Population Size and Factors

  • Created by: Megan
  • Created on: 23-02-14 11:46

Population Definitions

Population: the total number of orgnaisms of a species in a habitat

Carrying Capacity: the maximum stable populations size that an ecosystem can support over an extended period of time 

Limit Factors - the factor that is in shortest supply for that given population

Abiotic Factors:- water, space, soil pH, Light Intensity, Temperatures, Nitrate avaliablity, Oxygen and Carbon dioxde concentrations

Biotic Factors:- Prey/Food avaliability, predators, competiton, parasites, age of the population

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r-Strategist Populations - Mice, Jellyfish, Dandel

  • They have small bodies and rapid maturation
  • They are in large populations with rapid fluctuations - the population goes above the carrying capacity before limiting factors can take effect
  • Mating is random and large numbers of offspring produced which have low parental input
  • They live in unstable environments with low biodiversity
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k-Strategists - Humans, Elephants, Oak Trees

  • THey have large bodies and long juvenile periods
  • They live in smaller and more stable population sizes - the population gradually levels out as the populations gets closer tot he carrying capacity, with limiting factors having an ever increasing effect
  • Live in stable but diverse environments
  • Mates are chosen, the pairs bond and produce few offspring - they are born more dependant and require large parental input
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Prey/Predator Relationships

1) Prey Population increases when the predator population decrease of food is plentiful

2)Increased prey population provides increased food supplies for the predator

3) Predator population increases after a period of time

4) Prey population decreases due to increasing numbers of predators

5)over time the population of both decline

6) the cycle is repeated

The prey populatyion is always greater than the predator population

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Example of Prey/Predator relationship in Biologica

Aphids controlled with Aphidius - this targets Ahpids as they are parasitic insects. 


  • Specific to Aphids so other organsms are unharmed
  • inexpensive
  • Non toxic


  • Strong Flier so there is no guarantee they will prey on your infestation
  • Aphidius is not as effective in hairy leaved plants as they prefer non hairy leaves.
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Competition Definitions

it occurs when resources are not in large enough supply to satisfy the needs of all the individuals who depend on those resources - as the intensity of competition increases, the death rate increases while reproduction decreases.

Competitive Exclusion Principle:-

  • if two species have the same niche, one will out compete the other and the it will die out in that habitat
  • The more overlap between two species niches, the more competition there is 
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Intraspecific Competition

...between indiviudals of the same species

  • The best adapted to obtain food will survive and reproduce while the poorly adapted die out or fail to reproduce. This slows population growth (stationary phase)
  • There are slight fluctuations:- population decreases so there is less competiton and population increases. -population increases so there is more competiton and population decreases
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Interspecific Competition

...between individuals of different species - it can affect both the population size of a species and the distribution of the species

  • It can result in one population simply being smaller than the other but both stay relatively stable populations or one population may become extinct in that habitat
  • In reality, limiting facotrs change regularly, possibly even daily and one variable acts as a limiting factor for the growth of different species
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Phases of Population growth and decline

Lag Phase - the population is constant as organisms adjust to the environment they are in

Log Phase - population size doubles as nutrients and space become plentiful

Stationary Phase - the carrying capacity is reached so the population plateuas

Decline Phase - the carrying capacity is exceeded so the death rate increaeses and the population size decreases again

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