Population and Community Ecology Summary - Gr 11 & Gr12 IEB Mind Action Series
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Life Sciences
Institution
12th Grade
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Life Sciences
This is a detailed, comprehensive and well-organised summary of of the Population and Community Ecology chapter in the Gr 11 & Gr12 IEB Mind Action Series textbook. Diagrams and class notes are included throughout the summary.
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1
POPULATION ECOLOGY
Ecology: the study of interactions of organisms with their physical and biological
environments and how these determine the distribution and make up of populations
within an ecosystem
Population Ecology: is concerned with fluctuations in the size of a population and the
factors, both physical and social, that regulates these fluctuations
Population size: the total number of individuals in a population
Individuals → populations → community → ecosystem → biosphere
Biosphere Part of earth where living organisms are found - all ecosystems make up biosphere
ecosystem Made up of groups of different communities that interact with each other and with the non-
living environment
organism An individual form of life, like bacterium/protists/fungus/plant/animal composed of a single
cell or complex of cells that are capable of growing and reproducing
community A group of different species/populations that inhabit and interact in a particular area
species A group of closely related organisms that are very similar to each other and are usually
capable of interbreeding and producing fertile offspring
individual A single organism capable of independent existence
population A group of organisms of the same species that occupy the same area and can breed freely
with each other
Biome A group of different ecosystems characterized by the dominant plant life and the same
climate
WHAT AFFECTS POPULATION SIZE
NATALITY: birth rate/production of seeds
- Breeding seasons affect natality rates
MORALITY: death rate
IMMIGRATION: individuals move into a population and stay
EMIGRATION: individuals leave a population and do not return
- Popul. Size + when immigration exceeds death and emmigration
- Popul. - when death & emigration exceed birth and immigration
, 2
- Popul. Size in a closed population will only be affected by birth/death
REGULATION OF POPULATION GROWTH
ENVIRONMENTAL RESISTANCE: the total number of factors that stop a population
from reproducing at its maximum rate
CARRYING CAPACITY: the population density that the environment can support
- If individuals enter an unoccupied area w/ all essential resources, they will
reproduce and more demands for the increased population size must be met,
- thus building environmental resistance, stabilizing the population to be able to
survive at its current carrying capacity.
- Population fluctuates around carrying capacity until environ changes again
(seasons/annually)
- Population size is self-regulating caused by negative-feedback mechanisms
LIMITING FACTORS
- The factors that help to regulate the growth of a population
- Build up environmental resistance
TYPES:
1. Density independent factors
a. Limit growth of population because of natural factors
b. PHYSICAL FACTORS: rainfall/temp/humidity/acidity
c. CATASTROPHIC FACTORS: floods/fires/drought/tsunami/earthquakes
2. Density dependent factors
a. Have greater effect when PD = high; when organisms are more crowded:
b. Compete for more resources: food/light/02/water/space/shelter
c. More easily found by predators
d. Spread disease & parasites faster and to a greater amount
STABLE AND UNSTABLE POPULATIONS
STABLE POPULATIONS: numbers decrease when size exceeds carrying capacity, &
increases when numbers fall below carrying capacity
- Fluctuation
UNSTABLE POPULATIONS: population far exceeds carrying capacity; resulting in the
habitat:
- Deteriorating rapidly, leading to lowering the carrying capacity
, 3
- Being unable to support population, thus they will decrease rapidly/go extinct
HOW POPULATION SIZES ARE ESTIMATED
1. Direct method
2. Indirect methods
1. DIRECT METHODS:
- literally counting every single individual in population = CENSUS
- Can only be used when:
- Individuals are large enough to see
- Area isn't too large
- Individuals are: slow moving/stationary/generally stay in a fixed position
- If area is too large:
- Aerial photography shows whole area in which population occurs
- Helicopters can count large animals
2. INDIRECT METHODS:
- Counting a sample number of population, then calculating estimates of the size
1. Quadrat Method:
a. Purpose = get comparable samples from areas w/ consistent size and
shape
Total population = Numbers in sample X size
of a whole
Size habitat
of
METHOD: quadrat
1. Measure size of total area
2. Use meters squared or .5meters squared as a quadrat (same size for each
sample)
3. Quadrats should be distributed randomly
4. Count individuals in each quadrat (number in sample)
5. Calculate using formula
[Quadrats are easy to use, inexpensive and suitable for studying plants, slow-moving
animals and faster-moving animals with a small range. However, they require the
researcher to perform the work in the field and, without care, are prone to study
errors. Randomly-spaced quadrats that are too small might miss too many individuals,
resulting in under-representative estimates of population size. ]
THE IMPORTANCE OF RANDOM SAMPLING
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