Lesson 10 - Communities and Interactions

Lesson Objectives

Content Objective: Demonstrate an understanding of the dynamics and relationships within ecological communities, including intraspecific and interspecific interactions, the impact of competition and cooperation, predator-prey relationships, and the role of top-down and bottom-up control.

Language Objective: Explain the concepts of community ecology, differentiate between different types of interspecific relationships, examine the effects of resource competition, analyze predator-prey dynamics, and discuss the role of allelopathy and antibiotic secretion in community interactions using IB command terms to understand the depth needed in an answer.

Syllabus Details: 

• C4.1.9—A community as all of the interacting organisms in an ecosystem - "Communities comprise all the populations in an area including plants, animals, fungi and bacteria.

• C4.1.10—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"

• C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as

categories of interspecific relationships within communities - "Include each type of ecological interaction using at least one example."

• 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."

• 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."

• C4.1.14—Tests for interspecific competition - "Interspecific competition is indicated but not proven if one species is more successful in the absence of another. Students should appreciate the range of possible approaches to research: laboratory experiments, field observations by random sampling and field manipulation by removal of one species" 

• 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."

• 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."

Activites

Activity 1  - Interspecific Relationships 

Copy this Table

Activity 2 - Predator-Prey Relationships

The reintroduction of wolves into Yellowstone National Park is often cited as a leading example of how apex predators can shape ecosystems through trophic cascades—a process that illustrates top-down control in action. In the early 20th century, wolves were eradicated from Yellowstone, leading to a series of ecological changes. Without this top predator, the populations of herbivores, particularly elk, grew substantially. These large ungulates grazed heavily on young willow and aspen trees, affecting the park’s vegetation and altering habitats for other species.

In 1995, wolves were reintroduced to Yellowstone, and their presence initiated a chain of events known as a trophic cascade:

• Predation Pressure: Wolves began preying on elk, which reduced the elk population and changed their grazing behavior. Elk became more wary and moved around more, avoiding certain areas where they could be easily hunted by wolves.

• Vegetation Recovery: With the reduced grazing pressure, willow and aspen groves began to recover. These areas provided food and habitat for beavers, whose activities create ponds and wetlands that support a diverse range of species.

• Ripple Effects: The resurgence of vegetation also helped stabilize riverbanks and alter stream flows. This benefited aquatic organisms and led to the return of birds and other wildlife that had declined in the absence of suitable habitat.

• Biodiversity Increase: The changes in habitat and the reduction of elk allowed for other species, such as pronghorn and bison, to thrive. The carcasses of wolf-killed animals provided food for scavengers, supporting a complex web of life.

By acting as a controlling force at the top of the food chain, wolves indirectly contributed to an increase in biodiversity and the restoration of ecological balance in Yellowstone. It's an instructive case of how the reintroduction or conservation of a key species can lead to large-scale environmental improvements.

Links to Videos:

Wolves in Yellowstone National Park
Other Effects of Wolves on the Yellowstone National Park Ecosystem

Activity 3  - Testing for Interspecific Competition

Task Overview: Design a concise experiment to investigate interspecific competition between two species using one of the specified techniques. Clearly define your research question, variables, and outline your method.

You will be assigned one of the following techniques:

• Laboratory Experiments

• Field Observations by Random Sampling

• Field Manipulation by Removal of One Species

Include in Your Design:

• Research Question: Formulate a question that your experiment will answer regarding the interspecific competition between two specific species.

• Dependent Variable(s): Identify what you will measure to assess the competition (e.g., growth rate, reproductive success, population density).

• Independent Variable: Determine the variable you will change to understand its effect on the competition (e.g., the number of species A individuals, presence/absence of species B).

• Control Variables: List at least three factors you will keep the same to ensure a fair test (e.g., soil quality, light conditions, water availability).

• Method Outline: Write a brief outline explaining how you will conduct the experiment, ensuring it is clear and replicable.

Example Outline:

Research Question: How does the presence of Species A affect the nutrient uptake of Species B in a shared soil environment as measured by the amount of nitrogen remaining in the soil after 30 days?

Dependent Variable: Nutrient uptake by Species B.

Independent Variable: Presence of Species A.

Control Variables:

• Soil composition and volume.

• Amount of water and frequency of watering.

• Light exposure duration and intensity.

Method Outline: In a controlled laboratory setting, plant Species B in pots with a uniform soil mix. Introduce Species A to half of these pots while keeping the other half as controls with only Species B. All pots should receive equal amounts of water, light, and be kept at the same temperature. After a growth period, measure the nutrient content of the soil and the biomass of Species B to assess the impact of interspecific competition on nutrient uptake.

Home Learning

A. Strengthen Your Skills

 B.  Expand Your Knowledge

Hundreds of Raindeer died. It changed the ecosystem