Populations and communities - UWCSEA - IB Biology 2025

C4.1.1— What are populations? Define

Populations consist of individuals of the same species that live in a same area at the same time;

Members of a population typically breed within their group, ensuring the continuation of the species.

Reproductive isolation is a key factor in distinguishing one population from another;

as it prevents gene flow between populations, leading to evolutionary divergence.

C4.1.2—How can we estimate the size of populations? Why might this not be perfect? How can we tell whether differences are significant?

Random sampling is essential for estimating population size because it is often impractical or impossible to count every individual in a population;

Sampling error occurs because a sample may not perfectly represent the entire population, leading to differences between the estimated population size and the actual size.

The concept of statistical significance is important to understand when interpreting the results of random sampling;

Significant differences can be established using statistical tests e.g. Chi squared

C4.1.3—How can we sample populations of organisms that don't move? what are these organisms known as?

Sessile organisms (e.g., plants and certain invertebrates) are those that do not move and are fixed in one location

Quadrat sampling involves placing a square frame (quadrat) at random locations in a study area and counting the number of individuals within the frame to estimate population size.

The standard deviation of the mean number of individuals per quadrat provides a measure of variation within the population, indicating how evenly the organisms are spread across the area.

C4.1.4—How can we estimate the size of populations of organisms that move? What is the formula? What assumptions are we making?

The Lincoln index is a method used to estimate the size of a population of motile organisms by capturing, marking, releasing, and then recapturing a sample of the population;

The formula for the Lincoln index is

Population size estimate=(M×N)/R

where M is the number of individuals initially marked when captured, N is the total number of individuals in the second sample (that were captured), and R is the number of marked individuals that were recaptured.

Assumptions for this method include that the marked individuals mix evenly with the population;

that no marks are lost, and that there are no significant births, deaths, or migrations during the study period;

C4.1.5—What is carrying capacity and why does it arise?

Carrying capacity refers to the maximum number of individuals of a species that an environment can support sustainably.

Limited resources such as food, water, space, and shelter are factors that determine the carrying capacity of an environment.

Competition for these resources can lead to a struggle for existence, influencing which individuals survive and reproduce.

C4.1.6—How does negative feedback control population sizes?

Density-dependent factors are factors whose effects on the population vary with population density, such as competition, predation, disease, and waste accumulation.

Negative feedback mechanisms occur when an increase in population density leads to a decrease in population growth rate, pushing the population back towards the carrying capacity.

Examples include increased predation in dense populations and higher transmission rates of disease.

C4.1.7—What are population growth curves? What are the different phases of them?

Population growth curves typically exhibit an exponential phase;

where the population grows rapidly due to abundant resources and low competition.

As resources become limited, growth slows, leading to a logistic or sigmoid curve, where the population stabilizes at the carrying capacity;

population growth models are simplifications and may not fully represent real-world complexities;

C4.1.8—How can we model population growth curves?

Modelling involves using mathematical or computer-based simulations to represent population growth over time;

Organisms like yeast and duckweed are often used in experiments to model population growth under controlled conditions;

The sigmoid curve represents the typical pattern of population growth, with a lag phase, exponential phase, deceleration phase, and stationary phase.

C4.1.9—What is intrapecific competition? How can it include both co-operation and competition?

Intraspecific competition occurs within a species, where individuals compete for the same resources such as food, mates, and territory;

Cooperation can also occur within a species, where individuals work together for mutual benefit, such as in social animals like bees and wolves;

Examples of competition include plants competing for sunlight, while cooperation is seen in hunting packs of wolves.

C4.1.10—What is a community?

A community includes all the different populations of organisms living and interacting in a specific area;

Ecosystem interactions within a community include predation, mutualism, competition, and parasitism;

The community structure is influenced by biotic factors (e.g., interactions between species) and abiotic factors (e.g., climate, soil).

C4.1.11—What are herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity?

Herbivory involves animals feeding on plants (e.g., cows grazing on grass).

Predation involves one organism (predator) hunting and consuming another (prey) (e.g., lions hunting zebras).

Interspecific competition occurs between different species for the same resources (e.g., foxes and owls competing for rodents).

Mutualism is a relationship where both species benefit (e.g., bees pollinating flowers).

Parasitism involves one organism benefiting at the expense of another (e.g., ticks feeding on mammals).

Pathogenicity refers to the ability of an organism to cause disease in another (e.g., bacteria causing infections in humans).

C4.1.12—Explain mutualism with examples

Mutualism is an interaction where both participating species benefit

Examples include root nodules in Fabaceae (legume family) that fix nitrogen, mycorrhizae in Orchidaceae (orchid family) that enhance nutrient uptake, and zooxanthellae in hard corals that provide nutrients through photosynthesis.

Benefits include enhanced nutrient acquisition and protection.

C4.1.13—Resource competition between endemic and invasive species

Invasive species are species which are not native to a country or region;

they often outcompete endemic species for resources such as light, nutrients, or space, leading to the decline or extinction of endemic species, which are those that are native to the region;

An example could be the American bullfrog in Singapore, which has outcompeted native species for food and habitat.

C4.1.14—How can we test for interspecific competition?

Interspecific competition is indicated if one species is more successful in the absence of another, but this does not prove causation.

Research approaches include laboratory experiments, field observations by random sampling, and field manipulation by removing one species to observe the effects on the other.

Students should understand the difference between experiments and observations: experiments involve controlled variables, while observations are made in natural settings.

C4.1.15—How can we test for the association between two species?

The chi-squared test is a statistical method used to determine if there is an association between the presence/absence of two species in several sampling sites.

A significant chi-squared result may suggest interspecific competition or cooperation.

What is the formula for Chi squared?

C4.1.16—What are predator-prey relationships? What is an example of one?

Predator-prey relationships are a classic example of density-dependent control, where the population size of predators and prey are interlinked.

A real case study could involve the lynx and snowshoe hare populations, which show cyclical fluctuations based on predator-prey dynamics.

C4.1.17—What are top-down and bottom-up control in terms of population sizes?

Top-down control refers to regulation by predators (e.g., wolves controlling deer populations).

Bottom-up control involves regulation by resource availability (e.g., plant abundance controlling herbivore populations).

Understanding which type of control is dominant can help explain the structure of an ecosystem.

C4.1.18— Explain allelopathy and secretion of antibiotics

Allelopathy involves the release of chemicals by a plant to inhibit the growth of other plants (e.g., black walnut tree releasing juglone).

Secretion of antibiotics by organisms, such as Penicillium fungi producing penicillin, inhibits the growth of bacteria.

These processes are examples of how organisms compete indirectly by altering their environment.