BIOLOGY NOTES
KIARA DURKIN
ENVIRONMENTAL STUDIES
Population Ecology
The study of interactions of organisms within their physical and biological environment & how
these determine the distribution and make up of a population within an ecosystem
• Concerned with fluctuations in the size of the population and the factors, both physical and
social that regulates these fluctuations
• Biosphere: part of the earth where living organisms are found
• Ecosystem: groups of different species that interact with each other and the environment
• Organism: an individual form of life
• Individual: an organism capable of living independently
• Population: group of organisms in a species that occupy the same area and can breed
freely
• Community: group of different species that inhabit and interact in a particular area
What affects the size of a population?
Population size: the total number of individuals in a population
• Natality: birth rate in animals/ production of seeds in plants
• Mortality: death rate
• Immigration: individuals move to a different population and stay
• Emigration: individuals leave a population and do not return
- Populations will grow if increase in natality and immigration exceed mortality and
emigration
- Populations will decrease if mortality and emigration exceed natality and immigration
- Remain stable when all are approximately in balance
*Closed population = no immigration or emigration = only parameters affecting population size are
mortality and natality
How is population growth regulated?
• If individuals enter & many resources and a lack of predators = exponential growth
• Numbers increase = more demand on resources = environmental resistance (the total
number of factors that stop a population from reproducing at its maximum rate) causes
death rate/ emigration rate to increase
• Eventually a balance is reached & population stabilises at a certain number
- This number = carrying capacity (the population density that the environment can
support)
• Population fluctuates around carrying capacity until the environment changes again
• Population size fluctuates seasonally/ annually depending on available resources
• Population in an ecosystem is self-regulating (negative-feedback mechanisms)
Limiting factors
Factors that regulate the growth of a population
*Collectively build up environmental resistance
, • Limiting factors may be:
1. Density independent factors (limit growth because of natural factors, not size/ density of
population)
- Physical factors (rainfall, temperature, acidity, humidity, etc)
- Catastrophic events (floods, fire, drought, tsunami, earthquakes, etc)
2. Density independent factors (greater effect when population density is high)
- Compete for more resources (food, shelter, light, water)
- More easily found by predators
- Spread diseases/ parasites more readily
Stable and unstable populations
• Stable population: numbers decrease when size exceed carrying capacity but increase
once numbers fall below carrying capacity
• Unstable population: population far exceeds carrying capacity
- Results in habitat deteriorating rapidly (lowers carrying capacity); eventually not being
able to support population = extinction/ rapid population decrease
How is population size estimated?
1. Direct method:
- Counting every individual in a population through a census
- Can only be used where organisms are large enough to be seen & where area is not too
large
• Direct methods can be used for individuals that are:
- Slow moving (tortoise)
- Stationary (plants)
- Usually stay in fixed positions (barnacles, mussels)
• If area is too large to count every individual at one time:
- Aerial photograph (show whole area in which population occurs)
- Helicopters (count larger animals)
- Humans: census forms
2. Indirect methods
- Counting a sample size to estimate the total size of population
a) Quadrat method
• Count individuals in small areas that are consistent in size and shape (quadrats)
numbers in sample x size of whole habitat
N=
size of quadrat
N = total population
• Method:
- Measure the size of the total area
- Use wooden frame of known dimensions (e.g. 0.5m2) as a quadrat – keep size
consistent
- Quadrats should be distributed at random
- Count individuals in each quadrat – several samples taken
- Numbers in sample = total number of individuals in each quadrat
• Why is random sampling so important?
- Distribution of individuals are not uniform throughout area = sample quadrats from as
many areas as possible in the habitat = true reflection of size & distribution
KIARA DURKIN
ENVIRONMENTAL STUDIES
Population Ecology
The study of interactions of organisms within their physical and biological environment & how
these determine the distribution and make up of a population within an ecosystem
• Concerned with fluctuations in the size of the population and the factors, both physical and
social that regulates these fluctuations
• Biosphere: part of the earth where living organisms are found
• Ecosystem: groups of different species that interact with each other and the environment
• Organism: an individual form of life
• Individual: an organism capable of living independently
• Population: group of organisms in a species that occupy the same area and can breed
freely
• Community: group of different species that inhabit and interact in a particular area
What affects the size of a population?
Population size: the total number of individuals in a population
• Natality: birth rate in animals/ production of seeds in plants
• Mortality: death rate
• Immigration: individuals move to a different population and stay
• Emigration: individuals leave a population and do not return
- Populations will grow if increase in natality and immigration exceed mortality and
emigration
- Populations will decrease if mortality and emigration exceed natality and immigration
- Remain stable when all are approximately in balance
*Closed population = no immigration or emigration = only parameters affecting population size are
mortality and natality
How is population growth regulated?
• If individuals enter & many resources and a lack of predators = exponential growth
• Numbers increase = more demand on resources = environmental resistance (the total
number of factors that stop a population from reproducing at its maximum rate) causes
death rate/ emigration rate to increase
• Eventually a balance is reached & population stabilises at a certain number
- This number = carrying capacity (the population density that the environment can
support)
• Population fluctuates around carrying capacity until the environment changes again
• Population size fluctuates seasonally/ annually depending on available resources
• Population in an ecosystem is self-regulating (negative-feedback mechanisms)
Limiting factors
Factors that regulate the growth of a population
*Collectively build up environmental resistance
, • Limiting factors may be:
1. Density independent factors (limit growth because of natural factors, not size/ density of
population)
- Physical factors (rainfall, temperature, acidity, humidity, etc)
- Catastrophic events (floods, fire, drought, tsunami, earthquakes, etc)
2. Density independent factors (greater effect when population density is high)
- Compete for more resources (food, shelter, light, water)
- More easily found by predators
- Spread diseases/ parasites more readily
Stable and unstable populations
• Stable population: numbers decrease when size exceed carrying capacity but increase
once numbers fall below carrying capacity
• Unstable population: population far exceeds carrying capacity
- Results in habitat deteriorating rapidly (lowers carrying capacity); eventually not being
able to support population = extinction/ rapid population decrease
How is population size estimated?
1. Direct method:
- Counting every individual in a population through a census
- Can only be used where organisms are large enough to be seen & where area is not too
large
• Direct methods can be used for individuals that are:
- Slow moving (tortoise)
- Stationary (plants)
- Usually stay in fixed positions (barnacles, mussels)
• If area is too large to count every individual at one time:
- Aerial photograph (show whole area in which population occurs)
- Helicopters (count larger animals)
- Humans: census forms
2. Indirect methods
- Counting a sample size to estimate the total size of population
a) Quadrat method
• Count individuals in small areas that are consistent in size and shape (quadrats)
numbers in sample x size of whole habitat
N=
size of quadrat
N = total population
• Method:
- Measure the size of the total area
- Use wooden frame of known dimensions (e.g. 0.5m2) as a quadrat – keep size
consistent
- Quadrats should be distributed at random
- Count individuals in each quadrat – several samples taken
- Numbers in sample = total number of individuals in each quadrat
• Why is random sampling so important?
- Distribution of individuals are not uniform throughout area = sample quadrats from as
many areas as possible in the habitat = true reflection of size & distribution
