6.3.2(a) population size

spec points

  • the factors that determine size of a population

  • To include the significance of limiting factors in determining the carrying capacity of a given environment and the impact of these factors on final population size.

population: number of a certain species in particular area

population growth: human population is growing exponentially birth rate is higher than death rate

what affects birth rate

  • Economic conditions

  • Cultural and religious backgrounds

  • Social pressures and conditions

  • Birth control

  • Political factors 

what affects death rate

  • Age profile  - more elderly people = more deaths

  • Life expectancy

  • Food supply

  • Safe drinking water and sanitation

  • Medical care

  • Natural disasters

  • war

describe how each population is changing

Carrying capacity: maximum stable population size that an ecosystem can support

Limiting factor: A factor that slows down the rate of a natural process

population growth curve

  1. Slow growth -  (lag phase) small number of individuals reproduce. BR higher than DR

  2. Rapid growth – (log phase) Ever-increasing number of  individuals reproduce exponentially (no constraints on population size)

  3. Stable state - (reached carrying capacity) population declines until it remains stable with cyclic fluctuations. BR=DR

two types of strategist

k-strategist

r-strategist

Limiting factors exert more of an effect as the population grows closer to the carrying capacity 

Gradually levels out

Population size increases so rapidly that it can exceed the carrying capacity

There are then not enough resources to survive/ reproduce or a build up of waste products poisons the species.

Called ‘boom and bust’

k- strategists often:

Low reproductive rate, slow development, long life span, large body mass

r- strategists often:

High reproductive rate, fast development, short life span, small body mass

what will happen to the Algae population if:

  • Lots of light 

  • Warm

  • Lots of nutrients

No limiting factor means that growth can continue at an exponential rate

now what happens?

  • Nutrients are used up

  • Light doesn’t get to lower levels

  • Other species may enter the pond

  • Winter comes – low temp and less light

Limiting factors affect the population size

bacterial growth + limiting factors

  • bacteria will grow until a factor limits growth. no population can grow indefinitely

  • Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth

    • In reality, however, there are many abiotic and biotic factors that prevent every individual in a population from reaching adulthood and reproducing

  • This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)

    • This is why no single species has a population size that dominates all other species populations on Earth, with the possible exception of humans

  • The combined effects of limiting factors that prevent the further increase of a population is known as environmental resistance

Limiting Factor Type Examples

Abiotic

Shelter, Temperature, pH, Light, Water, Oxygen, Humidity, Accumulation of toxic waste

Biotic

Disease, Predators, Competition, Food

3 types of biotic factors

  • Interspecific competition (between different species)

    • This is competition for the same resources between individuals from different species

      • For example, red squirrels and grey squirrels in the UK compete for the same habitat, nesting sites and food resources

  • Intraspecific competition (between different members of one species)

  • Predation

  • In some cases, competition between species means that the sizes of both populations are limited

    • This can occur as each species has access to fewer resources and therefore less chance of survival and reproduction

    • This usually occurs if the two species are similarly well-adapted to the habitat

  • In other cases, competition between species can lead to a decrease in the population size of one of the species and an increase in the population size of the other

    • This can occur if one species is able to out-compete the other for resources and therefore has a greater chance of surviving and reproducing

    • This usually occurs if one of the species is better adapted to the habitat

Temperature

  • Each species has a different optimum temperature - further away from the optimum the fewer can survive. 

  • if the temperature of their surroundings is significantly lower or higher than the optimum body temperature for that species, individuals will have to use up a significant amount of energy to maintain their optimum body temperature (via homeostasis)

    • As each individual now has less energy available for growth and reproduction, fewer individuals reach reproductive age and successfully reproduce, meaning that population growth is limited

light

greater light intensity = greater rate of photosynthesis

  • more seeds

  • higher population

  • more animals can be supported

pH

  • affects enzymes, too high/low pH causes active site to denature so less e-s complexes

water and humidity

Where water is scarce only species that are adapted to dry conditions will increase/maintain their population.

Increased humidity affects transpiration rates – How and why will this affect other species?

adaptations to abiotic factors

otters: webbed feet – can walk on land and swim in water

whales: thick blubber – keep warm in cold seas where food is plentiful

bears: hibernate in winter – increase chance of survival as they conserve energy in coldest months 

adaptations to biotic factors

chimpanzees: twigs used to fish for termites – another source of food

scorpions: dancing before mating – ensures that they are the same species

bacteria: some bacteria produce antibiotics – kills other species of bacteria

migration

immigration: movement of individuals into an area

emigration: movement of individuals away from an area

density-independent factors

Density-independent factors are limiting factors that include things like unusual weather, natural disasters, and human activity. Nature can sometimes create circumstances that reduce population size relatively quicker.

  • food or nutrient limitation

  • pollutants in the environment

  • climate extremes, including seasonal cycles such as monsoons