Biology A2: Unit 4-Populations

Contains: Populations & Ecosystems

Investigating Populations

Variation in Population Size

Human Populations

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Populations & Ecosystems


Habitat: The place where an organism lives.

Population: All the organisms of one species in a habitat.

Community: Populations of different species in a habitat.

Ecosystem: All the organisms  living in a particular area and the abiotic factors.

Abiotic factors: The non-living features of the ecosystem.

Biotic factors: The living features of the ecosystem.

Niche: The role of a species within its habitat.

Adaptation: A feature  that members of a species have that increases their chance of survival and reproduction.

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Populations & Ecosystems

Every species occupies a different niche.

The niche a species occupies includes:

a. Biotic interactions

b. Abiotic interactions

Every species has its own unique niche - a niche can only be occupied by one species.

It may look like two species are filling the same niche  but there will be slight differences.

If two species try to occupy the same niche, they will compete with each other (interspecific competition). One species will be more succesful than the other and as a result, only one species will survive.

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Populations & Ecosystems

Organisms are adapted to biotic and abiotic factors, these adaptations can be:

a. Physiological: Processes inside the organism.

b. Behavioural: The way the organism acts.

c. Anatomical: Structural features of the organism.

Organisms with better adaptations are more likely to survive, reproduce and pass on the alleles for their adaptations, which then have a higher frequency within the population.

This is called natural selection.

Every species is adapted to use an ecosystem in a way that no other species can.

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Populations & Ecosystems

Examples of adaptations to abiotic conditions:

1. Otters have webbed paws - this means they can walk on land and swim effectively. This means theycan live and hunt on land and in water.

2. Whales have a thick layer of blubber - this helps to keep them warm. This means they can live in places where food is plentiful.

3. Brown bears hibernate - They lower their metabolism over winter. This means they can conserve energy.

Examples of adaptations to abiotic conditions:

1. Sea otters use rocks to smash open shellfish and clams. This gives them access to another food source.

2. Scorpions dance before mating - this ensures they attract a mate of the same species. 

3. Some bacteria produce antibodies that kill other species-less competition.

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Investigating Populations

Investigating populations involves looking at the abundance and distribution of a species in a particular area.

1. Abundance - the number of individuals of one species in an area.

a. Frequency - the number of samples a species is recorded in.

b. Percentage cover - how much of the area is covered by a species(plants only).

c. Count samples - the number of individuals within the samples taken (mobile organisms and plants).

2. Distribution - where a particular species is within the area you are investigating

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Investigating Populations

Random sampling:

1. Choose an area to sample - a small area within  the area being investigated.

2. Samples should be random to avoid bias - pick random samples by dividing the area into a grid and using a random number generator to select coordinates.

3. Use an appropriate technique to take a sample of the population (see revision card 7)

4. Repeat the process, taking as many samples as possible. This gives a more reliable estimate for the whole area.

5. The number of individuals for the whole area  can be estimated by taking an average of the data in each sample and multiplying it by the size of the whole area. The percentage cover can be estimated by calculating the average of all the samples.

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Investigating Populations

Different methods are used to investigate different organisms:

1. Pitfall Traps (ground insects):

a. Pitfall traps are steep-sided containers that are sunk into a hole. The top is partially open.

b. Insects fall into the container and cannot get out - they are protected from rain and some predators by a raised lid.

c. The sample can be affected by predators also falling into the trap - they may eat other insects, affecting the results.

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Investigating Populations

Different methods are used to investigate different organisms:

2. Pooters (ground insects):

a. Pooters are jars that have rubber bungs sealing the top and two tubes stuck through the bung.

b. The shorter tube has mesh over the end that is in the jar. The longer tube is open at both ends.

c. When you inhale through the shorter tube, air is drawn in through the longer tube. If you place the longer tube over an insect, it will be sucked into the jar.

d. This can take a long time (and is labour intensive) to get a large sample using pooters. Some species may be missed if the sample is not large enough.

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