Populations & Communities

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Populations in Ecosystems

A population is a group of organisms of the same species living in an area at one time.
Members of a population interact with each other and can breed together.
A population can be isolated from other populations of the same species due to living in a different area.
Isolation means that members of separate populations cannot breed together and gene exchange cannot take place between them.
Populations interact with their abiotic environment to form an ecosystem.

Estimating Population Size

Sampling involves measuring small samples of a population that act to represent the whole population.
Random sampling selects sampling points at random, avoiding bias by the person carrying out the sampling.
Systematic sampling places sampling points at fixed intervals, avoiding accidentally missing out sections of habitat.
A transect is a line along which samples are taken, often used to investigate the effect of environmental features on species distribution.
A sampling error is the difference between an estimated population size and the true population size.
Sampling error can be minimised by good investigation design, e.g. carrying out the right type of sampling and taking a large enough sample size.

Random quadrat sampling

Random Quadrat Sampling

A frame quadrat is a square frame placed within the area to be studied to provide a sample.
Quadrats are used to study the distribution of sessile organisms.
Quadrats can be different sizes depending on the species being studied, e.g. 1 m² for small organisms and 400 m² for large organisms such as trees.
Frame quadrats can be placed randomly, e.g. using random co-ordinates, or systematically, e.g. along a transect.
Data recorded from a frame quadrat can include presence or absence, species frequency, species abundance (ACFOR scale), and percentage cover.
Quadrats can be divided into smaller squares to allow percentage cover to be assessed more easily.

Estimating Population Size: Motile Organisms

The mark-release-recapture method is used for motile organisms.
The first large sample is taken; as many individuals as possible are caught, counted and marked in a way that won’t affect their survival.
The marked individuals are returned to their habitat and allowed to randomly mix with the rest of the population.
After a sufficient time period, a second large sample is captured and the number of marked and unmarked individuals is counted.
The Lincoln index is used to estimate population size: Population size = (M × N) / R, where M = number marked in first sample, N = total number in second sample, R = number of marked individuals recaptured.
The Lincoln index assumes that marked individuals disperse and mix fully, marking doesn't affect survival, marking remains visible, and the population size stays the same during the study period.

Limiting Population Size

Carrying capacity is the maximum number of individuals of a species that an ecosystem can support, represented by the letter K.
Abiotic factors that affect carrying capacity include light availability, temperature, and soil mineral availability.
Biotic factors that affect carrying capacity include competition for resources, predation, and disease.
Density-dependent factors have a different effect at different population densities, e.g. pathogens, competition, and predation have a greater effect at high densities.
Density-independent factors have the same effect on a population at any population density, e.g. a natural disaster such as a flood.
Density-dependent factors tend to act to keep a population at or below its carrying capacity; this is a negative feedback effect.
A population controlled by positive feedback will respond to a change in population size by continuing to change in the same direction.

Carrying capacity

Limiting Population Size: Examples

Predators are consumers that kill and eat other animals; the animals that are eaten are prey.
In a stable community, predator and prey population sizes rise and fall in a predator-prey cycle that limits the population sizes of both.
In a predator-prey cycle: the number of predators increases when there is more prey available; the number of prey decreases in response to an increase in predators; the number of predators decreases in response to a decrease in prey; the number of prey increases in response to a decrease in predators.
The relationship between the Canada lynx and the snowshoe hare is a famous example of the predator-prey interaction.
Populations can be controlled by top-down control (limited by predators) or bottom-up control (limited by availability of resources).
A population that is limited by predators is controlled by a top-down control, e.g. snowshoe hare limited by lynx, or plant populations limited by herbivory.
A population that is limited by the availability of resources is controlled by a bottom-up control, e.g. lynx limited by prey availability, or plant populations limited by light intensity.

Allelopathy & Antibiotic Secretion

Interspecific competition occurs when species compete with each other for resources.
Some species have strategies that increase their ability to outcompete other species, e.g. camouflage increases survival chances, while secretion of harmful chemicals decreases the survival chances of a competitor.
Harmful chemicals secreted into the environment are known as secondary metabolites (as opposed to primary metabolites, which are essential for survival).
Allelopathy is a strategy that involves damaging the survival of a competing species.
Organisms that carry out allelopathy secrete secondary metabolites that harm other organisms into their surroundings, e.g. plants may secrete harmful chemicals via roots into the soil, release harmful gases via stomata, or store harmful chemicals in leaves that are released when leaves fall and break down.
Examples of plant species that carry out allelopathy include garlic mustard (produces sinigrin), bracken ferns (release toxins into soil), and Himalayan balsam (secretes allelochemicals into soil).
Antibiotic secretion in some bacteria is a well-known example of allelopathy; antibiotics kill bacteria by preventing cell wall formation or inhibiting protein synthesis, reducing interspecific competition.

Population Growth Curves: Skills

Populations of living organisms tend to follow a set growth pattern over time, giving rise to a population growth curve.
A sigmoid (s-shaped) growth curve contains three phases: exponential phase, transition phase, and plateau phase.
Exponential phase (logarithmic phase): no factors limit population growth, so the population increases exponentially; the number of individuals increases, and so does the rate of growth.
Transition phase: limiting factors start to act on the population, e.g. competition increases and predators are attracted to large prey populations; the rate of growth slows, though the population is still increasing.
Plateau phase (stationary phase): limiting factors cause the death rate to equal the birth rate and population growth stops; this plateau occurs at the carrying capacity.
Population growth is exponential when the speed of growth is proportional to the number of individuals, i.e. a population of 20 individuals will reproduce at twice the rate of a population of 10 individuals.
An exponentially growing population plotted with a log scale on the y axis will appear as a straight line.
Organisms such as yeast and duckweed can be used to model the sigmoid population growth curve under laboratory conditions.

The sigmoid population growth curve

Populations: Intraspecific Relationships

Intraspecific relationships involve interactions between individuals of the same species ('intra' = within).
Intraspecific relationships can involve cooperation (both members benefit) or competition (one member outcompetes the other).
Intraspecific cooperation examples include orcas working together to catch prey, meerkats dividing roles, and ants working together to build nests and provide food.
Individuals of the same species have the same needs, so they frequently compete for the same resources.
Plants compete with members of the same species for light, minerals, water, and space.
Animals compete with members of the same species for food, mates, and territory.
Examples of intraspecific competition include male red deer fighting for access to females, robins defending territory aggressively, and oak trees competing for light, water, and minerals.

A predator-prey relationship

Community: Interspecific Relationships

A community is multiple populations of different species living and interacting in the same area.
Communities include populations from all groups of living organisms, e.g. plants, animals, fungi, and bacteria.
Living communities interact with their abiotic environment to form an ecosystem.
Interspecific relationships are interactions between different species within a community ('inter' = between).
Types of interspecific interaction include herbivory, predation, interspecific competition, mutualism, parasitism, and pathogenicity.
Mutualism occurs between members of different species; both members benefit, e.g. bacteria in root nodules, mycorrhizal relationships between fungi and plants, and coral polyps and algae.
In root nodules (e.g. in Fabaceae), bacteria convert nitrogen gas into ammonia, which is then converted into nitrates; the bacteria gain carbohydrates from the plant.
Mycorrhizae are relationships between plant roots and fungi; fungal hyphae increase surface area for water and mineral ion absorption, and the fungi receive organic compounds such as glucose.

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1.What is the definition of a population?
Easy
- AA group of organisms of the same species living in an area at one time
- BAll the organisms living in a habitat together with their non-living environment
- CMultiple populations of different species living and interacting in the same area
- DAll the living organisms in a habitat
2.What is the definition of a community?
Easy
- AA group of organisms of the same species living in an area at one time
- BMultiple populations of different species living and interacting in the same area
- CAll the organisms living in a habitat together with their non-living environment
- DThe non-living parts of an ecosystem
3.What is the definition of an ecosystem?
Easy
- AA group of organisms of the same species living in an area at one time
- BMultiple populations of different species living and interacting in the same area
- CA community of living organisms interacting with their non-living environment
- DThe physical environment in which organisms live
4.Which of the following is an abiotic factor?
Easy
- APredation
- BCompetition
- CDisease
- DLight availability
5.Which of the following is a biotic factor?
Easy
- ATemperature
- BCompetition for resources
- CSoil mineral availability
- DLight availability
6.Which of the following statements about quadrat sampling to determine population size are correct? I. Random sampling avoids bias. II. Random sample sites can be selected by throwing a quadrat with eyes closed. III. Quadrat sampling is only useful for small organisms.
Easy
- AI only
- BI and II only
- CI and III only
- DI, II, and III
7.Which of the following correctly describes a population?
Easy
- AAll of the organisms living in a habitat, together with their living and non-living interactions.
- BAll of the small birds living in a habitat.
- CAll of the blue tits living in a habitat.
- DAll of the living organisms in a habitat.
8.Which of the following statements about the mark-release-recapture method is correct?
Medium
- AMarked individuals must be allowed to randomly mix with the rest of the population before taking a second sample.
- BAny marking method can be used to mark organisms in the first sample.
- CPopulation size can be estimated using quadrat sampling.
- DSimpson's index is used to calculate population size.

Thêm câu hỏi

9.What is a sampling error?
Medium
- AThe difference between an estimated population size and a true population size.
- BThe process of counting small samples of a population that will represent a whole population.
- CWhen some members of a population are more likely to be counted than others.
- DWhen positions of the sampling points are not selected at random.
10.Which of the statements relating to data models is/are correct? I. Models can incorporate all factors affecting biological systems. II. The sigmoid growth curve model is a realistic representation of population growth in a real-life situation. III. Computer modelling can allow scientists to make predictions about future population changes.
Medium
- AI and II only
- BI, II and III
- CII and III only
- DIII only
11.Why does exponential growth occur in the early stages of population increase?
Easy
- AThere is competition for resources and increasing predation.
- BDeath rate is equal to birth rate.
- CBirth rate is higher than death rate.
- DResources are readily available and there is little or no predation.
12.What is the carrying capacity of an ecosystem?
Easy
- AThe maximum number of individuals of a species that an ecosystem can support
- BThe number of individuals present per unit area of habitat
- CThe rate at which a population grows exponentially
- DThe total number of species in a community
13.Which of the following is a density-dependent factor?
Medium
- AA flood
- BA drought
- CCompetition for resources
- DA volcanic eruption
14.Which of the following statements about mutualism is correct?
Medium
- ABoth species benefit from the relationship.
- BOne species benefits while the other is harmed.
- COne species benefits while the other is unaffected.
- DBoth species compete for the same resources.
15.In the relationship between mycorrhizal fungi and plant roots, what does the plant receive from the fungus?
Medium
- ASugars for respiration
- BMineral ions and water
- CNitrogen gas
- DAntibiotics
16.Which of the following are assumptions of the mark-release-recapture method? (select all that apply)
Medium
- AMarked individuals mix randomly with the rest of the population.
- BThe marking does not affect the survival of the marked individuals.
- CThe marking remains visible throughout the sampling period.
- DThe population size changes significantly due to births and deaths during the study.
- EThere is significant migration into or out of the population during the study.
17.Which of the following are types of interspecific relationships? (select all that apply)
Medium
- APredation
- BMutualism
- CParasitism
- DIntraspecific competition
- EHerbivory
18.A community includes all the populations of different species living and interacting in the same area.
Easy
True or false?
19.Carrying capacity is represented by the letter R.
Easy
True or false?
20.In a predator-prey cycle, the number of predators increases when there is more prey available.
Easy
True or false?
21.Density-independent factors have a greater effect on dense populations than on sparse populations.
Easy
True or false?
22.Match each term with its correct definition.
Medium
- Population
- Community
- Ecosystem
- Carrying capacity
- A group of organisms of the same species living in an area at one time
- Multiple populations of different species living and interacting in the same area
- A community of living organisms interacting with their non-living environment
- The maximum number of individuals of a species that an ecosystem can support
23.Place the phases of a sigmoid population growth curve in the correct order from first to last.
Medium
- Exponential phase
- Transition phase
- Plateau phase
24.Which of the following correctly describes a community in this woodland?
Medium
- AAll of the blue tits living in the habitat
- BAll of the small birds living in the habitat
- CAll of the organisms living in the habitat, together with their living and non-living interactions
- DAll of the living organisms in the habitat
25.Which statement about estimating population size in motile organisms is correct?
Easy
- AAny marking method can be used to mark organisms in the first sample.
- BPopulation size can be estimated using quadrat sampling.
- CMarked individuals from the first sample must be allowed to randomly mix with the rest of the population before taking a second sample.
- DSimpson's index is used to calculate population size.
26.Match each term with its correct description.
Medium
- Herbivory
- Predation
- Mutualism
- Parasitism
- An organism feeding on a plant
- An organism catching and consuming an animal
- Organisms of different species work together for the benefit of both
- A parasite lives in or on a host, causing harm
27.Which of the following correctly shows a dotted line that represents carrying capacity?
Easy
- AA horizontal line at the plateau of a population growth curve
- BA vertical line at the start of population growth
- CA line that rises steeply during exponential growth
- DA line that decreases as population size increases
28.Which of the following statements about top-down and bottom-up control of populations are correct? (select all that apply)
Medium
- AA population limited by predators is controlled by top-down control.
- BPlant populations limited by light intensity are controlled by bottom-up control.
- CA population limited by the availability of resources is controlled by top-down control.
- DTop-down control may shape an ecosystem through both lethal and non-lethal effects.
- EBottom-up control is always dominant in every ecosystem.
29.Certain plants have developed a mutualistic relationship with soil fungi, in which the plant provides sugars to the fungus for respiration. In what way does the plant receive a benefit in return?
Medium
- AThe fungus helps to stabilise the plant's root network in poor / stony soils.
- BThe fungus acts like a sponge to retain water in dry soils after (infrequent) rainfall.
- CThe fungus fixes atmospheric nitrogen and supplies nitrate ions to the plant.
- DThe fungus accesses mineral ions in mineral-deficient soils and supplies them to the plant's roots.
30.In a woodland habitat, blue tits and other small birds feed on various species of caterpillar and beetle, which themselves feed on the leaves of trees and herbaceous plants. Which of the following statements accurately describes a population within this example?
Medium
- AAll of the organisms living in the habitat, together with their living and non-living interactions.
- BAll of the small birds living in the habitat.
- CAll of the blue tits living in the habitat.
- DAll of the living organisms in the habitat.
31.A student grew Elodea canadensis (Canadian pondweed) and a native UK pondweed species separately and together in water tanks. Which of the following is the best explanation for why Elodea grew taller in the mixed tank than the native species?
Medium
- AElodea is a better interspecific competitor for resources such as light and minerals.
- BElodea benefits from a mutualistic relationship with the native pondweed.
- CThe native pondweed is a predator of Elodea.
- DElodea has a lower carrying capacity than the native pondweed.
32.In a mark-release-recapture study, 150 woodlice were caught, marked and released. Later, a second sample of 120 woodlice was caught, of which 30 were marked. Which of the following is the best estimate of the population size?
Hard
- A600
- B375
- C480
- D150
33.Which of the following are examples of density-dependent factors that can limit population size? (select all that apply)
Medium
- ACompetition for food
- BSpread of disease
- CPredation
- DA flood
- EA volcanic eruption
34.In a predator-prey cycle, the number of prey increases in response to a decrease in the number of predators.
Medium
True or false?
35.In a sigmoid growth curve, the plateau phase occurs when the birth rate exceeds the death rate.
Medium
True or false?
36.What does an increase in turbidity of the broth culture indicate?
Medium
- AAn increase in the number of yeast cells.
- BA decrease in the number of yeast cells.
- CAn increase in the amount of light passing through the suspension.
- DA decrease in the amount of nutrients in the broth.
37.The student plotted the yeast population size against time on a graph with a logarithmic scale on the y-axis. The points formed a straight line during the early stage. What does this indicate?
Hard
- AThe population is growing exponentially.
- BThe population has reached its carrying capacity.
- CThe population is decreasing exponentially.
- DThe growth rate is constant.
38.A chi-squared test was carried out to test for association between species A and species B. The results were: chi-squared value = 5.89, degrees of freedom = 1. Critical values table: df 1 — P=0.1: 2.71, P=0.05: 3.84, P=0.01: 6.64, P=0.001: 10.83. What can be concluded?
Medium
- AThere is no significant association between species A and species B
- BSpecies A competes with species B for resources
- CThere is a statistically significant association between species A and species B at the 5% probability level
- DThere is a statistically significant association between species A and species B at the 1% probability level
39.A water quality investigation recorded 26 cases of heavy metal poisoning north of a river and 12 cases south of the river. A chi-squared test gave a chi-squared value of 5.2 with 1° of freedom. Critical values for df 1: P=0.05: 3.83, P=0.01: 6.64, P=0.001: 10.83. What conclusion can be drawn?
Medium
- AThe chi-squared value is greater than the critical value, so there is an association between heavy metal poisoning and location
- BThe chi-squared value is lower than the critical value, so there is no association between heavy metal poisoning and location
- CPoisoning from heavy metals is worse in villages
- DHeavy metals are found in both locations, north and south of the river
40.In a quadrat study of clover, five 1 m² quadrats each divided into 100 small squares contained clover in 12, 39, 35, 85 and 27 squares respectively. What is the average percentage cover of clover across the five quadrats, to the nearest whole number?
Medium
- A40%
- B50%
- C20%
- D198%
41.What is the term for the maximum number of individuals of a species that an ecosystem can support?
Easy
42.What is the name of the relationship in which both species benefit?
Easy
43.What is the term for the interaction where one organism feeds on a plant?
Easy
44.In a mark-release-recapture study, 236 leafhoppers were caught and marked. In the second sample, 244 were caught, of which 71 were marked. Estimate the population size using the Lincoln index. Give your answer to the nearest whole number.
Medium
45.In a quadrat study, clover was found in 5 quadrats. Each quadrat was divided into 100 squares. The numbers of squares containing clover were 12, 39, 35, 85, and 27. Calculate the average percentage cover of clover. Give your answer to the nearest whole number.
Medium
46.Complete the sentence about sampling.
Easy
A sampling error is the difference between an estimated population size and the ____ population size.
47.Complete the sentence about population growth.
Easy
In the ____ phase of a sigmoid growth curve, the population grows exponentially because there are no limiting factors.
48.Complete the sentence about allelopathy.
Medium
Allelopathy involves the secretion of harmful chemicals known as ____ metabolites into the environment.
49.State the term used to collectively describe all the interbreeding blue tits in the woodland.
Easy
50.State the term used to collectively describe all the interbreeding foxes in the woodland.
Easy
51.A student used a 1 m² quadrat divided into 100 small squares to estimate the percentage cover of clover. In five randomly placed quadrats, clover was present in 12, 39, 35, 85 and 27 squares. Calculate the average percentage cover of clover across the five quadrats. Give your answer to the nearest whole number.
Medium
52.State the term used to describe the maximum number of individuals of a species that an ecosystem can support.
Medium
53.Define the term species.
Hard
54.Describe how a quadrat could be used to study the abundance of a species of clover plant in a meadow compared to a woodland.
Hard
55.State two abiotic factors the student should have controlled throughout the experiment.
Medium
56.Explain how the mark-release-recapture method can be used to estimate the population size of motile organisms.
Hard
57.Explain how a quadrat could be used to study the abundance of a species of clover plant in a meadow compared to a woodland.
Hard
58.In a mark-release-recapture study, 236 leafhoppers were caught and marked. In the second sample, 244 were caught, of which 71 were marked. Estimate the population size using the Lincoln index. Give your answer to the nearest whole number.
Medium
- A974 leafhoppers
- B812 leafhoppers
- C893 leafhoppers
- D731 leafhoppers
59.In a quadrat study, clover was found in 5 quadrats. Each quadrat was divided into 100 squares. The numbers of squares containing clover were 12, 39, 35, 85, and 27. Calculate the average percentage cover of clover. Give your answer to the nearest whole number.
Medium
- A44 %
- B48 %
- C40 %
- D36 %
60.A student used a 1 m² quadrat divided into 100 small squares to estimate the percentage cover of clover. In five randomly placed quadrats, clover was present in 12, 39, 35, 85 and 27 squares. Calculate the average percentage cover of clover across the five quadrats. Give your answer to the nearest whole number.
Medium
- A48 %
- B36 %
- C40 %
- D44 %

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