Biology - C4.1 Populations and communities

C4.1.1—Populations as interacting groups of organisms of the same species living in an area

Students should understand that members of a group usually mate with each other, and being unable to breed with other groups helps to tell one population of a species apart from another.

C4.1.2—Estimation of population size by random sampling

It’s better to estimate the size of a population rather than counting every single individual. ‍Counting all the organisms in a population may be too expensive in terms of time and money, or it may simply not be possible. For these reasons, scientists often estimate a population's size by taking one or more samples from the population and using these samples to make inferences about the population as a whole. 

Randomness is also important in sampling. The use of simple random sampling removes all hints of bias—or at least it should. Because individuals who make up the subset of the larger group are chosen at random, each individual in the large population set has the same probability of being selected.  

Nature of Science (NOS): Students should realize that using random sampling instead of measuring the entire population will always lead to some error. This error is the difference between the estimated population size and the actual size of the whole population. 

C4.1.3—Random quadrat sampling to estimate population size for sessile organisms
Both sessile animals and plants, where the numbers of individuals can be counted, are suitable.

Application of skills: Students should understand what the standard deviation of a mean shows. They don’t need to remember the formula to calculate it. For example, the standard deviation of the average number of individuals in each quadrat can be found using a calculator. This gives an idea of how much the numbers vary and how evenly the population is spread out. 

C4.1.4—Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms

Application of skills: Students should use the Lincoln index to estimate population size.
Population size estimate = M × NR , where M is the number of individuals caught and marked initially, N is

the total number of individuals recaptured and R is the number of marked individuals recaptured. Students should understand the assumptions made when using this method. 

The assumptions: 

1. Closed population: 

No births, deaths, immigration, or emigration during the study

2. Equal catchability: All individuals have the same chance of being captured

3. Marks are not lost or overlooked: All marks remain detectable

4. Marking does not affect behavior or survival

5. Marked individuals mix randomly with the unmarked population

6. Sampling is random with respect to marked and unmarked individuals

7. Sufficient time between sampling periods for mixing, but not too long to violate closed population assumption

C4.1.5—Carrying capacity and competition for limited resources

A simple definition of carrying capacity is sufficient, with some examples of resources that may limit carrying capacity. 

Carrying capacity: the maximum population size of a species that can be sustained by a given environment. 

Competition for limited resources: struggle between organisms for shared, finite resources. The size of a population is influenced by both density-dependent and density-independent limiting factors. 

Types of resources:

-Food, water, shelter, mates, territory

       -Intraspecific competition: between members of the same species

       -Interspecific competition: between different species 

Outcomes: Reduced growth, reproduction, or survival of some individuals. 

C4.1.6—Negative feedback control of population size by density-dependent factors

Negative feedback control: mechanisms that slow population growth as density increases 

Helps maintain population size near carrying capacity
• Prevents overexploitation of resources and population crashes

Examples:

• Increased predation

• Higher disease transmission

• Reduced birth rates due to stress
  
  

The number of individuals in a population can change due to factors that don’t depend on how crowded it is (like weather). However, factors that depend on population size, like competition for resources, higher chances of being hunted, and the spread of diseases, tend to keep the population near its natural limit (carrying capacity). 

C4.1.7—Population growth curves

• Students need to study at least one example of an ecosystem.

• They should understand why populations grow quickly at first (exponential growth) and that in sigmoid population growth, there isn’t a slow start (lag phase).

• Nature of Science (NOS): The curve is a simplified model. Students should know that models don’t show all the details of real-world systems.

• Skills practice: Students should test how a population grows compared to the exponential growth model. They will graph it, using a vertical axis (population size) with a logarithmic scale and a horizontal axis (time) with a regular scale.

• Rapid, unchecked growth

• J-shaped curve
  
  • Real-world example: Bacterial growth in ideal conditions   (slide 7 - graph)

C4.1.8—Modelling of the sigmoid population growth curve

Application of skills: Students should collect data regarding population growth. Yeast and duckweed are recommended but other organisms that proliferate under experimental conditions could be used.

Sigmoid growth means Population growth pattern in environments with limited resources

• initial rapid growth, then slowing as carrying capacity is approached

• S-shaped or sigmoid curve

• Three phases:

  1. Initial exponential growth

  2. Slowing growth due to density-dependent factors

  3. Stabilization around carrying capacity +/-

• Importance: Reflects natural population dynamics and resource constraints

C4.1.9—Competition versus cooperation in intraspecific relationships

Include reasons for intraspecific competition within a population. Also include a range of real examples of competition and cooperation. 

Reasons for Intraspecific Competition

Intraspecific competition happens when individuals of the same species compete for the same limited resources. It occurs because:

1. Food scarcity – Not enough food for all members.

2. Space – Limited living or breeding areas.

3. Mates – Competition to reproduce and pass on genes.

4. Light or nutrients – For plants in crowded areas.

Examples of Competition

1. Lions compete for prey like zebras.

2. Birds compete for nesting sites in trees.

3. Trees in forests compete for sunlight by growing taller.

Examples of Cooperation

1. Wolves hunt in packs to take down large prey.

2. Meerkats work together to guard their group from predators.

3. Ants build and defend their colony together.

C4.1.10—A community as all of the interacting organisms in an ecosystem
Communities are all the living things in an area, like plants, animals, fungi, and bacteria, living together.

C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities

Symbols: 

(+): The species benefits.

(-): The species is harmed.

Interspecific relationship within communities: relationships between different species living in the same community

There are six types.

Herbivory (+-)

Primary consumer eating producers.

Herbivores (+)  feeding on plants (-)

(e.g., caterpillars eating leaves, 

sheep grazing, aphids eating plant sap).

May kill the producer.

Predation (+-)

One species kills and eat another 

species. Predator - prey relationship.

One benefits, the other is harmed 

(e.g., lion hunting a zebra,

Lady bug eating aphid).

Interspecific competition:

Two different species competing for resources or mates.

Competition (--): Both species are negatively affected (e.g., lions and hyenas fighting for prey,

Ivy climbing an oak tree to get to the light).Interspecific competition:

Two different species competing for resources or mates.

Competition (--): Both species are negatively affected (e.g., lions and hyenas fighting for prey,

Ivy climbing an oak tree to get to the light).

Mutualism

Mutualism (++): Both species benefit (e.g., bees pollinating flowers, fungi on plant roots, algae and corals) 

Parasitism (+-): 

Parasites live in or on a host.

Parasite benefits, host is harmed (e.g., tapeworms in humans, ticks on deer)

Pathogenicity (+-): 

Pathogen lives inside a host. Causes a disease.

Pathogen benefits (?), host suffers (e.g., bacteria causing disease, HIV in humans,

Potato blight)

C4.1.12—Mutualism as an interspecific relationship that benefits both species

Include these examples: root nodules in Fabaceae (legume family), mycorrhizae in Orchidaceae (orchid family) and zooxanthellae in hard corals. In each case include the benefits to both organisms.

Mutualism is a relationship between two different species where both benefit. Examples include:

1. Root Nodules in Fabaceae (Legumes)

   • Legume benefit: The bacteria in the root nodules (like Rhizobium) fix nitrogen from the air, turning it into a form the plant can use for growth.

   • Bacteria benefit: The plant provides sugars and a safe place to live.

2. Mycorrhizae in Orchidaceae (Orchids)

   • Orchid benefit: Fungi in the roots help orchids absorb water and nutrients from the soil.

   • Fungi benefit: Orchids provide sugars made during photosynthesis.

3. Zooxanthellae in Hard Corals

   • Coral benefit: The algae (zooxanthellae) produce oxygen and sugars through photosynthesis, which the coral uses for energy.

   • Algae benefit: The coral offers protection and nutrients like carbon dioxide and waste products that help the algae grow.

Note: When students are referring to organisms in an examination, either the common name or the scientific name is acceptable.

C4.1.13—Resource competition between endemic and invasive species

Choose one local example to illustrate competitive advantage over endemic species in resource acquisition as the basis for an introduced species becoming invasive.

Endemic species: Species that are native to and found only in a specific area.
Invasive species: Species that are not native and spread quickly, often competing with local species.

Effects on Niches:
Invasive species can limit the space or resources (like food, light, or shelter) available to endemic species by taking over.
Example: Grey squirrels in the UK (invasive) take more food and space from the native red squirrels (endemic).

Dangers: 

Extinctions of native plants and animals (due to predation, new diseases, and competition)

disrupting food chains

reducing the variety of life

changing natural habitats

threatening food security by invading farmland

C4.1.14—Tests for interspecific competition

Interspecific competition is suggested but not confirmed if one species does better when the other species is absent. Students should understand that there are different ways to conduct research: through controlled lab experiments, observing species in nature using random sampling, and by removing one species from an area to see what happens.

NOS: Students should understand that hypotheses can be tested through both experiments and observations, and they should know the difference between the two methods. 

Interspecific competition is when: 

• Different species compete for the same limited resources in an ecosystem

• Understanding competition helps explain species distribution and community structure

Experiments

A controlled environment for testing competition means setting up a situation where certain factors can be changed to test a theory. The aim is to figure out if changing one factor causes a change in another.

Advantages:

• Can control different factors

• See how changes directly affect the success of species

Limitations:

• Might not reflect real-life conditions completely

Example: Testing how two plant species grow under different nutrient levels

Field Observations and Random Sampling
This is the process of collecting data from natural environments by observing and recording things as they happen naturally, without changing anything.

Process:

1. Choose the study area.

2. Use random sampling methods.

3. Record the number and spread of species.

Benefits:

• Gives real-world data on how species interact.

• Helps spot patterns that suggest competition.

Challenges:

• Many environmental factors can affect the results.

(SLIDE 46)

Species Removal Experiments

Remove one species from a group of organisms and see how the others react.

For example, in 1966, Robert Paine removed starfish from intertidal zones and observed what happened to the rest of the community. He tracked how the community changed without the starfish.

What we learned:

• If some species do better without another, it might mean they are competing.

• Just because two things happen together doesn't mean one caused the other.

• This helps us understand how competition affects the way communities work.

C4.1.15—Use of the chi-squared test for association between two species

Application of skills: Students should be able to use chi-squared tests to examine whether two species are present or absent at different sampling sites. This can help them explore the differences or similarities in how the species are spread, which may provide evidence of competition between the species. 

In community ecology, we often see how species are related to each other. But it's important to figure out if these relationships are meaningful or just random. 

The chi-squared test is a useful method to check if the connections between two species in an ecosystem are significant.

What is the Chi-Squared Test?

The chi-squared (χ2) test is a statistical analysis used to determine if there's a significant association between two variables. 

In ecology, it helps scientists assess whether the presence of two species is related.  

 The formula of the Chi-squared is χ² = ∑(O_i – E_i)²/E_i, 

O_i = observed value (actual value) 

E_i = expected value.

∑= the sum 

 Χ^{2   =} Chi-Square, the test statistic 

Data for the chi-squared test is often gathered using quadrat sampling.

• Positive association: Two species are often found in the same quadrats.

• Negative association: Species are less likely to be found together, usually because they compete for resources.

These associations can be explained by ideas like competitive exclusion (where one species outcompetes the other) or resource partitioning (where species share resources in a way that reduces competition).

2 Hypotheses in Chi-Squared Test

When doing a chi-squared test, we set up two hypotheses:

• Null Hypothesis (H0): There is no relationship between the distribution of species X and Y.

• Alternative Hypothesis (HA): There is a relationship between the distribution of species X and Y (it could be positive or negative).

The test looks at the actual data compared to what we would expect to find, to see if the difference is big enough to suggest a meaningful relationship.

Steps of the Chi-Squared Test

1. Construct a table with observed values

2. Calculate expected frequencies

3. Determine degrees of freedom

4. Identify the critical region using a chi-squared distribution table

5. Calculate the chi-squared statistic

6. Interpret the result

Interpreting Chi-Squared Results
After calculating the chi-squared value:

• If the chi-squared value is higher than the critical value (from the table), it suggests a possible link between the two species.

• If it's lower, there isn't enough evidence to reject the null hypothesis, meaning no clear link has been found.

Limitations of the Chi-Squared Test

• It can be influenced by sample size (ideally, at least 50 samples).

• Assumes the samples are independent and random.

• With very large sample sizes, small, unimportant relationships might seem significant.

C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations Include a real case study.

C4.1.17—Top-down and bottom-up control of populations in communities

Students should understand that both of these types of control are possible, but one or the other is likely to be dominant in a community.

Top-down and bottom-up control refer to the ways populations in a community are affected or controlled.

• Top-down control happens when predators or higher-level consumers control the population of lower-level organisms (like herbivores or plants). For example, if wolves are removed from an ecosystem, the deer population may increase because there are fewer predators.

• Bottom-up control happens when the availability of resources (like food, water, or space) limits the population of organisms at higher levels in the food chain. For example, if plants (the food for herbivores) are limited, then herbivores won't be able to grow in numbers, which in turn affects predators that rely on herbivores for food.

Both types of control can be important, but in any given community, one is usually more dominant than the other.

C4.1.18—Allelopathy and secretion of antibiotics

These two processes are similar in that a chemical substance is released into the environment to deter potential competitors. Include one specific example of each—where possible, choose a local example.

Allelopathy and the secretion of antibiotics are both processes where an organism releases a chemical substance into the environment to stop other organisms from growing or thriving nearby.

• Allelopathy: This happens when plants release chemicals to inhibit the growth of other plants around them. For example, black walnut trees produce a chemical called juglone, which can prevent nearby plants from growing.

• Secretion of antibiotics: This process occurs when microorganisms (like bacteria or fungi) release substances that kill or stop the growth of other microbes. An example is Penicillium fungi, which produces penicillin to kill bacteria.

Both processes help these organisms reduce competition and increase their chances of survival.