biology 4

a2 aqa biology

  • Created by: rebecca
  • Created on: 07-06-12 09:29

Populations - definitions

Ecosystems - a species occupies a niche, goverened by adaptations to abiotic and biotic factors

Community - all the species of a habitat

Population - all the organisms of one species in a habitat

birth rate = (no births per year/pop in same year) x 1000

death rate = (no deaths per year/pop in same year) x 1000

% pop growth rate = (pop change / pop at start) x 100

immigration - individuals joining a pop

emmigration - individuals leaving a pop

life expectancy - the average life expectancy is that age at which 50% of the individuals in a particular pop are still alive

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populations - definitions

increasing population - birth rates > death rates

decreasing population - birth rates < death rates

stable population - death rate = birth rate - no change in pop size

pop growth = (B+I) - (D+E)

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investigating ecosystems

random sampling 

  • quadrats - consider size of quadrat, number of sample needed & position of each quadrat
  • transects - abundance & distribution of a species
  • quantative data


  • percentage cover - estimate the area within a quadrat that a species covers
  • frequency - the likelihood of a species occuring in a quadrat


  • estimated pop size = (total in 1st sample x total in 2nd sample)/no marked in 2nd
  • no immigration/emmigration
  • few births/deaths
  • mark not rubbed off
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variation in pop size


  • one organism consumed by another
  • predator-prey relationships: predator eats prey & decreases no of prey. more competiton for fewer prey. predator pop decreases. fewer predators = fewer prey eaten. prey pop increases. predator pop increases.

interspecific competition - individuals of DIFFERENT species

intraspecific competition - individuals of SAME species

abiotic factors

  • temperature - enzymes & optimum
  • light - energy source for ecosystems
  • pH - affects enzyme activiy
  • water & humidity - affects transpiration in plants
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inheritance & selection


  • genotype: genetic constitution of an organism
  • phenotype: observable characteristics
  • gene: a section of DNA that determines a single characteristic. exists in 2 or more forms
  • allele: one of the different forms of a gene. only one can occur


  • the evolution of a new species from an existing species
  • geographical isolation: physical barrier prevents 2 pops breeding with each other, conditions in 1 area change, mutations arise & are passed down generations. LONG period of time new species evolves, & is unable to produce fertile offspring with original species
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inheritance & selection

sex linkage

  • controlled by recessive alleles
  • on X or Y chromosome

Multiple alleles

  • only 2 can be present in an individual
  • more than 2 alleles for a gene


  • two alleles are dominant
  • both expressed in phenotype
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  • environmental conditions remain same
  • phenotype closest to mean favoured
  • eliminates extremes


  • if environmental conditions change phenotypes needed to survive will change
  • phenotypes at extremes are favoured
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p + q = 1

p2 + 2pq + q2 = 1

  • used to predict the frequencies of the alleles of a particular gene in a population
  • predicts that the proportion of dominant & recessive alleles remain the same for each generation 

can only be used if:

  • population is large
  • no mutations
  • population is isolated
  • no selection
  • mating is random
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energy & ecosystems

energy transfer

  • photosynthesis: main route that energy enters the ecosystem
  • energy transferred through trophic levels
  • energy transfer = (energy available after / energy available before) x 100


  • numbers - no account of size, may not be pyramid shape/ may be inverted, maybe impossible to represent on same scale
  • biomass - gm-2, only organisms at a particular time are shown, total mass of the plants/animals at a particular place
  • energy - most accurate, energy stored in organisms, difficult & complex
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energy & ecosystems - food production

Pest control

  • pest = organism that competes with humans for food - may be a danger to health
  • effective pesticide should: be specific, biodegrade, be cost effective, not accumulate
  • biological control: pests don't develop resistance, control organism may become a pest, doesn't work quickly (time lag), controls the pest - doesn't eradicate it, reproduces itself, specific
  • chemical pesticides: pests develop genetic resistance - new pesticides need to be developed, must be reapplied at intervals - expensive, always has some effect on non-target species
  • integrated pest-control systems: removal of pests mechanically-time, using biological control, pesticides as last resort.
  • pests & productivity - monoculture crop allows rapid spread, pests create a limiting factor, photosynthesis reduces. ∆ productivity decreases.
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energy & ecosystems - food production

intensive rearing

  • reduce energy losses: feeding controlled, predators excluded, selective breeding, hormones used to increase growth, animals confined, movement reduced - less energy lost in muscle contraction, temperature controlled - reduced heat losses

agricultural ecosystems

  • energy input: food for workers, fossil fuels for machinery 
  • natural ecosystem only input is sun & productivity is low
  • agricultural: productivity increased - reducing effect of limiting factors - reducing competition

net productivity = gross productivity - respiratory losses

net productivity: kJm-2year-1, area of ground covered by leaves of crop, efficiency of photosynthesis

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primary              pioneer species

barren land->primary colonisers->secondary->tertiary--shrub land->climax

hostile, instable

community less hostile, more stable & higher biodiversity

secondary(faster): land altered due to fire/disease/grazing/agriculture -               re-colonisation to climax community

  • non-living environment becomes less hostile
  • a greater number and variety of habitats
  • increased biodiversity
  • more complex food webs
  • increased biomass
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conservation of habitats

  • management of Earths natural resources
  • managing succession & preventing next stage
  • why? - ethical(some species here before humans) -economic -cultural & aesthetic(habitats & organisms enrich our lives
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  • dead & decaying remains
  • animal waste


  • mined from rocks & deposits
  • converted into different forms
  • blended together to form appropriate balance for particular crop


nutrients removed from soil

rain water dissolves nitrates & carries them beyond reach of roots

find way to watercourse

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little nitrate in lakes & rivers = limiting factor

nitrate conc increases, not limiting factor, plant & algae grow on surface & absorb light

no light to lower depth 

light limiting factor for deeper plants, plants die

lack of dead plants not limiting factor so saprobiotic microorganisms grow

increased demand for oxygen - conc decreases - limiting factor - aerobic organisms die

less competition for anaerobic organisms - populations increase

anaerobic organisms decomposed - more nitrates released

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global warming


  • extinction of organisms
  • rise in sea level
  • higher temp & less rainfall = failure of crops
  • greater rainfall & intense storms = life cycles & pop of insects alters
  • methane produced by decomposers
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