Biology - Chapter 19: Populations in Ecosystems

Ecology

The study of the inter-relationships between organisms and their environment

What does the environment involve?

Biotic (living) and abiotic (non-living) factors

Ecosystems

A dynamic system made up of a community and all the non-living factors of its environment

2 major processes that take place in ecosystems

1. The flow of energy through the system

2. The cycling of elements within the system

Carrying capacity

A certain size of population that an ecosystem is able to support

What causes the size of a population to vary? (2)

1. The effect of abiotic factors

2. Interactions between organisms

Community

All the populations of different species living in the same habitat at the same time

Habitat

The place where an organism normally lives

Microhabitats

Smaller units within habitats with their own microclimate

Niche

How an organism fits into its environment: where the organism lives and what it does there

What does the niche of an organism include?

All the biotic and abiotic conditions to which an organism is adapted to in order to survive, reproduce and maintain a viable population

Explain the competitive exclusion principle (4)

• If two species are in competition with each other, one will have an advantage over the other.

• That population will increase in size, while the other diminishes

• This will then lead to the complete removal of the disadvantages species

• Therefore, no two species occupy exactly the same niche

What two interactions between organisms can affect the size of a population?

1. Interspecific and intraspecific competition

2. Predation

Population size

The number of individuals in a population

How might you plot a growth curve for a population of bacteria and why?

By using a logarithmic scale - the population grows very rapidly

What determines the carrying capacity of a population?

Its limiting factors

Give 4 examples of abiotic factors

1. Temperature (affects rate of reaction)

2. Light (ultimate source of energy - intensity affects rate of photosynthesis)

3. pH (affects rate of enzyme-controlled reactions)

4. Water and humidity (affects rate of transpiration / water loss from animals)

Why are ecosystems described as dynamic?

They are in a constant state of change as populations fluctuate (e.g. flow of energy + cycling of elements)

2 types of competition

Interspecific and intraspecific competition

Intraspecific competition

When individuals of the same species compete with one another for resources

What might organisms compete for?

Food, water, breeding sites

What happens as the availability of resources (like food, water, breeding sites, et) increase?

The size of the population increases

Interspecific competition

When individuals of different species compete for resources

Predator

An organism that feeds on another organism

Prey

An organism fed on by a predator

When does predation occur?

When one organism is consumed by another

How does the predator-prey relationship affect the population size? (4)

1. Predators eat prey - prey population falls

2. Less prey so increased competition between predators - predator population falls

3. Fewer prey eaten so more survive and reproduce - prey population increases

4. More prey available as food means reduced competition between predators - predator population increases

Abundance

The number of individuals of a species in a given space

Why are only small samples of habitats used when investigating abundance?

Counting every organism would be too time-consuming and would cause damage to the habitat

What type of sampling do you use for slow-moving or non-motile organisms?

Random sampling or systematic sampling

2 types of quadrat

1. Point quadrat

2. Frame quadrat

What type of sampling do you use for motile organisms?

Mark-release-recapture

What is a point quadrat?

Horizontal bar supported by legs, ten holes along bar where a pin may be dropped - count organisms touched by pin

What is a frame quadrat?

Square frame divided into subsections - count number of organisms within frame

3 factors to consider when using quadrats

1. Size of quadrat used - depends on size of organisms + their distribution

2. Number of sample quadrats used - larger the number, the more reliable the results

3. Position of quadrats - must be random

How do you carry out random sampling? (5)

1. Lay out two tape measures at right angles to one another

2. Obtain co-ordinates (at least 10) using an RNG

3. Place a quadrat (1m x 1m) at each co-ordinate and count the number of organisms within each

4. Calculate the mean number of organisms per m²

5. Calculate the total area and multiply by mean number of organisms to estimate total number of organisms

How do you carry out systematic sampling? (3)

1. Place belt transect along area in a straight line

2. Place a quadrat along it at regular intervals

3. Estimate the population / count the number of organisms in each

Give 3 ways to measure abundance

1. Estimating total population from sample

2. Estimating frequency of species in area

3. Estimating percentage cover of area by species

How does the mark-release-recapture method work? (4)

1. Known number of animals are randomly caught

2. Ensure marking is not harmful of fish / does not affect survival of fish

3. After some time, new sample collected randomly

4. Size of population is calculated using formula (attached)

How is the size of the population calculated with the mark-release-recapture method?

6 assumptions the mark-release-recapture method relies on

1. Proportion of marked to unmarked in second sample is the same as the proportion of marked to unmarked in the first

2. Marked individuals released from first sample distribute themselves evenly amongst the rest of the population

3. Population has a definite boundary so no immigration/emigration

4. Few deaths or births in the population

5. Method of marking is not toxic to the individual or makes them more liable to predation

6. Mark or label is rubbed off or lost during the investigation

Succession

The change in community over time either due to changes in the abiotic factors or due to changes in the species present

How might different species alter an environment? What does this cause? (2+1)

1. By making it less suitable for existing species

2. By making it more suitable for other species with different adaptations

This means that the new species can outcompete the old one, so take over the area

Pioneer species

The first species to colonise the new land

What common features emerge as a result of succession? (5)

1. Abiotic environment becomes less hostile

2. Greater number and variety of habitats and niches

3. Increased biodiversity (different species can occupy the habitats/niches)

4. More complex food webs (as greater biodiversity)

5. Increased biomass (as more complex food webs)

Climax community

Stable community where no further succession takes place

Describe the stages of succession (4)

1. Pioneer species colonise an inhospitable environment

2. The species change the environment, making it less hostile for others (e.g. less hostile abiotic factors, more food sources)

3. More species colonise so there is an increase in biodiversity

4. Conditions change further and a more stable community is formed - the climax community

Why might a pioneer species be suited for colonisation? (6)

1. Reproduce asexually (single organism can rapidly multiply)

2. Produce vast number of wind-dispersed seeds/spores (can easily reach isolated situations)

3. Rapid germination of seeds upon arrival (don’t need a dormancy period)

4. Ability to photosynthesise (light is available - food is not)

5. Ability to fix nitrogen from the atmosphere (little soil so few nutrients)

6. Tolerance to extreme conditions

Primary succession

A type of succession that takes place on newly-formed or newly-exposed land

Secondary succession

A type of succession that occurs when land that has already sustained life is altered

Why does secondary succession occur more rapidly than primary succession? (2)

Soil already exists in which spores and seeds often remain alive in the soil + there is an influx of animals and plants through dispersal and migration from the surrounding area

Conservation of resources

The management of the Earth’s natural resources by humans in such a way that maximum use of them can be made in the future

What does conservation involve?

The maintenance of ecosystems and biodiversity

5 reasons for conservation

1. To protect habitats

2. To protect species / increase biodiversity

3. To reduce global warming

4. Source of medicines

5. To reduce erosion

How might habitats be conserved?

By managing succession so that a change to the next stage is prevented

How does succession result in the species present in an ecosystem changing? (6)

1. Pioneers colonise land

2. They change the environment - give an example

3. This causes more habitats / niches to be present OR

4. Some species are better competitors

5. This enables a change in the species present and other organisms can become established

6. Conditions change further to favour climax community

Why might you do systematic sampling instead of random sampling?

To see the effect of the gradient of an environmental factor

Why is it advantageous for a plant that colonises after 50 years to have a high rate of photosynthesis at low light intensities?

Plant will grow in the shade

Why might the number of animal species be higher at one habitat compared to another? (2)

1. More food sources present

2. More habitats/niches present

Why might the mark-release-recapture method produce unreliable results in large lakes? (2)

1. Less chance of recapturing fish

2. Unlikely fish distribute randomly

How might you be able to tell succession is taking place from a table? (2)

The data shows an increase in species richness / species diversity / total number of living organisms

Then talk about how stages of succession are shown

Give three features of a climax community

1. Stable community

2. Abiotic factors constant

3. Populations stable around carrying capacity

Density-dependent factors + give an example

Factors that are affected by the density of the population - normally biotic factors e.g. predation, disease, competition

Density-independent factors + give an example

Factors that are not affected by the density of the population - normally abiotic factors e.g. temperature

How does succession result in a wide variety of species living in an ecosystem? (2)

1. Results in increase in variety/diversity of species OR in number of species

2. This provides more habitats / types of food

What does it mean if a population is stable?

It has more complex food webs, so a change in one species would have little effect on others as there are alternative food sources.

When do you use a transect?

When there is a graduent of an environmental factor

Community

Populations of different species that live in the same environment

3 ethical considerations related to the use of the mark-release-recapture method

1. Animal should not be marked in a way that increases predation

2. Capturing the animal may cause it distress

3. The animal may not reintegrate back into the population

What size must the population be in order for mark-release-recapture to work?

Large