Biology Unit 4- Populations

For A2, AQA

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Ecology- Study of inter-relationships between organisms and their environment

Biosphere- Layer of land, air and water that surrounds the Earth

Ecosystem- Self contained functional units, made up of interctign biotic and abiotic features in a specific area

Habitat- Place where an organism usually lives, characterised by physical conditions and species of other organisms present

Population- Group of individuals of the same species that occupy the habitat at the same time

Community- All popultions living in the same area at the same time

Ecological niche- All conditions and resources required for an organism to survive, reproduce and maintain it's population. Niche can only be occupied by one species

Abundance- Number of individuals of one species in a particular area

Distribution- Where  species is within a particular area

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Mark- Release- Recapture

1. Catch known number of individuals and mark

2. Release and wait a while

3. Catch known number of individuals and record number of marked individuals recaptured

Population size= (total no. in 1st sample x total no. in 2nd) / no. of marked recaptured


1. Mark not rubbed off/ lost

2. Mark not toxic or causes predation on individuals

3. Few "deaths" and "births" - stable population

4. No immigration or emigration 

5. Marked have time to redistribute into population

6. Ratio of marked: unmarked is the same in 2nd sample and whole population

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1. Organisms should be studied in situ, or the minimum number removed

2. Organisms removed should be returned asap. Even if dead so they can be eaten, providing nutrients to the ecosystem

3. Leave time for site to recover. Further data would then be collected under "normal" conditions

4. Disturbance and damage avoided- protect ecosystem

Kids should be let loose in a field because...

Practical learning is better than theoretical, everyone has to start somewhere, it will aid awareness f conservation

Kids shouldn't be let loose because...

They are inexperienced and it would cause damage to environment, can watch a video about a field instead, ecology is boring so no-one will study it further. 

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Factors that should be considered are:

1. Size of the quadrat to be used

2. Number of quadrats to record in an area

3. Position of each quadrat in the study area

Frame Quadrat:

A square frame that is divided into subdivisions by wire etc. Normally the number of subdivisions is 100 to make calculations easier. 

Point Quadrat:

A horizontal bar with two legs. There re holes in the bar, and you drop a pin through the holes, and record whatever the point touches (used for plants). 

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More quadrat stuff...


The likelihood of  species occuring in a quadrat. 

 e.g. bug appears in 15/30 squares so the frequency = 50%

Good where species are hard to count, and gives a quick idea of presence and distribution. 

Bad as it doesn't provide information on density or detail of distribution

Percentage Cover:

An estimate of the area within a quadrat that a species covers (plants).

Good where species are abundant, rapid collection as individuals not counted

Useless when overlapping layers of plants etc. exist

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Random sampling:

1. Lay tape measures at right angles

2. Obtain co-ordinates from random number generator

3. Place the quadrat at the co-ordinates and record

Systematic sampling:

- Good where some transition in a community takes place e.g. tidal seashore

- Shows stages of zonation with transects

 -Line transect = string stretched on the ground, and organisms that the line passes over are counted (normally plants). 

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Abiotic Factors

They are the non-iving part of the environment

Abiotic factor --> pH: Population is larger when appropriate pH exists. Population is smaller when the pH is far from the enzyme optimum

Abiotic factor --> temperature: Temp too low = slows enzymes. Temp too high = denatured. Ectotherms (cold blood) affected and also plants = slow population growthEndotherms regulate their own temp. If temp moves far away from enzyme optimum, more energy is spent maintaining temp less energy on growth and reproduction

Abiotic factor --> water and humidity: If water is scarce= small population only with those well adapted. Humidity affects transpiration and evaporation in plants

Abiotic factor --> light: Increased light intensity= increased photosynthesis = increased growth increased reproduction. More plants in turn means more food for animals. 

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Biotic factors= activities of living organisms--> competition/ predation

Intraspecific Competition

Competition of individuals of the same species e.g. food, water, breeding space

Availability determines the population size (can be a time lag in population size changing after competition)

Interspecific Competition

Competition of individuals of different species e.g. food, light, water

Competitive exclusion principle:

When two species occupy the same niche, one will have the competitive advantage. If conditions remain the same then one species will be removed.

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Predation occurs when one organism is consumed by another

Predator-prey relationship:

1. Prey eaten = smaller prey population

2. Fewer prey = predators have increased competition

3. Predator population reduced due to competition

4. Fewer predators= less prey eaten so population of prey increases

5. More prey now available, so predator population increases.

Not realistic though as organisms eat a range of foods

Predation in a lab:

Predator would exterminate prey as range and variety of habitat is limited to the confines of a lab. 

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Population Growth Curve









1. Slow, as initial small number of individuals reproduce

2. Increasing population - more reproduction

3. Decline due to predation/ food supply etc- limiting factors. Fluctuations die to variations in factors e.g. food supply. 

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Stable population               Decreasing population         Increasing population

BR and DR in balance           Lower BR and DR             High BR, fewer oldies

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Human populations

Population growth rate = (pop. change during period/ pop. at start of period) x 100

Explosion in the human population has been caused by the development of agriculture and the manufacturing of industry and trade development

Demographic Transition:

Large increase in life expectancy in the most developed countries. Societies there have changed from a short life expectancy and high birth rates to a long life expectancy and low birth rates

Factors affecting human populations:

-Birth rate  -Death rate    -Immigration -Emigration

(Also there are unpredictable factors such as HIV, Aids and obesity)

Population growth= (birth + immigration) - (death + emigration)

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Factors affecting birth rates:

*Economic conditions --> low per capita income means increased birth rate

* Culture/ Religion --> countries encourage larger families/ no birth control

*Social pressures --> large family = better social standing/ more income

*Birth control --> contraception + abortion influences birth rate

* Political factors --> government influence through education and taxation 

Birth rate = (no.of births per year / total population in the same year) x 1000

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Factors affecting death rates:

*Age profile --> high population of oldies = increased death rate

*Life expectancy --> Longer in MEDC's than LEDC's 

*Food supply --> good supply and diet means lower death rate

*Water and sanitation --> less risk of disease means lower death rate

*Medical care --> good healthcare and education = decreased death rate

*Natural disasters --> in more prone countries there is a higher death rate

*War --> immediate drop in popluation, and delayed response as less members of population around to reproduce

Death rate = (no.of deaths per year/ total pop. in same year) x 1000

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This is incredibly helpful, thanks so much !!!

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