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Which bacteria oxidises nitrites to nitrates?
Nitrobacter bacteria
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Does nitrification require oxygen and why?
Yes because oxygen is required for this oxidation
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What happens to the nitrates produced in nitrification?
They are absorbed from the soil by plants and used to make nucleotide bases and amino acids
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What happens in denitrification?
Nitrates are converted back to nitrogen gas
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What is produced when bacteria involved in denitrification growing under anaerobic conditions use nitrates as a source of oxygen?
Nitrogen gas (N2) and Nitrous oxide (N2O)
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Define the term 'carrying capacity'
The maximum population size that can be maintained over a period of time in a particular habitat
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What are the stages in a graph of population size against time?
Lag, Log, Stationary, Death
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What happens in the lag phase and why?
Few individuals, acclimatising to habitat causing a low rate of reproduction
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What happens in the log phase and why?
Resources are plentiful and conditions are good. Rate of reproduction exceeds rate of mortality so population size rapidly increases
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What happens in the stationary phase and why?
Population size levels out at the carrying capacity and the rates of reproduction and mortality are equal
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When the rate of a natural process is affected by a number of factors, what is the limiting factor?
The one whose magnitude limits the rate of the process. It is often the factor in the shortest supply
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Give 5 examples of limiting factors
Food, water, light, oxygen, predators, intensity of competition
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How does predation act as a limiting factor on a population?
More predators = more prey eaten. Less prey then available, fewer predators can survive, predator pop. goes down so then more prey. Cyclic.
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When does competition occur?
When resources are not present in adequate amounts to satisfy the needs of all the individuals who depend on those resources
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Where does intraspecific competition occur?
Between members of the same species
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How constant is the population if it is dependent on intraspecific competition?
Remains relatively constant
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Where does interspecific competition occur?
Between members of different species
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Give an example of interspecific competition
Paramecium aurelia and Paramecium caudatum, where P. aurelia is more successful
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What is the competitive growth principle?
The more overlap between two species the more compeition that results until one is outcompeted and dies out
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What are the two methods of managing small scale timber production?
Coppicing and pollarding
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What is coppicing?
Cutting a deciduous tree trunk close to the ground to encourage new shoots to grow
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What is pollarding?
Coppicing but cutting the trunk higher up
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When and why is pollarding used?
Used when there are lots of deer around as the deer eat the new shoots if they are too near to the ground
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How is a continuous supply of wood provided?
By rotational coppicing
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What are standards?
Trees that have been left without coppicing to be harvested later in order to supply larger pieces of timber
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Why is rotational coppicing good for biodiversity?
Because it causes different areas of woodland to provide different habitats increasing the number and diversity of species present
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Give an example of a sustainably managed woodland in the UK
Bradfield Woods
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Which process used to be used for large scale timber production?
Clear felling
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When an area is clear-felled, the soil often runs off into waterways, polluting them. Why is this?
Trees are usually there to absorb water and prevent the soil being washed away. Trees also usually maintain soil nutrient levels through the carbon and nitrogen cycles
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List the 3 principles which modern sustainable forestry is based upon
1) Any tree which is harvested is replaced by another 2) The forest must maintain its ecological function 3) Local people should derive benefit from the forest
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What are the 3 things that must be done in order to ensure successful timber production?
1) Control of pests and pathogens 2) Only plant fast growing plants 3) Position trees an optimal distance apart
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Define conservation
Conservation involves the maintenance of biodiversity, including diversity between species, genetic diversity within species and maintenance of a variety of habitats and ecosystems
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Give ethical reasons why conservation is important
Every species has a value in its own right and a right to survive and humans have a duty to look after them
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Give economic reasons why conservation is important
Ecotourism, pollinating crops, maintain water quality, reduction in biodiversity could reduce climatic stability - associated costs
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Give social reasons why conservation is important
Food sources, drugs, predators of pests (biological control agents)
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Distinguish between conservation and preservation
Conservation involves management and reclamation but preservation is keeping areas in their 'untouched' forms
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Give some examples of biodiversity management strategies
Providing extra food to increase carrying capacity, control of predators and poachers, vaccinations against disease, coppicing/pollarding
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How have humans disturbed the habitat of the Galapagos Islands?
High demands on water/energy/sanitation, more waste and pollution, oil spill in 2001, felling for agricultural land
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How did humans over exploit the Galapagos Islands' resources?
Harvesting species faster than they could replenish themselves e.g. Pinta tortoises and sea cucumbers
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Which tree has out-competed all competition on Santa Cruz?
The red quinine tree
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Which mammal has been the most damaging to the forests?
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How has the Charles Darwin Research Station combated the introduction and dispersion of new species?
1) Quarantine system 2) Exploitation of natural predators 3) Cu0lling wild goats and pigs
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What is a biotic factor?


Any living component of an ecosystem that affects the life of another organism

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Give an example of a biotic factor


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What is an abiotic factor?


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Give an example of an abiotic factor


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