IB DP Biology Populations and communities Study Notes

IB DP Biology Populations and communities Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

How do interactions between organisms regulate sizes of populations in a community?
What interactions within a community make its populations interdependent?

Standard level and higher level: 5 hours

IBDP Biology 2025 -Study Notes -All Topics

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

Population refers to a group of individuals of the same species living in a given area. They normally interbreed with each other and don’t interbreed with other species. Populations can range from a few individuals to billions. They are often separated by geographical barriers, like the sea separating islands.

Population interactions include competition for resources and cooperation. Emergent properties arise from these interactions. For instance, in a herd, individuals interact to minimize predation risk. Predator-prey interactions often occur at the herd’s edges. Individuals within the herd can move to reduce their chance of predation.

A single individual, like Lonesome George, the last Pinta Island tortoise, cannot be considered a population as it cannot interbreed.

C4.1.2—Estimation of population size by random sampling

Estimating population size is often challenging due to the vastness of populations and the difficulty of counting individuals. Random sampling is a technique used to estimate population size by selecting a representative sample. This involves randomly choosing individuals from the population, ensuring that each individual has an equal chance of being selected. By analyzing the sample, researchers can make inferences about the entire population.

C4.1.3—Random quadrat sampling to estimate population size for sessile organisms

Quadrat sampling is a method used to estimate the population size of sessile organisms like plants. A quadrat is a square frame placed randomly within a habitat. The number of organisms within the quadrat is counted, and this data is used to estimate the total population size. This method is suitable only for organisms that don’t move.

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

The capture-mark-release-recapture method is a technique used to estimate population size for motile organisms. In this method, a sample of individuals is captured, marked, and released back into the population. After a period of time, another sample is captured, and the number of marked individuals¹ is recorded. The Lincoln Index formula is then used to estimate the total population size.

Assumptions of this method include:

No migration into or out of the population
No births or deaths during the study period
Marked individuals mix back into the population and have the same chance of recapture as unmarked individuals
Marks remain visible and do not affect survival or behavior

C4.1.5—Carrying capacity and competition for limited resources

Carrying capacity refers to the maximum number of individuals that a particular environment can support. When a population exceeds its carrying capacity, resources become limited, leading to competition among individuals. This competition can result in increased mortality, reduced reproduction, or emigration.

Factors that can limit population size and determine carrying capacity include:

For plants: water, light, and soil nutrients
For animals: food, water, space, and suitable breeding sites

The image of the creosote bush population illustrates the concept of carrying capacity. Despite the large gaps between individuals, the population is likely at carrying capacity due to limited water availability. Each bush has extensive root systems, effectively utilizing all available water in the soil.

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

Population size fluctuates over time due to various factors. Density-independent factors, like natural disasters, affect populations regardless of their size. Density-dependent factors, such as competition for resources, predation, and disease, have a greater impact on larger populations. These factors act as negative feedback mechanisms, stabilizing population size around the carrying capacity of the environment.

C4.1.7—Population growth curves

Population growth curves illustrate the patterns of population change over time. Exponential growth occurs when resources are abundant and there are no limiting factors, leading to rapid population increase. However, as resources become limited, the growth rate slows down, and the population eventually stabilizes at its carrying capacity. This pattern is known as logistic growth.

The J-shaped curve represents exponential growth, while the S-shaped curve represents logistic growth. Factors like competition, predation, and disease can limit population growth and contribute to the S-shaped curve. Understanding population growth patterns is crucial for managing populations and conserving biodiversity.

C4.1.8—Modelling of the sigmoid population growth curve

The sigmoid population growth curve can be modeled experimentally using organisms like duckweed or yeast. A small number of organisms are introduced into a controlled environment with abundant resources. The population size is then monitored over time.

Duckweed: This small, aquatic plant reproduces asexually and can quickly populate a water body. By controlling factors like light, nutrients, and container size, the carrying capacity of the environment can be determined.

Yeast: This single-celled fungus reproduces by budding. Yeast populations can be grown in controlled environments to study the effects of factors like temperature, pH, and nutrient availability on population growth.

By studying these model organisms, researchers can gain insights into the factors that influence population dynamics and the shape of the sigmoid growth curve.

C4.1.9—Competition versus cooperation in intraspecific relationships

A community is a group of populations of different species living together in an area and interacting with each other. These interactions can be complex and varied, including competition, predation, mutualism, and commensalism.

Coral reefs are an example of a complex community with many interactions between species. The coral polyps provide a habitat for a variety of organisms, including algae, fish, and invertebrates. The algae living within the coral polyps provide them with nutrients through photosynthesis. This mutualistic relationship benefits both the coral and the algae.

C4.1.10—A community as all of the interacting organisms in an ecosystem

Intraspecific Competition and Cooperation

Individuals within the same species often compete for resources like food, water, mates, or territory. This competition can lead to natural selection, favoring individuals with traits that allow them to outcompete others. Examples include competition for light among plants, competition for pollinators among flowering plants, competition for food among animals, and competition for breeding sites among animals.

However, cooperation can also occur within species. For example, social insects like ants and bees exhibit complex cooperative behaviors, such as division of labor and mutual defense. Cooperative breeding, where individuals help raise offspring that are not their own, is another example of cooperation within species.

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

Interspecific Relationships in Communities

Interspecific relationships refer to the interactions between individuals of different species within a community. These relationships can be categorized into several types:

1. Herbivory:

A primary consumer feeds on a producer.
Examples: Aphids feeding on plant sap, limpets grazing on algae

2. Predation:

One species (the predator) kills and eats another species (the prey).
Examples: Dingoes hunting kangaroos, starfish eating oysters

3. Interspecific Competition:

Two or more species compete for the same resource.
Examples: Barnacles competing for space on rocky shores, plants competing for light

4. Mutualism:

A relationship where both species benefit.
Examples: Nitrogen-fixing bacteria in plant roots, lichens, coral and zooxanthellae

5. Parasitism:

One species (the parasite) lives on or inside another species (the host), harming it.
Examples: Ticks feeding on deer blood, roundworms living in the intestines of animals

6. Pathogenicity:

One species (the pathogen) infects another species (the host), causing disease.
Examples: Potato blight fungus, tuberculosis bacterium

These interspecific relationships shape the structure and dynamics of communities, influencing species diversity, abundance, and distribution.

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

Mutualism is a type of interspecific relationship where both species benefit. In many cases, the two species are from different taxonomic kingdoms and have different capabilities, allowing them to provide different services.

Examples of Mutualism:

Root Nodules in Fabaceae: Legumes form a symbiotic relationship with nitrogen-fixing bacteria called Rhizobium. The bacteria convert atmospheric nitrogen into a form usable by the plant, while the plant provides the bacteria with a protected environment and carbohydrates.
Mycorrhizae in Orchidaceae: Orchids form a mutualistic relationship with fungi. The fungi help the orchid seedlings obtain nutrients, while the orchid provides the fungi with carbohydrates.

These examples demonstrate how mutualistic relationships can be essential for the survival and growth of both species involved.

C4.1.13—Resource competition between endemic and invasive species

When invasive species are introduced to new environments, they can outcompete native species for resources. This can lead to a decline in the populations of native species and even local extinction.

Key factors contributing to the success of invasive species:

Lack of natural enemies: Invasive species may not have natural predators or competitors in their new environment, allowing them to proliferate unchecked.
Rapid reproduction and dispersal: Many invasive species have high reproductive rates and efficient dispersal mechanisms, enabling them to spread quickly.
Competitive advantage: Invasive species may have traits that give them a competitive advantage over native species, such as efficient resource use or tolerance to environmental stressors.

Examples of invasive species that outcompete native species include:

Red lionfish: This invasive fish species has spread throughout the Caribbean and Atlantic, outcompeting native fish for food and habitat.
Water hyacinth: This aquatic plant can rapidly cover the surface of water bodies, blocking sunlight and reducing oxygen levels, harming native aquatic plants and animals.

To mitigate the negative impacts of invasive species, it is essential to prevent their introduction, control their spread, and promote the conservation of native species and their habitats.

C4.1.14—Tests for interspecific competition

Interspecific Competition: Occurs when two or more species compete for the same limited resource, such as food, water, or space.
Statistical Tests: Chi-squared tests can be used to determine if the distribution of species is independent or if there is evidence of competition.
Field Manipulation Experiments: Removing one species from a habitat can reveal the competitive effects on the remaining species.
Laboratory Experiments: Controlled laboratory experiments can be used to study competition under specific conditions.
Observational Studies: Long-term observations of natural systems can provide valuable insights into competition and other ecological interactions.

Additional Considerations:

Competitive Exclusion Principle: This principle states that two species cannot coexist indefinitely in the same niche. One species will eventually outcompete the other.
Resource Partitioning: Species can coexist by dividing resources, reducing competition. For example, different bird species may feed on insects from different parts of a tree.
Character Displacement: The evolution of differences in traits, such as beak size or shape, can reduce competition between species.

By understanding interspecific competition, we can better appreciate the complex dynamics of ecological communities and the factors that shape species distributions and abundance.

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

The chi-squared test is a statistical method used to determine if there is an association between two species. It compares observed and expected frequencies of species occurrences in quadrats. If the calculated chi-squared value is greater than the critical value, the null hypothesis of independence is rejected, suggesting an association between the two species. This test is useful for studying ecological interactions like competition and habitat preferences.

C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations

Predator-prey interactions are fundamental to ecological dynamics. When a predator population increases, it exerts pressure on the prey population, leading to a decline in prey numbers. As prey becomes scarce, the predator population also declines due to reduced food availability. This cyclical relationship, known as a predator-prey cycle, can be observed in many natural systems.

The predator-prey relationship between red foxes and mountain hares in Sweden. The graph shows fluctuations in the populations of both species over time. As the hare population increases, the fox population also increases due to abundant food. However, as the fox population grows, it exerts greater predation pressure on the hares, leading to a decline in their population. This, in turn, reduces the food supply for foxes, causing their population to decline. The cycle then repeats.

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

It illustrates the concept of top-down and bottom-up control in ecological communities.

Top-down control occurs when a higher trophic level regulates the populations of lower trophic levels. For example, an increase in predator numbers can lead to a decrease in herbivore populations, which in turn can affect the abundance of producers. This is often referred to as a “trophic cascade.”

Bottom-up control occurs when a lower trophic level influences the populations of higher trophic levels. For example, the availability of nutrients or resources can limit the abundance of producers, which in turn limits the populations of herbivores and predators.

Communities can be dominated by either top-down or bottom-up control, or a combination of both. The relative importance of these two control mechanisms can vary depending on the specific ecological context. Understanding these interactions is crucial for managing ecosystems and predicting the effects of disturbances, such as climate change or invasive species.

C4.1.18—Allelopathy and secretion of antibiotics

Allelopathy:

Definition: The secretion of chemicals by plants that inhibit the growth of other plants.
Mechanism: Allelopathic substances can interfere with germination, root growth, or nutrient uptake of neighboring plants.
Example: The tree of heaven (Ailanthus altissima) releases allelopathic chemicals that can inhibit the growth of other plants.

Antibiotic Production:

Definition: The production of antimicrobial substances by microorganisms.
Mechanism: Antibiotics target specific cellular processes in bacteria, such as cell wall synthesis or protein synthesis.
Example: Penicillium fungi produce penicillin, an antibiotic that inhibits bacterial cell wall synthesis.

Both allelopathy and antibiotic production are examples of chemical warfare between organisms, allowing them to compete for resources and reduce competition from other species. These strategies play important roles in shaping ecological communities and influencing the distribution and abundance of species.

IB DP Biology Populations and communities Study Notes | New Syllabus