Topic 7: Populations in Ecosystems

Question 1

Define ecology

Answer 1

The study of the relationships between organisms and their environment

Question 2

Define environment

Answer 2

The conditions surrounding an organism. Can be split into abiotic and biotic components.

Question 3

Define ecosystem

Answer 3

Dynamic systems made from a community and all abiotic factors of its environment.

Has two major processes:
- Flow of energy through the ecosystem
- Cycling of elements in the ecosystem

Question 4

Define population

Answer 4

A group of individuals of the same species occupying the same habitat at the same time

Question 5

Define carrying capacity

Answer 5

The size of a population of a species an ecosystem can support.

Can vary due to abiotic factors and interactions between organisms

Question 6

Define community

Answer 6

All the populations of different species living and interacting in the same place at the same time

Question 7

Define habitat

Answer 7

The place where an organism normally lives, characterised by the physical conditions and other organisms present.

Have microhabitats - smaller units with their own microclimate

Question 8

Define ecological niche

Answer 8

Where an organism lives and what it does their (its role within the ecosystem)

Question 9

What is the competitive exclusion principle?

Answer 9

No two species can occupy exactly the same niche in the same ecosystem indefinitely when resources are limiting (the species that uses the resources most effectively will eliminate the other)

Question 10

How do we plot population growth on a graph and why?

Answer 10

Plot population size vs time logarithmically so all values can be plotted clearly (some very small and some very large values)

Question 11

Give some abiotic factors that can affect population size

Answer 11

• Temperature
• Light
• pH
• Water / humidity

Question 12

Describe how temperature affects population size

Answer 12

Each species has a different optimum temperature, and further away from that, fewer individuals can survive.

For plants and cold-blooded animals:
- As temp drops, enzymes work more slowly, so metabolic rate is reduced so a smaller carrying capacity
- As temp increases, enzymes denature

For warm-blooded animals:
- The further away from optimum temp, the more energy expended in maintaining internal conditions, so less energy for growth
- There is a slower reproductive rate, so a reduced carrying capacity

Question 13

Describe how light can affect population size

Answer 13

The ultimate source of energy for most ecosystems.

More light means more photosynthesis, so more growth, so more spores/seeds, increasing carrying capacity.

A greater population of plants means more food for other organisms, increasing carrying capacity for them too.

Question 14

Describe how pH can affect population size

Answer 14

pH affects enzyme action. The further away from the organism’s optimum pH, the smaller the population as enzymes are less efficient.

Question 15

Describe how water / humidity levels can affect population size

Answer 15

When water is scarce, only small populations that are adapted to dry conditions can survive.

Humidity affects transpiration and the evaporation from animal bodies. In dry air, only adapted species can survive

Question 16

Give some biotic factors that affect population size

Answer 16

• Competition
• Predation
• Disease
• Food availability

Question 17

Describe how competition can affect population size

Answer 17

Individuals (within and between species)
outcompeting each other for shared resources decreases population size

Question 18

Describe how predation can affect population size

Answer 18

More predation reduces population size of the prey, but increases the amount of predators, and vice versa

Question 19

Describe how disease can affect population size

Answer 19

The more disease, and more susceptible the population is to disease, the lower the population size

Question 20

Describe how food availability can affect population size

Answer 20

More food available increases the carrying capacity of the population, and vice versa

Question 21

When does competition occur?

Answer 21

When two or more individuals share a resource that is insufficient to fully satisfy all requirements

Question 22

Describe intraspecific competition

Answer 22

Individuals of the same species compete for resources like food, water, breeding sites, habitats, mates, mineral ions, light etc.

Greater resource availability means less competition, so a greater population size

Question 23

Describe interspecific competition

Answer 23

Individuals of different species compete for resources.

When two species are competing, one will normally have a competitive advantage and its population will increase as the other decreases.

If conditions stay the same, one species will die out due to the competitive exclusion principle.

Question 24

What is predation?

Answer 24

Predators are organisms that feed on other organisms known as their prey.

Predation occurs when one organism is consumed by another.

Question 25

What happens when predator-prey populations are brought together in a laboratory and why is it not representative?

Answer 25

In a laboratory, the prey is usually exterminated because they aren’t adapted to escape predation in that environment.

In nature prey rarely goes extinct because the area over which prey population is spread is greater and there are more refuges to escape predators.

Question 26

Describe how predator-prey relationships affect population sizes

Answer 26

• Predators eat prey, decreasing prey population size
• Fewer prey means predators are in greater competition with each other, so predator population is reduced.
• Fewer predators means less prey is eaten, so more survive and reproduce, increasing prey population
• Increased prey means predator population increases, and the cycle repeats.

Question 27

What is the benefit of periodic population crashes?

Answer 27

Important in evolution as there is a selection pressure (those able to escape predators / withstand disease or adverse climate will survive and reproduce)

Question 28

Why are samples used to investigate populations?

Answer 28

The abundance of species is often necessary to study habitats, but is virtually impossible to know exactly, as it would be too time consuming and would likely damage the habitat.

Means that samples representative of the whole population must be used.

Question 29

How would you ensure a sample is representative of the whole population?

Answer 29

• A greater sample size means more reliable data
• Sample must be random to prevent bias

Question 30

Give three experimental methods used to investigate populations

Answer 30

• Quadrats
• Transects
• Mark-release-recapture

Question 31

What are the two types of quadrat?

Answer 31

• Point quadrat
• Frame quadrat

Question 32

How does a point quadrat work?

Answer 32

It is a horizontal bar supported by two legs, with ten holes at regular intervals along the bar.
Long pins are dropped through the bar, and each species the pin touches is recorded

Question 33

How does a frame quadrat work?

Answer 33

It is a square frame divided by string / wire into equal subdivisions.
Often can be folded for storage and transport.

The quadrat is places in random locations within the area being investigated and the abundance of each species is recorded.

Question 34

What are some factors to consider when investigating populations using quadrats?

Answer 34

• Size of quadrat - depends on the size of the species being investigated and their distribution (uneven distribution = a large number of small quadrats is more representative)
• Number of samples - more samples is more reliable, but must be balanced with the time available
• Position of each quadrat - must be random (lay out 2 tape measures in a grid and use a random number generator for the coordinates of the next location)

Question 35

When would you use a transect, and how?

Answer 35

Appropriate when an area varies over a short distance (a gradual change), for example the stages of succession.

Stretch a tape along the ground and put a frame quadrat at regular intervals along it. Record the species within each quadrat.

Question 36

When investigating populations, how do you know how many repeats are needed?

Answer 36

For results to be reliable, a minimum of 15 repeats is needed (enough for a statistical test).

While running the experiment, calculate a running mean, and when it shows little change, there is enough samples.

Question 37

Describe three ways to measure abundance of a species using quadrats

Answer 37

• Counting individuals when species isn’t too abundant
• Frequency - the likelihood of a species occurring in a quadrat (% of quadrat it appears in). Useful for species hard to count + provides quick info on species present and their distribution. Gives no info on density or a detailed distribution
• Percentage cover - estimate of the area within a quadrat that the species covers. Useful when the species is abundant or difficult to count. Data collected rapidly and no counting involved. Less useful when organisms occur in several overlapping layers

Question 38

Describe how mark-release-recapture works

Answer 38

Used when measuring mobile animals as quadrats can’t be used.

• Capture a number of individuals and mark them (avoiding increasing chances of detection)
• Release them and allow time to re-integrate with the population
• Collect the second sample and record the number of marked and unmarked organisms

Question 39

Give the mark-release-recapture equation

Answer 39

Estimated total population = (num organisms sample 1 x num organisms sample 2) / (num marked organisms recaptured)

Question 40

What are some assumptions that the mark-release-recapture method relies on?

Answer 40

• Proportion of marked to unmarked individuals in second sample is the same as the whole population
• Marked individuals from the first sample distribute themselves evenly among the population
• Population has a definite boundary - no immigration/emigration
• Few, if any, births or deaths within population
• Method of marking is not toxic and doesn’t make individual more susceptible to predation
• Marks / labels not lost or rubbed off during investigation

Question 41

What events can eventually lead to succession?

Answer 41

Glaciers retreating, erupting volcanoes and sand piled into dunes by the wind create bare rock, leading to primary succession.

Events like forest fires can create barren land ready for secondary succession.

Question 42

What is succession?

Answer 42

The gradual change in a community over time due to changes in abiotic factors / species present

Question 43

In general, how does the environment and community change over the stages of succession?

Answer 43

At each stage new species colonise the area and change the abiotic factors in a way that makes it less suitable for existing species but more suitable for others, so the existing species gets outcompeted.

Overall, it creates a less hostile environment.

Question 44

What is a pioneer species? Give an example

Answer 44

The first species to colonise an area

e.g lichens

Question 45

Give some features that many pioneer species have

Answer 45

• Produce many wind-dispersal seeds / spores to easily reach isolated situations
• Rapid germination of seeds on arrival (no dormancy period)
• Ability to photosynthesise as light is available but food isn’t - not dependent on animals
• Ability to fix nitrogen from the atmosphere because even if there is soil, it has few / no nutrients
• Tolerance to extreme conditions

Question 46

Describe the stages of primary succession

Answer 46

• Pioneer species colonise an area.
• Weathering of rock and death of pioneers produces soil.
• Death and decomposition adds nutrients to the soil until it can support a small colony of plants
• Mosses colonise the area, then ferns.
• Environment becomes less hostile and biodiversity increases
• Soil gets thicker with more death. Small flowering plants arrive, then grasses, shrubs and trees
• The climax community is a balanced equilibrium. Biodiversity decreases as species outcompete each other. It is a stable community where no further succession occurs

Question 47

What are some common features of succession?

Answer 47

• Abiotic features become less hostile (more nutrients in soil, plants provide shelter)
• Greater number and variety of habitats and niches
• More complex food webs
• Increased biomass (especially in mid-succession)
• Increased biodiversity - peak at mid-succession but decreases at climax community as dominant species outcompete others

Question 48

Describe secondary succession

Answer 48

Organisms re-colonise a devastated ecosystem (e.g forest fires, clearance for agriculture).

Ecosystem returns to a climax community more rapidly as there is no pioneer stage, and soil already exists often with live seeds/spores.

The climax community may be different to the original.

Question 49

Describe how abiotic factors change as succession progresses

Answer 49

• Dead organic matter increases
• Humidity increases
• pH decreases
• Rate of evaporation decreases
• NaCl concentration decreases
• Soil temperature increases
• Wind speed decreases

Question 50

As succession progresses, explain why:

• Dead organic matter increases
• Humidity increases

Answer 50

• DOM increases as more plants are decomposing and there is more animal excretion
• Humidity increases as rate of transpiration increases, and plant shelter leads to decreased air movement

Question 51

As succession progresses, explain why:

• pH decreases
• Rate of evaporation decreases

Answer 51

• pH decreases because rain leaches basic ions (from sea spray) out sandy soil
• Rate of evaporation decreases because humidity increases, so there is a lower water potential gradient

Question 52

As succession progresses, explain why:

• NaCl concentration deceases
• Soil temperature increases
• Wind speed decreases

Answer 52

• NaCl concentration decreases because less sea spray reaches the ground
• Soil temperature increases because there is more organic matter, less drainage and a higher water content
• Wind speed decreases because plants provide a windbreak, so there is less air movement

Question 53

What is conservation?

Answer 53

The management of Earth’s natural resources by humans so maximum use can be made in the future.

Humans intervene to maintain ecosystems and biodiversity

Question 54

What are the main reasons humans invest in conservation?

Answer 54

• Personal - to maintain our planet and life support system
• Ethical - other species have been here longer and should be able to coexist. Should have respect for living things
• Economic - living organisms contain a huge gene pool that could make many valuable substances in the future. Natural ecosystems increase long-term productivity
• Cultural/aesthetic - habitats enrich our lives and add interest. Inspires writers, artists, composers etc

Question 55

What is the main way we conserve species?

Answer 55

Through managing succession, as many species would be out-competed in the later stages.

Question 56

Give some examples of humans managing succession for conservation

Answer 56

• Moorland: burning heather and grazing sheep to destroy young saplings and prevent succession
• Chalk downland: clearing of forest and sheep/rabbits eat saplings, preventing growth
• Grassland: mowing and grazing