populations in ecosystems

Question 1

What is a community?

Answer 1

All the populations of different species living in
the same place (habitat) at the same time.

Question 2

What is an ecosystem?

Answer 2

A community and the non-living (abiotic)
components of its environment.
Ecosystems can range in size from very small to very large.
They are dynamic systems (populations rise / fall over time).

Question 3

What is a niche?

Answer 3

● The specific role of a species within its habitat, eg. what it eats, where and when it feeds
● Governed by its adaptation to both abiotic (non-living) and biotic (living) conditions

Question 4

Explain the advantage of species occupying different niches

Answer 4

● Less competition for food / resources
● If two species tried to occupy the same niche, one would outcompete the other

Question 5

What is carrying capacity?

Answer 5

The maximum (stable) population size of a species that an ecosystem can support

Question 6

List the factors that influence carrying capacity

Answer 6

Abiotic factors
Eg. light intensity, temperature, soil pH & mineral content, humidity

Interactions between organisms
a. Interspecific competition - between organisms of different species
b. Intraspecific competition - between organisms of the same species
c. Predation (predators kill and eat other animals, called prey)

Question 7

Explain how abiotic factors may affect population size / carrying capacity

Answer 7

● If conditions favourable, organisms more likely to survive & reproduce → increasing carrying capacity
● Eg. increasing light intensity increases rate of photosynthesis in plants
○ This increases carrying capacity of a variety of plant species
○ So increases the number and variety of habitats, niches and food sources for animals
○ So increasing carrying capacity of a variety of animal species

Question 8

Explain how interspecific competition may affect population size

Answer 8

● Reduces [named resource] available to both species, limiting their chances of survival & reproduction
○ So reduces population size of both species
● If one species is better adapted, it will outcompete the other
○ So population size of less well adapted species declines, potentially leading to extinction

Question 9

Explain how intraspecific competition may affect population size

Answer 9

1. As population size increases, resource availability per organism decreases, so competition increases
   ● So chances of survival & reproduction decrease → population size decreases
2. As population size decreases, resource availability per organism increases, so competition decreases
   ● So chances of survival & reproduction increase → population size increases

Question 10

Explain the changes which occur in populations of predators & prey

Answer 10

Populations fluctuate in cycles, the predator population peaking after the prey (lag time):
1. Prey population increases so predators have more food
● So more predators survive & reproduce
2. Predator population increases so more prey killed & eaten
● So less prey survive & reproduce
3. Prey population decreases so predators have less food
● So less predators survive & reproduce
4. Predator population decreases so less prey killed & eaten
● So more prey survive & reproduce (cycle repeats)

Question 11

Describe how the size of a population of slow-moving or non-motile
organisms can be estimated

Answer 11

1. Divide area into a grid / squares eg. place 2 tape measures at right angles
2. Generate a pair of coordinates using a random number generator (eg. on a calculator)
3. Place a quadrat here and count number / frequency of [named species]
4. Repeat a large number of times (10 or more) and calculate a mean per quadrat
5. Population size = (total area of habitat / quadrat area) x mean per quadrat

Question 12

Describe how the mark-release-recapture method can be used to estimate
the size of a population of motile organisms

Answer 12

● Capture sample of species, mark and release
● Ensure marking is not harmful / does not affect survival
● Allow time for organisms to randomly distribute before collecting second sample
● Population = (number in sample 1 x number in sample 2) / number marked in sample 2

Question 13

A student collected 17 lizards and marked them before releasing
them back into the same area. Later, she collected 20 lizards, 10 of
which were marked. Calculate the number of lizards in this area.

Answer 13

(17 x 20)/10 = 34

Question 14

What assumptions does the mark-release-recapture method make?

Answer 14

1. Sufficient time for marked individuals to mix / distribute evenly within the population
2. Marking not removed and doesn’t affect chances of survival / predation
3. Limited / no immigration / emigration
4. No / few births / deaths / breeding / change in population size (or birth & death rate are equal)

Question 15

Suggest why the mark-release-recapture method can produce unreliable
results in very large areas

Answer 15

● Unlikely that organisms will distribute randomly / evenly
● Less chance of recapturing organisms (that were marked initially)

Question 16

Describe and explain how primary succession occurs

Answer 16

Succession = change in a community over time due to change in abiotic factors / species

1. Colonisation by pioneer species (first to colonise)
2. Pioneer species (and other species at each stage in succession) change abiotic conditions
   ● Eg. they die and decompose, forming soil which retains water (humus / organic matter)
3. So environment becomes less hostile / more suitable for other species with different adaptations
   AND less suitable for previous species, so better adapted species outcompete previous species
4. As succession goes on, biodiversity increases
5. Climax community reached - final stable community (no further succession)

Question 17

Describe features of a climax community

Answer 17

● Same species present / stable community over a long time
● Abiotic factors (fairly) constant over time
● Populations (fairly) stable (around carrying capacity)

Question 18

Explain how conservation of habitats involves management of succession

Answer 18

● Further succession can be prevented to stop a climax community forming
○ By removing or preventing growth of species associated with later stages eg. by allowing grazing
● This preserves an ecosystem at a certain point / in its current stage of succession (plagioclimax)
● So early species are not outcompeted by later species and habitats / niches are not lost

Question 19

Describe the conflict between human needs and conservation as well as the
importance of managing this

Answer 19

● Human demand for natural resources (eg. timber) is leading to habitat destruction / biodiversity loss
● Conservation is needed to protect habitats / niches / species / biodiversity
● Management of this conflict maintains the sustainability of natural resources
○ Meeting current needs without compromising the ability of future generations to meet theirs