Population size may vary as a result of • the effect of abiotic factors • interactions between organisms: interspecific and intraspecific competition and predation.

The size of a population changes over time and is effected by many different factors.

They key factors are:

• Birth and death rates
• Immigration and emigration- individuals moving in and out of the habitat
• Biotic potential- the maximum rate that a species can reproduce at
• Carrying capacity (K)- biotic and abiotic limiting factors on population size

Average population growth curve

A species moves into an environment, and begins to breed, birth rate is above death rate which means there is population growth. At first there are a small number of individuals so only a few individuals are born meaning the growth rate is slow, but as more as the population grows the birth rate increases meaning population growth increases.

The population grows so large that the environment can no longer support it, this means it has surpassed the carrying capacity, for example there are too many individuals for the amount of food, this causes the death rate to increase. An increase in death rate above birth rate means the population starts to decrease.

The population decreases below the carrying capacity meaning that it is no longer limited by the environment, e.g. there is enough food for all the individuals. This means that the death rate decreases below the birth rate so population beings to increase. The population will increase above the carrying capacity and the cycle will repeat its self so the population fluctuates around the carrying capacity.

Abiotic limiting factors

These are the non-living factors that stop a population from growing above a certain point:

• Water- involved in respiration
• Humidity- low humidity means water evaporates out of organisms
• PH- effects enzyme action which changes the speed of chemical reactions
• Temperature- ''
• Light- energy source
• Hummus (organic matter)- nutrients available for reactions

Biotic limiting factors

These are the ways in which plants and animals limit the size of a population:

• Food- plants and animals supply energy for survival
• Disease- viruses, fungi and bacteria can cause death and infertility
• Competition- for food, water, shelter
• Predation- animals and plants are often killed as a result of being prey

Interspecific competition: a niche is the needs of a species e.g. the types of food they eat. If two species have the same niche then one of the species will out compete the other and the other will die out (competitive exclusion principle).

Intraspecific competition: individuals in a species have the same niche, therefore if the population grows above carrying capacity and there is not enough of a resource there will be fierce competition.

In species that have a metamorphosis the first stage is much slower (as there is a higher density of individuals) which means only a few will make it to adulthood, this is beneficial as the mature and immature individuals eat different food so there is less competition.

Predation: predators eat prey, so there is less prey available, this means the predators have less to eat so their population decreases, this means that the prey are killed less often so the population increases, which leaves more food for the predators so their population in turn increases and the cycle will repeat.