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Populations and ecosystems

Ecology- Study of the inter relationships between organisms and their environment (living and non-living)

Biosphere- Life supporting layer of land, air and water that surround the Earth

Ecosystem- A self contained system in ecology made up of both biotic and abiotic factors.

Population- Group of interbreeding organisms of one species in a habitat. Populations of different species form a community

Community- All the populations of different species living and interacting in a particular place at the same time

Habitat- Place where a community of organisms lives.  There are many habitats within an ecosystem. Within each habitat there are smaller units each with their own microclimate/microhabitats

Ecological niche- How an organism fits into the environment- where it lives and what it does there. No two species occupy the same ecological niche

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Investigating populations

Number of individuals in a given space= abundance. It would damage the habitat to sample a whole population so we use a sample representative of the whole population

 Quadrats- Need to consider- 

  • Size of the quadrat to use- depends on size of plant or animals being counted and how they are distributed within the area.
  • Number of sample quadrats to record within the study area- larger number of sample quadrats the more reliable the results but can be time consuming. Greater number of species the greater number of sample quadrats needed to give valid results
  • Position of each quadrat within the study area- random sampling must be used

 Random sampling- Used to avoid bias data. To get truly random sampling we must lay out 2 long tape measures at 90° angle along two sides of study area, obtain coordinates by using a random number generator and place quadrat at intersection of each pair of coordinates and record data

Systematic sampling along transect- More informative to measure abundance and distribution of a species in a systematic way.

  • Line transect- a piece of string or tape is used and anything that passes the line is recorded
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Measuring abundance

Random sampling is used to obtain measures of abundance. Measured using:

  • Frequency- likelihood of a particular species occurring in a quadrat 
  • Percentage cover- estimate the area within a quadrat that a plant covers

Mark-release-recapture technique- Used to measure the abundance of animals in a population

  • Known number are caught, marked in some way, and then released back into the community and sometime later a given number are recaptured randomly and the number marked is recorded.
  • Population size =Number of animals marked X total caught second time /Number of marked animals caught second time

Relies on some assumptions:

  • Marked sample distribute themselves evenly amongst the whole population
  • Population has definite boundaries so no immigration or emigration
  • Few if any deaths or births within population
  • Marking is not toxic nor does it make the individual more liable to predation
  • Mark is not lost or rubbed off during investigation
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Ethics and fieldwork

Where possible the organisms should be studied in situ-if it is necessary to remove them, the number taken should be kept to a minimum

Any organism removed should be returned to the site ASAP even if they are dead

Sufficient period of time should elapse before the site is used again

Disturbance and damage to the habitat should be avoided

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Variation in population size

Number of individuals in the population is the population size

Population growth curves- Usual pattern of growth for a natural population has three phases:

  • period of slow growth- initially small number of individuals reproduce to slowly uild up numbers
  • period rapid growth- ever-ever increasing numbers continue to reproduce- population size doubles
  • population growth declines until size remains stable- decline may be due to food supply limiting numbers or to increased predation

Population size

  •  A population may grow or or decrease due to limiting factor-  these can be biotic (e.g. predation) or abiotic (e.g. temp, light) factors
  • After some time the ‘good life ends and the going gets tough’ therefore the growth of population slows, possibly ceasing altogether
  • No population continues to grow indefinitely because certain factors limit growth
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Factors effecting population size...

Abiotic factors

  • Temperature- each species has a different optimum temperature at which survival is best- further away from optimum the smaller the population- enzymes work less efficiently at too low temperatures or too high temperatures and often denature
  • Light- ultimate source of energy for ecosystems- rate of photosynthesis increases as the light intensity increases- greater rate of photosynthesis the faster plants grow
  • pH- affects enzyme action- population of organisms is larger where the appropriate pH exists
  • Water and humidity- where water is scarce populations are small and consist only of species that are well adapted to living in dry conditions


  • Where two or more individuals share any resource that is insufficient to satisfy all their requirements fully, competition results
  • Intraspecific competition- competition between members of the same species
  • Interspecific competition- competition between different species- two species occupy the same niche
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...Factors effecting population size


  • Predator- organism that feeds on another organism known as a prey
  • The predator and prey have evolved together- if the other had not matched the improvements of the other, it probably would have become extinct
  • Data for the relationship between predator and prey collected in a lab doesn’t reflect what happens in the wild- it’s difficult to collect reliable data on natural populations as it is not possible to count all individuals

 Effect of predator-prey relationship on population size

  • Predator eat prey thereby reducing population of prey
  • Fewer prey so the predators are in greater competition
  • Predator population is reduced as some are unable to consume prey so the prey population increases
  • With more prey available the predator population increases
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Human population size and growth rate

There are two factors that have caused an explosion in the population size of human:

  • Development of agriculture
  • Development of manufacturing and trade the created the industrial revolution

Factors affecting the growth and size of human populations

  • Basic factors that affect the growth and size of the human population is the death and birth rates. These two factors determine whether the population increases, decreases or stays the same

The population is also affected by the two types of migration:

  • Immigration- individuals join a population from outside
  • Emigration- individuals leave a population
  • Population growth = (births + immigration) – (death + emigration)

% population growth rate = population change during the periodx 100/ Population at the start of the period

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Factors affecting birth and death rates

Birth rate =number of births per yearx 1000/ Total population in the same year

  • Economic conditions- countries with a low per capita income tend to have higher birth rates
  • Cultural and religious backgrounds-some countries encourage larger families etc.
  • Social pressures and conditions- some countries having a large family improves social standing
  • Birth control- extent to which contraception and abortion are used
  • Political factors- influence birth rates through education and taxation policies

Death rate =number of deaths per yearx 1000/ Total population in same year

  • Age profile- greater the proportion of elderly people in the population the higher the death rate
  • Life expectancy at birth- people in economically developed countries live longer
  • Food supply- adequate and balanced diet reduces death rate
  • Safe water and effective sanitation- reduced death rate by reducing the risk of water-borne diseases
  • Natural disasters- more prone a region is to draught, famine or disease means a higher death rate
  • War- an immediate drop in population and a longer term fall as a result of fewer fertile adults
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Population structure

  • Demographic transition- The change from short life expectancy and high birth rate to life expectancy being high and birth rates being low 
  • The size of the human population depends on the number of females of child bearing age
  • Stable population- the birth rate and death rates are balanced so there is no increase or decrease in population size
  • Increasing population- there is a high birth rate giving a wider base to the pyramid and fewer older people giving a narrower apex to the pyramid – typical for less economically developed countries
  • Decreasing population- lower birth rate (narrower base) and a lower mortality rate leading to more elderly people (wider apex) - typical for certain more economically developed countries
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What is energy?

  • The ability to do work’.  Measures in joules (J)

Why do organisms need energy?

  • Without input energy natural processes tend to break down in disorder
  • Movement- both within an organism and of the organism itself
  • Active transport- ions and molecules against a concentration gradient across plasma membranes
  • Maintenance, repair and division- of cells and organelles within the cells
  • Production of substances- used within organisms (hormones and enzymes)
  • Maintenaning body temperature- birds and mammals need energy to replace lost heat 

Energy and metabolism

  • Light energy from the sun is converted into chemical energy during photosynthesis
  • Chemical energy in the form of organic molecules converted into ATP during respiration in all cells
  • ATP is used by cells to perform useful work
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  • Not good long term energy store and is therefore an immediate energy store of a cell
  • Each ATP molecule releases less energy than each glucose molecule
  • Hydrolysis of ATP to ADP is a single reaction - very fast release of energy

How ATP stores energy

  • ATP has three phosphate groups, these are the key to how ATP stores energy- bonds between the phosphate groups are unstable and so have a low activation energy which means they are easily broken- When they break they release energy
  • ATP + H2O=ADP + Pi + E
  • As water is used to convert ATP to ADP the reaction is known as a hydrolysis reaction

Synthesis of ATP

Conversion of ATP to ADP is reversible therefore energy can be used to add an inorganic phosphate to ADP to re-form ATP. As water is removed in this process, the reaction is a condensation reaction

Synthesis of ATP from ADP involves the addition of a phosphate molecule to ADP by: Photophosphorylation, oxidative phosphorylation and substrate level phosphorylation 

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The light dependent reaction

Involves the capture of light whose energy is used for two purposes:

  • Add an inorganic phosphate molecule to ADP making ATP
  • Split water into H+ ions and OH- ions known as photolysis

Making of ATP

  • Chlorophyll molecule absorbs light energy; excites a pair of electrons within the chlorophyll molecule, raising them to a higher energy level the electron leaves the chlorophyll molecule. The electrons are taken up by the electron carrier (electron carrier is reduced)
  • The electrons pass through the electron transport chain in a series of oxidation-reduction reactions in the membranes of the thylakoids. The electrons lose energy at each stage- used to combine Pi with ADP to make ATP

Photolysis of water

  • In order for the chlorophyl to continue absorbing light the electron needs to be replaced by the electron being released during photolysis: 2H2O=4H+ + 4e + O2
  • The H+ ions are taken up by electron carrier NADP which becomes reduced.
  • Reduced NADP enters the light independent reaction; acts as a chemical energy source 
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The light independent reaction

  • ATP and NADP are used to reduce carbon dioxide in the second stage of photosynthesis
  • Occurs whether or not there is light available- requires products of the light dependent stage
  • Takes place in the stroma of the chloroplasts

Calvin cycle

  • CO2 from the atmosphere diffuses into the leaf and dissolves in water and then diffuses through the plasma membrane, cytoplasm and chloroplast membranes into the stroma
  • CO2 then combines with 5 carbon ribulose bisphosphate (RuBP) using an enzyme this produces 2 molecules of glycerate 3-phosphate (GP)
  • ATP and NADP reduce the 2 GP to 2 triose phosphate (TP) and NADP is reformed and returns to the light dependent reaction
  • Some TP is converted into organic molecules (glucose) but most is used to regenerate (RuBP) using ATP from stage 1. (RuP + ATP= RuBP + ADP)
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Limiting factors

  • It is a variable that limits the rate of a chemical reaction . At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value

Effect of light intensity on the rate of photosynthesis- Rate of photosynthesis is measured by vol of oxygen released or the vol of carbon dioxide taken in

  • As light intensity increases, the vol of O2 produced and CO2 absorbed due to photosynthesis increases to a point at which it is balanced. No net exchange of gases- compensation point
  • Increase in light intensity will have no effect 

Effect of  CO2 on the rate of photosynthesis- CO2 conc. affects the enzyme activity (e.g enzyme catalysing the combination of RuBP with CO2)

Effects of temperature on the rate of photosynthesis- Between 0-25°c, the rate of photosynthesis is roughly doubled for each 10°c rise in temperature- in most plants the optimum is 25°c after which the rate tends to decrease due to enzyme denaturation

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Respiration: Glycolysis

Initial stage for both aerobic and anaerobic respiration- occurs in cytoplasm in all living cells and is the process by which Glucose, a hexose sugar(6 carbons) is split into two molecules of pyruvate (3 carbons)

(1) Glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP

(2) This creates 2 molecules of triose phosphate(3 carbons) and 2 molecules of ADP

(3) Triose phosphate is oxidised (loses hydrogen), forming 2 molecules of pyruvate(3 carbons)

(4) NAD collects the hydrogen ions so it's reduced, forming 2 reduced NAD

(5) 4 ATP are produced, but 2 were used up in step one, so there's a net gain of 2 ATP

What happens next?

  • Two molecules of reduced NAD go to the last stage (oxiative phosphorylation)
  • Two pyruvate molecules go into the matrix of the mitochondria for the link reaction
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Respiration: The Link Reaction

The link reaction takes place in the matrix of the mitochondria. Two puyruvate molecules are made for every glucose molecule that enters glycolysis. This means the link reaction happens twice for evey glucose molecule.

(1) Pyruvate is decarboxylated- one carbon atom is removed from pyruvate in the form of CO2

(2) NAD is reduced- it collects hydrogen from pyruvate, changing pyruvate into acetate(2 carbons)

(3) Acetate is combined with coenzyme A (coA) to form acetylcoenzym A (acetyl coA)

(4) Pyruvate + NAD + CoA=acetyl CoA + reduced NAD + CO2

What happens next?

  • One molecules of acetyl coA goes into the Krebs cycle
  • One molecules of CO2 are released as waste products of respiration
  • One molecules of reduced NAD formed go to the last stage (oxidative phosphorylation)
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Respiration: The Krebs Cycle

The Krebs cycle involves a series of oxidation-reduction reactions, which take place in the matrix of the mitochondria. The cycle happens twice for every glucose molecule

(1) Acetyl coA combines with a 4-C compound to form a 6-C compound- CoA is regenerated 

(2) Decarboxylation occurs, where CO2 is removed and a 5-carbon compound is formed

(4) Oxidation also occurs, where hydrogen is removed

(5) The hydrogen is used to reduce NAD to form 1 reduced NAD

(6) Decarboxylation + oxidation occurs- forms a 4-C compound, reduced FAD and 2 reduced NAD

(7) A phosphate group is directly transeferred from intermediate to ADP forming 1 ATP- this is called substrate level phosphorylation

What happens next?

  • One reduced FAD and three reduced NAD go to the last stage
  • 4-C compund regenerated for next Krebs cycle, CoA reused for link reaction, one ATP used for energy and two CO2 released as a waste product
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Respiration: Oxidative Phosphorylation

Oxidative phosphorylation is the process where energy carried by electrons, from reduced coenzymes (reduced FAD + reduced NAD) is used to make ATP- involves the ETC and chemiosmosis

(1) H atoms are released from reduced FAD and reduced NAD as they are oxidised to FAD and NAD, the H atoms split into electrons (e-) and protons (H+)

(2) The electron move along the ETC(made up of 3 electron carriers), losing energy at each carrier

(3) This energy is used to pump protons from the mitochondrial matrix into the intermembrane space

(4) The conc. of protons is now higher in the intermembrane space- electrochemical gradient forms

(5) Protons move down this gradient, back into the matrix via ATP synthase

(6)The movement of H+ ions across the membrane generates synthesis of ATP from ADP and Pi- this is called Chemiosmosis

(7) In the matrix at the end of the ETC, the protons electrons and O2 (from the blood) combine to form water

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Anaerobic respiration

In anaerobic respiration (where there's no oxygen), pyruvate is connverted into ethanol (in plants and yeast) or lactate (in animal cells and some bacteria

production of ethanol in plants and some organisms

  • Pyruvate molecule formed in glycolysis loses carbon dioxide and accepts hydrogen from reduced NAD to produce ethanol and regenerate NAD- pyruvate + reduced NAD = ethanol + carbon dioxide + NAD

Production of lactate in animals

  • Occurs in animals to overcome the temporary shortage of oxygen- occurs mostly in muscles
  • In the absence of oxygen there is a build-up of reduced NAD and for glycolysis to continue this need to be removed- pyruvate + reduced NAD = lactate + NAD
  • The lactate is oxidised back to pyruvate at some point which can be further oxidised into glycogen
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Food chains and food webs

Photosynthesis is the main route by which energy enters the ecosystem. Energy is transferred in trophic levels in food chains and food webs. Energy is recycled back into the ecosystem by microorgansims

Key words:

Producers- photosynthetic organisms that make organic substances

Consumers- obtain energy by feeding on other organisms- animals are consumers

Decomposers- organisms that break down complex materials into simple components again

Trophic levels- the position of the organism in a food chain

Food chain- feeding relationship in which the producers are eaten by primary consumers and they are eaten by seconadary consumers...yada yada

Food web- many food chains linked together

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Energy Transfer between trophic levels

Energy transfer

 1-3% of suns energy captured by green plants. 5-10% used by primary consumers for growth. 15-20% used by secondary and tertiary consumers

Where did all the energy go?

  • Over 90% of the suns energy is reflected back , not all wavelengths of light can be absorbed and used for photosynthesis, light may not fall on the chlorophyll molecule,  factors limit the rate 
  • There is only a low % of energy transfer at each stage because some of the organism is not eaten, some parts cannot be digested and so lost in faeces, excretion, loss due to heat from respiration
  • Efficiency of energy transfer= energy available after transferX100 /Energy available before transfer

Gross production- the total quantity of energy that is absorbed so that it can be assimilated. Most of this is lost during respiration for heat and movment- its called repiratory loss. Energy thats left after respiratory loss is the net productivity (amount of energy available for the next trophic level) which becomes biomass. Net production = gross production – respiratory loss

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Ecological pyramids

pyramids of number

  • Used to measure the number organisms at each trophic level
  • no account taken of size, number of individuals can be so great it's impossible to represent accurately

Pyramids of biomass

  • Shows total biomass of organisms at each trophic level. Measured in gm-2 (area) or gm-3 (volume)
  • Biomass- total mass of living material in a specific area at a given time. Usually measured as dry mass 
  • Seasonal differences are not apparent

Pyramids of energy

  • Shows energy stored in organisms at each trophic level. Measured in KJ m-2 year-1
  • More reliable than POB as 2 organisms of the same dry mass may store different amounts of energy.
  • Collecting data difficult and complex
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Comparisons of natural and agricultural ecosystems

Natural ecosystems

  • Hasn't been changed by human activity
  • Solar energy only- no additional energy input, lower productivity, more species diversity, more genetic diversity within a species, nutrients recycled naturally, populations controlled by natural means, natural climax community

Agricultural ecosystems

  • Solar energy plus energy from food (for manual labour) and fossil fuels, higher productivity, less species diversity, less genetic diversity within species, natural recycling is limited and supplemented by addition of artificial fertilisers, populations are controlled, artificial community prevented from reaching its natural climax
  • Addititional energy is put into removing other species, add fertilisers, and pesticides. These reduce competition, provide mineral ions, destroy pests and reduce disease. This increases photosynethesis and hence productivity

Productivity- the rate at which something is produced

Net productivity= gross productivity - respiratory losses

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Farming practices and productivity...

Pests are organisms that reduce the productivity of crops by reducing the amount of energy available for growth

Chemical control-  poisenous chemicals kill pests e.g. pesticides. They kill:

  • Weeds- reduced competition, more energy available, faster + larger growth, higher productivity
  • Fungal infections- uses more energy to fight disease, larger growth, higher productivity
  • Insects- less biomass loss (eaten), grow to be larger, higher productivity
  • Issues: expensive, may idirectly affect non-pest species

Biological- Natural predators of the pest introduced to reduce their number. E.g. parasites of the pest species, pathogenic bacteria and viruses. The aim is to reduce and control pest NOT eradicate

  • cheap, specific, reproduces itself, no resistance
  • Issues: may directly affect non-pest species, may become pests, less cost effective (takes ages)

Intensive pest control- uses both so it reduces pest number reduced even more. Can reduce cost and enviromental impacts

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...Farming practices and productivity

Intensive farming- involves controlling the conditions DLs live in, so more energy is used for growth and less for other activities:

  • movement restricted, warm enviroment, predators excluded, optimum food, selective breeding, use of hormones
  • This means more food in a shorter period of time, higher productivity
  • Issues: Pain and stress, restricts natural behaviour, low quality of food

Fertilisers- chemicals that provide crops with minerals needed for growth. Natural fertilisers are organic matter and artificial fertilisers are inorganic

  • Replaces lost minerals so more energy can be used for growth, increasing efficiency of energy conversion.
  • Issues: can be washed away, too much kills other crops, money wasted if excess used, too little and productivity is reduced so less is profit made
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The Carbon Cycle

The carbon cycle is how carbon moves through living organisms and thenon-living enviroment

(1) CO2 is absorbed by plants when they carry out photosynthesis- it becomes organic carbon compunds in plant tissue

(2) Carbon is passed on to primary consumers when they eat plants- passes through the food chain

(3) Carbon compounds in dead organisms digested by microorganisms called decomposers which release it into soil (taken up by plants). Feedind on dead organic matter is called saprobiotic nutrition

(4) Carbon is returned to the air (and water) as all living organisms (including decomposers) carry out respiration, which produces CO2 

(5) If dead organic matter is in places with no decomposers, e.g. deep oceans, carbon compunds are turned into fossil fuels which are released as CO2 when they're burnt in factories- combustion

(6) CO2 is also released by house and car emissions

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Global warming and CO2

Fluctuations in CO2 concentrations

  • Daily: CO2 concentration increases at night because it's no longer being removed (no photosynthesis happening), but all organisms are still respiring and adding CO2 to the atmosphere
  • Yearly:  CO2 conc. increases throughout autumn and winter because less is being removed from the atmosphere, as fewer plants are photosynthesising (low light intensity, no leaves)

Global warming

  • the increase in average global temperature over the last century.
  • An increase in human activities like burning fossil fuels, farming and deforestation has increased atmospheric conc of CO2 and methane. The greenhouse effect enhances causing global warming
  • Increasing CO2 means crop grow faster, increasing crop yields (CO2 not a limiting factor)
  • Change may mean life-cycle of some insects decrease (e.g. butterflies), and some species become abundant (e.g. mosquitos)
  • The distribution and number of wild animals and plants change (e.g. polar bears)
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Nitrogen cycle...

Nitrogen fixation

Nitrogen gas converted into nitrogen containing compounds- can be carried out industrially but also by lightening, bacteria in soil, or bacteria living in root nodules of plants


Production of ammonia from organic ammonium containing compound. Saprobiotic microorganisms, feed on these materials releasing ammonia, which forms ammonium compounds in the soil

Nitrification (Oxidation reaction)

  • The conversion of ammonium ions to nitrate ions. carried out bynitrifying bacteria
  • Oxidation of ammonium ions to nitrate ions (NO2+)
  • Oxidation of nitrite ions to nitrate ions (NO3+) which producers can absorb


In oxygen defient soil, fewer aerobic nitrifying and nitrogen fixing bacteria are found. There is an increase in denitrifying bacteria- they convert soil nitrates into N2

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...Nitrogen cycle


  • When soil becomes waterlogged and short of oxygen fewer aerobic nitrifying and nitrogen fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria-convert soil nitrates into gaseous nitrogen

Leaching- when water soluble compunds like nitrogen in the soil are washed away e.g. by rain. They pollute bodies of water causing eutrophication

Eutrophication (nutrients build up in bodies of water- caused by leaching)

  • Nitrates leached stimulate growth of algae in ponds and water
  • Large amounts of algae block sunlight from reaching the plants below (algae float)
  • The plants die as light becomes a limiting factor- unable to photosynthesise
  • Bacteria feed on the dead matter
  • Increased number of bacteria reduce oxygen concentration in the water (they respire)
  • Fish etc. die as there isn't enough dissolved oxygen
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  • The change over time in the species that occupy a particular area
  • First stage of succession is the colonisation of the pioneer species (can colonise inhospitable environments)
  • The non-living environment become less hostile (soil forms, nutrients are more plentiful and plants provide shelter from the wind)
  • Greater number and variety of habitats- increases biodiversity
  • Causing more complex food webs and increases biomass
  • Climax communities are in a stable equilibrium with the prevailing climate

Conservation of habitats

  • Conservation is the management of the Earth’s natural resources in such a way that maximum use of them can be made in the future- the main reasons for conservation are ethics, economic and cultural and aesthetic
  • It frequently involves management of succession
  • We can conserve habitats by creating man made climax communities
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  • Genotype- genetic constitution (make up) of an organism- describes all the alleles the organism contains. Any change to the DNA is called a mutation and it may be inherited if it occurs in the formation of gametes
  • Phenotype- Expression of this genetic constituition and its interaction with the enviroment. Any change to the phenotype is called a modification
  • Gene- section of DNA, that is, a sequence of nucleotide bases-determines a single characteristic 
  • Allele- one or more  alternative forms of the same gene- only one allele of a gene can occur at the locus of any one chromosome They may be homozygous or heterzygous
  • Homologous chromosomes- a pair of chromosomes that have the same gene loci and therefore determine the same features but not necessarily identical
  • Co-dominance- both alleles are equally dominant
  • Multiple alleles- there are more than two alleles and only two may be present at the loci
  • Sex Linkage- A gene which is carried on the X or Y chromosomes, however due to the X chromosome being longer many genes are only seen on the X and there is no equivalent gene on the Y
  • Haemophilia- The blood clots slowly. Mainly occurs in males
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Hardy Weinberg principle

Mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population. Predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next provided:

  • No mutations arise
  • Population is isolated
  • There is no selection
  • The population is large
  • Mating within the population is random

Predict allele frequency by: p +  q = 1, where = frequency of the dominant allele and = the frequency of the recessive allele

Predict genotype fequency by: p2 + 2pq + q2= 1.0, where p2= the frequency of the homzygous dominant genotype, 2pq= the frequency of the heterozygous gentotype and q= the frequency of the homozygous recessive genotype

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Selection and Speciation


  • Directional selection- favours individuals with alleles for characteristics on one side of the mean
  • Stabilising selection- favours individuals with alleles for characteristics about the mean


  • The evolution of two or more species from an existing speciess
  • Occurs when populations of the same species are geographically separated, and due to the different enviromental conditions, over time become a different species (geographical isolation)
  • Different alleles will be advantagous in different condition and so will increase in frequency
  • The change of allele frequency will lead to differences accumulating in the gene pools of the separate populations, causing changes in phenotype frequencies
  • Eventually they will become reproductively isolated- they wont be able to breed with one another to produce fertile offspring because they are separate species
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