Detailed, colourful, nicely displayed revision notes with images on AQA A-Level Biology on section 6, topic 14, Populations in ecosystems (Genetics, populations, evolution, and ecosystems ). According to the AQA A-Level specification, these notes are on section 3.7.4. these notes do not include suc...
Populations in ecosystems
Ecology- the study interrelationships between organisms and their environment. The
environment includes both non-living (abiotic) factors, such as temperature and rainfall and
living (biotic) factors, such as predation and competition.
Population – a group of individuals of one species that occupy the same habitat at the same
time and can interbreed.
Carrying capacity – the certain size of a population that an ecosystem can support
The size of a population can vary as a result of:
- The effect of abiotic factors
- Interactions between organisms e.g. interspecific and intraspecific competition and
predation
Intraspecific – between members of the same species
Interspecific – between members of different species
Community- all the populations of different species living and interacting in a particular
place at the same time.
Habitat- a place where an organism normally lives and is characterized by physical
conditions and the other types of organisms present. There are many habitats within an
ecosystem e.g. bushes and trees.
Within each habitat there are smaller units called microhabitats e.g. a crevice on the bark of
an oak tree.
Niche- describes how an organism fits into the environment. It refers to where an organism
lives and what it does there. It includes all the biotic and abiotic conditions that an organism
is adapted to in order to survive, reproduce and maintain a viable population.
Populations are dynamic, in that they vary in size and composition over time.
Variation in population size
In cases where a population grows rapidly you will not be able to plot a graph because the
curve will run off the curve at an early point in time and you may not be able to see the
pattern or point where they stop increasing in size. Therefore, a logarithmic scale is used so
you can see the information plotted on a graph for a longer time.
, In this example we can see that the rate of growth starts to slow down at 8 hours as we
were able to plot more points with the log scale. At the start we can see that the bacteria
were dividing rapidly as all the factors needed for the growth of the population were
present and there were no limiting factors. However in time things change and limiting
factors become a problem for example:
- Mineral ions are consumed as the population becomes larger
- The bacteria at the surface prevents the light from those at deeper levels
- Other species are introduced that feed on the bacteria or compete for light or
minerals
- winter brings lower temperatures and lower light intensity of shorter duration.
Abiotic factors:
- Temperature - each species has a different optimum temperature that it is best able
to survive at, the further away from this the fewer individuals that are able survive.
This can also affect the denaturation of enzymes in organisms.
- Light - this is a basic necessity of light, with the rate of photosynthesis increasing as
light intensity increases.
- pH - this can have an impact on the action of enzymes with each enzyme having an
optimum pH that it can work at. Where the appropriate pH exists there is a larger
proportion of organisms.
- Water and humidity - in instances where water is scarce only small populations of
adapted species will exist. Humidity affects transpiration in plants and therefore only
those that are adapted to environments where transpiration is high will survive.
Competition
There are a number of different factors that members of the same species compete for. The
availability of resources determines the size of a population; the greater the resources, the
greater the size of the population as there’s less competition etc. This type of competition is
called intraspecific competition with factors affecting it being as follows:
- Food
- Water
- Mates
- Shelter
- Minerals
- Light
An example of intraspecific competition is male robins. These maintain a large territory to
support their families, however in winter when food is short, they move into other males
territories to obtain enough food to survive.
Interspecific competition on the other hand is where members of different species compete
with one another for the same resources. This occurs most commonly when different
species occupy the same niche. An example is red and grey squirrels in the UK. When a
population of two species are in competition one will normally have a competitive
advantage over the other. The population of this species will gradually increase while the
other will diminish. This is known as the competitive exclusion principle. To show how a
factor influences the size of a population it’s necessary to link it to the birth and death rate
of the individuals in a population.
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