Biology 2B Ecosystems

Population dynamics

Factors affecting population size 1

• Births – number of new organisms• Deaths – number organisms dying• Migration

– Immigration – organisms entering the population

– Emigration – organisms leaving the population

These are all generally given as rates per 1000.

Growth = (births + immigration) – (deaths + emigration)

Factors affecting population size 2Density independent

Physical (abiotic) factors• Rainfall• Humidity• Temperature• Acidity• SalinityCatastrophic events• Flood• Fire• Drought• Volcanic eruptions• Earthquakes • Tsunamis

Density dependent (biotic factors)• Food supply (abundance or

distribution of prey species) - this becomes more critical as population size and density increases

• Disease - this is easier to spread as population size and density increases

• Parasites - these spread more easily as population size and density increases

• Competition - this becomes more critical as population size and density increases

• Predation (abundance or distribution of predator species) - this becomes more critical as population size and density decreases

The carrying capacity is the number of organisms that can be supported by the environment in which the population exists.

Population growthA population will grow exponentially unless its growth is limited by factors in the environment.

A new or colonising population will often show exponential growth initially until it is using all the resources present.If the population numbers are not controlled eg by a predator or disease the population will reach a size that uses all the resources in the ecosystem and the population size drops rapidly – a crash. As ecosystems may be damaged by such large numbers, it may take longer for the population to increase again.Populations controlled by competition in this way show large swings in population numbers – booms and busts.

Population growth 2Biotic potential is a term that describes how rapidly a population can increase. It is determined by the species ability to survive, breed and colonise new areas.Environmental resistance is a term that describes the forces that act to slow population growth. These include resources available and the interaction of other species with the population.

If biotic potential is greater than environmental resistance, populations will grow.If environmental resistance is greater than biotic potential, populations will shrink.Stable populations reach a size where biotic potential matches environmental resistance and population sizes fluctuate around the carrying capacity of the ecosystem.

Sustainable population growthMost populations are controlled by density dependent factors eg predators or disease.This limits their size to the carrying capacity that can be sustained in their particular environment.The population size tends to fluctuate just above and just below the levels of the carrying capacity.Biologists use the terms J-shaped (or J-curve) and S-shaped (or S-curve) to describe the growth patterns of populations.S-shaped populations are more stable than J-shaped populations as they are held in balance by other factors in the ecosystem.

Natural and artificial populations

Natural ecosystems are those that exist without human intervention

Artificial ecosystems are those that are produced by humans.

Calculating population growth

Birth rate = number of births per 1000 head of population number of births ÷ total population x 1000

Death rate = number of births per 1000 head of population number of deaths ÷ total population x 1000

Immigration rate = number of births per 1000 head of population number of individuals entering ÷ total population x 1000

Emigration rate = number of births per 1000 head of population number of individuals leaving ÷ total population x 1000

Growth rate = (birth rate + immigration rate) – (death rate + emigration rate)

Distribution patternsDistribution refers to the region in which members of a population can be found.Distribution patterns can be described as even (regular), clumped or random.Distribution is usually affected by the presence or absence of resources (eg near water) and social interactions between members of the same species (eg juveniles with adults).