Topic 7—C: Populations in Ecosytems- 2. Variation in population size

Question 1

Definition of population

Answer 1

A population is all the organisms of one species in a habitat

Question 2

Definition of community

Answer 2

Population of different species in a habitat make up a community

Question 3

Definition of population size

Answer 3

It’s the total number of organisms of one species in a habitat

Question 4

Definition of carrying capacity

Answer 4

The maximum stable population size of a species that an ecosystem can support

Question 5

Why does carrying capacity vary?

Answer 5

As a result of both abiotic and biotic factors

Question 6

Abiotic factors and population size

Answer 6

• ## population size of any species varies due to abiotic factors, e.g. the amount of light, water or space available, or the temperature or the chemical composition of their surroundings

Question 7

When abiotic conditions are ideal for a species what happens to organisms?

Answer 7

• Organisms can grow more quickly and reproduce successfully
  E.g.
• when the temperature of a mammal’s surroundings is the ideal temperature for metabolic reactions to take place, they don’t have to use up as much energy maintaining their body temperature
• this means more energy can be used for growth and reproduction, so their population size will increase

Question 8

When abiotic conditions aren’t ideal for species what happens to organisms?

Answer 8

• organisms can’t grow as fast or reproduce as successfully
  E.g. when the temperature of a mammals surroundings is significantly lower or higher than their optimum body temperature, they have to use a lot of energy to maintain the right body temperature
• this means less energy will be available for growth and reproduction so their population size decreases

Question 9

What biotic factors can vary population size?

Answer 9

• interspecific competition
• intraspecific competition
• predation

Question 10

Interspecific competition

Answer 10

• when organisms of different species compete with eachother for the same resources
• this can mean that the resources available to both populations are reduced
• this means both populations will be limited by a lower amount of food
• they’ll have less energy for growth and reproduction so the population sizes will be lower for both species
• if two species are competing but one is better adapted to its surroundings than the other, the less well adapted species is likely to be out competed- it wont be able to exist alongside the better adapted species

Question 11

Intraspecific competition

Answer 11

• when organisms of the same species compete with eachother for the same resources
• it can cause a cyclical change in population size around the ecosystems carrying capacity - where the population grows, shrinks, grows again
• this is because the population of a species increases when resources are plentiful
• as the population increases, they’ll be more organisms competing for the same amount of space and food
• eventually these resources become limiting
• if the population grows beyond the carrying capacity there won’t be enough resources for all the organisms and the population will begin to decline
• a smaller population then means there’s less competition for space and food, which is better for growth and reproduction- so the population starts to grow again
• cyclical pattern then continues

Question 12

Predation

Answer 12

• where an organism (predator) kills and eats another organism (prey)
• population sizes of predators and prey are interlinked- as the population of one changes, it causes the other population to change
• as the prey population increases, there’s more food for predators, so the predator population grows
• as the predator population increases more prey is eaten so prey population begins to fall
• this means there’s less food for the predators so the population decreases