A concise, bullet-pointed summary on Population Ecology. Covers strictly what you need to know for all tests and assessments (as dictated by the SAGS guidelines). Includes annotated diagrams and images to illustrate points mentioned, as well as additional research, helpful when understanding the wo...
• Population: a group of individuals of the same species who live together in the same habitat/
area, at the same time, and are likely to breed
• Population size governed by a number of factors, both physical and social
• Populations of animals exhibit different social organisations that enhance their survival within
the community
• The main goal of a population is to ensure survival of their species and therefore to maximise
their reproductive
• Community: all the different populations of organisms occupying the same area at the same
time
• Species: a group of individuals with similar characteristics which are capable of breeding and
producing fertile offspring
• Habitat: an environment where an organism lives and reproduces
• Ecological niche: the functional role the organism plays in its community, defines the resources
and conditions necessary for survival of each species
• Ecosystem: a unit of plants and animal communities in interaction with each other as well as
with non-living factors in a particular area
Changes in Population Size
Change in population density = ( births + immigration ) — ( deaths + emigration )
1. Natality (birth rate)
- Natural ability of a population to increase
- Percentage increase of the population per unit time
- In humans, it is expressed as the number of births per 1000 persons per year
2. Mortality (death rate)
- Number of deaths per 1000 people
- In a closed population (no immigration or emigration) the only factors affecting population size
are the birth rate and death rate
- Most natural populations are not closed, but are open, therefore individuals can emigrate or
immigrate as they wish
3. Immigration (entering a population)
- Individuals entering an area from another one
- Occurs especially when a population colonises a favourable new habitat
4. Emigration (exiting a population)
- Individuals leaving an area
- Occurs especially when an area become overpopulated, resulting in a lack of food or space
• Population size can also fluctuate because of seasonal or annual changes
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, Chloë van Beukering Life Sciences Notes 2020
Population Growth Patterns
• When food is abundant and growing conditions are favourable, a population has the potential
to increase in number from generation to generation
- Growth is geometric when each
generation’s increase is a constant
percentage of the total population size
- Also known as exponential growth
because the larger the population gets, the
faster it grows
- The “J-shaped” curve represents the typical
form of an exponential growth curve
- Eventually the population hits the
environmental carrying capacity
- As a population density approaches the
carrying capacity, competition becomes
more intense, mortality increase, the birth
rate drops and any of the following is
possible:
- The population may level out and stabilise
below the capacity
- This pattern is known as a logistic or “S-
shaped” growth curve
- The population may fluctuate around (or
below) the carrying capacity
- The population may briefly overshoot the
carrying capacity and then crash, resulting in
repeated cycles of “boom” and “bust”
Population Regulating Factors
• Populations would continue to grow if there weren’t any factors that limit or regulate their sized
• These factors include: competition, drought, disease, predation and climate-related changes
• These factors prevent a population from reaching its maximum reproductive growth rate and are
collectively called environmental resistance
• Carrying capacity is the maximum number of individuals of a certain species that an
environment can support indefinitely without depleting the resources
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