Adam Cook
Biology spec led revision
Chapter 12 - Populations and Sustainability
Factors affecting population size
Population size changes because of abiotic factors:
1) The population size is the total number of organisms of one species in a habitat
2) The population size of any species varies because of abiotic factors e.g. the amount of light,
water or space available, the temperature of their surroundings or the chemical composition of
their surroundings
3) When abiotic factors are IDEAL for a species, organisms can grow fast and reproduce
successfully
• When the environmental temperature is ideal for metabolic reactions organisms don’t
have use much energy to maintain their body temperature.
• Meaning more energy can be used for growth and reproduction, so population size will
increase
4) When abiotic conditions aren't ideal, organisms wont grow fast or reproduce as successfully
• When environmental temperatures are significantly lower or higher than the optimum
body temperature then they will have to use energy to maintain the right body
temperature.
• Meaning there will be less energy spent on growth and reproduction so the population
size with decrease.
And because of Biotic Factors
1) Inter-specific Competition
1) Interspecific competition is when organisms of DIFFERENT species compete for the
SAME resources. (e.g. red and grey squirrel compete for the same resources in the UK)
2) Interspecific competition between two species can mean resources available to both
species is reduced (if they share the same food then there will be less for both)
3) This means that both populations will be limited by a lower amount of food. Having
less energy for growth and reproduction, so population size will be lower for both
4) In areas where both red and grey squirrel live the populations would be smaller for both
compared to if only one lives here
5) Interspecific competition can also affect distribution of species. If two species are
competing but one is better adapted to the surroundings then it will be out-competed - it
wont exist alongside the other species.
6) Since introduction of grey squirrel into the UK means the red squirrel had disappeared
from larger areas as the grey squirrel is better adapted being larger and so able to store
more fat for over the winter.
2) Intra-specific competition
1) When organisms of the SAME species compete for the SAME resources
2) The population of a species will increase when a resource is plentiful
3) As the population increases there will be more organisms competing for the same
amount of space and food
4) Eventually space and food become limiting, there isn’t enough supply for demand
5) Population begins to decline as a result
6) A smaller population means LESS competition for resources, better for growth and
reproduction
7) So population grows again
1 of 2
Biology spec led revision
Chapter 12 - Populations and Sustainability
Factors affecting population size
Population size changes because of abiotic factors:
1) The population size is the total number of organisms of one species in a habitat
2) The population size of any species varies because of abiotic factors e.g. the amount of light,
water or space available, the temperature of their surroundings or the chemical composition of
their surroundings
3) When abiotic factors are IDEAL for a species, organisms can grow fast and reproduce
successfully
• When the environmental temperature is ideal for metabolic reactions organisms don’t
have use much energy to maintain their body temperature.
• Meaning more energy can be used for growth and reproduction, so population size will
increase
4) When abiotic conditions aren't ideal, organisms wont grow fast or reproduce as successfully
• When environmental temperatures are significantly lower or higher than the optimum
body temperature then they will have to use energy to maintain the right body
temperature.
• Meaning there will be less energy spent on growth and reproduction so the population
size with decrease.
And because of Biotic Factors
1) Inter-specific Competition
1) Interspecific competition is when organisms of DIFFERENT species compete for the
SAME resources. (e.g. red and grey squirrel compete for the same resources in the UK)
2) Interspecific competition between two species can mean resources available to both
species is reduced (if they share the same food then there will be less for both)
3) This means that both populations will be limited by a lower amount of food. Having
less energy for growth and reproduction, so population size will be lower for both
4) In areas where both red and grey squirrel live the populations would be smaller for both
compared to if only one lives here
5) Interspecific competition can also affect distribution of species. If two species are
competing but one is better adapted to the surroundings then it will be out-competed - it
wont exist alongside the other species.
6) Since introduction of grey squirrel into the UK means the red squirrel had disappeared
from larger areas as the grey squirrel is better adapted being larger and so able to store
more fat for over the winter.
2) Intra-specific competition
1) When organisms of the SAME species compete for the SAME resources
2) The population of a species will increase when a resource is plentiful
3) As the population increases there will be more organisms competing for the same
amount of space and food
4) Eventually space and food become limiting, there isn’t enough supply for demand
5) Population begins to decline as a result
6) A smaller population means LESS competition for resources, better for growth and
reproduction
7) So population grows again
1 of 2
