1.2.5 Ecosystems: interactions, energy, and stability

Helpful prior knowledge and learning objectives:

Helpful prior learning:

Learning objectives:

Wolves have an essential role in ecosystems, but are often considered a threat to farmers’ animals like sheep. In many parts of the world, wolves have been hunted to extinction, causing instability in ecosystems. But when wolves were reintroduced to Yellowstone National Park in the United States in 1995 as part of a rewilding project, researchers found that they improved ecosystem stability and regeneration. The video below explains.

What is an ecosystem?

An ecosystem is groups of living organisms that interact with each other and their physical environment.

Ecologists divide the natural world into biotic (living) and abiotic (non-living) parts. In an ecosystem, organisms like plants and animals (biotic) interact with each other and the abiotic parts like sun and water. For example in a forest, trees use sunlight and water to grow and they provide food and shelter to insects and birds (Figure 1). An organism's survival relies on its relationships with other living organisms and on life-supporting abiotic conditions.

A bird feeding her young in a nest in a tree

Figure 1. A forest provides a habitat and food for birds, and the birds also support other organisms in the forest

(Credit: Jen Goellnitz CC BY-NC-ND 2.0)

Humans are also part of ecosystems, but unlike most other organisms, human activities often disrupt ecosystem balance because we take too much and give too little in return. However, like all other living organisms our survival depends on balance and stability in ecosystems.

In an ecosystem, energy and matter are transferred and transformed by living organisms. For example, carbon and heat are released as organisms burn energy in activities to meet their needs. Nutrients are passed from one organism to another through feeding relationships.

How do organisms interact in ecosystems?

In ecosystems, organisms interact in ways that impact their population size and habitats, as you saw in the video about wolves in Yellowstone National Park at the start of this section. Some examples of interactions include:

Bee pollinating a purple flower

Figure 2. Bees and flowers have a mutualistic relationship

(Credit: Jillian Butolph CC BY-NC-ND 2.0)

How does energy move through ecosystems?

Every living organism needs energy, mainly sourced from the sun. Plants, called autotrophs or primary producers, transform radiant energy into chemical energy for food using photosynthesis. They form the first trophic level in food chains, supporting all other life. A food chain illustrates one path of feeding relationships among organisms, like those in Yellowstone National Park (Figure 3).

Herbivores, animals like rabbits or deer that eat plants, are at the next trophic level. They are called primary consumers, because they consume (eat) plants. Then come secondary consumers, animals that consume (eat) the primary consumers. Tertiary consumers eat the secondary consumers, and quaternary consumers eat tertiary consumers. Carnivores consume other animals, while omnivores, like humans, eat both plants and animals.

A food web (Figure 4) shows more feeding relationships, where organisms may consume more than one type of food.

Food chain from Yellowstone National Park

Figure 3. A food chain from Yellowstone National Park

A food web from Yellowstone National Park

Figure 4. A food web from Yellowstone National Park

Energy is transferred at each step in the food chain, but with significant losses of up to 90% in the form of heat. This is why in an ecosystem there are usually more plants than herbivores, and more herbivores than carnivores. The energy loss between trophic levels also explains why scientists suggest that humans consume a plant-based diet. Plant-based diets are more energy efficient, because we get calories from plants that retain more useful energy from the sun.

Food webs show how the extinction of one species, often due to human actions, causes a trophic cascade affecting other populations at other trophic levels and even nonliving elements. For instance, removing wolves from Yellowstone National Park led to an increase in deer populations, which consumed more plants, and eventually altered river flows.

Humans are a part of many food webs, eating plants and animals at all trophic levels. We also use organisms for other purposes, such as trees for building, and rely on ecosystems for the air we breathe, the water we drink and the food we eat. However, the impact of our economic activities can be so great as to cause tipping points that can cause entire ecosystems to collapse and change state, which threatens all life on Earth. The Amazon Rainforest faces a tipping point risk from deforestation, which reduces the transpiration needed for cloud formation (Figure 5), changing global weather, and global food and water supplies.

Satellite image of clouds forming over the Amazon Rainforest

Figure 5. Satellite image of clouds forming over the Amazon Rainforest as the land warms, water evaporates off the leaves, and the rising and cooling air condenses to form clouds

(Credit: Nasa Earth Observatory Public Domain)

What factors affect ecosystem stability?

Ecosystems are stable when they are resilient, or able to recover after a disturbance. There are a number of factors that affect ecosystem stability or resilience.

The role of limiting factors in ecosystem stability

To stay in balance, ecosystems can only support a certain number of living things, known as their carrying capacity. Interactions between organisms and abiotic conditions help to keep the populations of organisms in balance. These factors that limit the size of populations of organisms are called limiting factors and include food, water, shelter, and space. If a population increases beyond carrying capacity, competition for scarce resources leads to a decrease in population size until balance is restored. 

These interactions typically involve balancing feedback loops (Section S.x), which counteract changes in an ecosystem. 

Figure 6 shows a balancing feedback loop, where if there is an increase in the mongoose population, there will be a decrease in the mouse population. The negative symbol by the arrow indicates an inverse relationship. Then, if the mouse population declines, this will also cause the mongoose population to decline. The positive symbol indicates a positive, or direct, relationship. Overall, the feedback loop keeps the two populations balanced, indicated with a B.

Balancing feedback loop between a mongoose and a mouse

Figure 6. Balancing feedback loop showing the relationship between predator (mongoose) and prey (mouse), a feeding relationship that limits both populations to maintain a stable ecosystem

Population growth of a group of organisms typically follows one of two trends over time:

J-curve population growth graph

Figure 7. J-curve exponential population growth, followed by a crash

S-curve population growth graph

Figure 8. S-curve population growth, resulting in balance with the ecosystem’s resources

Healthy ecosystems limit population growth through interactions among organisms and available abiotic resources. However, human populations have used energy-rich fossil fuels and machines to overcome limiting factors like food scarcity and disease (Section 1.2.3). Rising human populations and increased use of energy and material resources to meet human needs and wants cause human populations to exceed Earth’s carrying capacity.  

The question is whether we will see a J-curve crash in human population or achieve an S-curve balance with Earth’s life-support systems by reducing our negative impact and regenerating ecosystems.

The role of biodiversity in ecosystem stability

Someone pulling a piece out of a Jenga tower

Figure 9. If you remove pieces from a game of Jenga, the tower becomes unstable. The same is true of losing organisms in an ecosystem.

(Credit: Martin Sharman CC BY-NC-SA 2.0)

Biodiversity is the variety of life in an ecosystem. Biodiversity improves ecosystem stability or resilience in response to disturbances in a number of ways:

The short video below captures the importance of biodiversity.

Activity 1.2.5

Concept: Systems

Skills: Communication skills

Time: Varies depending on the option

Type: Individual, pairs or group, depending on the option

Option 1 - Understanding a trophic cascade with the wolf in Yellowstone National Park

20 min

Human populations have regarded wolves as an enemy for centuries. Wolves feed on animals that humans raise for food, such as sheep. So humans have often attempted to eliminate wolves from the ecosystem to protect populations of farmed animals. This can set off a trophic cascade with wide-ranging impacts on ecosystems.

Food web from Yellowstone National Park, minus the wolf

Figure 4. A food web from Yellowstone National Park, minus the wolf - what happens to the rest of the ecosystem?

Option 2 - Exploring trophic cascades with an interactive about sea otters, and other case studies

40+ min - time varies depending on how many case studies students examine

Biointeractive has an online interactive exercise that walks students through a classic trophic cascade triggered by the loss of sea otters from a kelp forest ecosystem. Students then test their understanding of trophic cascades in four other case studies, where they predict the relationships among different species and the consequences of ecosystem changes.

Option 3: Discussion about the conflict between the wolf and animal farmers, or other conflicts between megafauna and economic interests

Though the reintroduction of wolf populations into ecosystems has had a positive impact on ecosystem stability, they can also harm the economic interests of farmers raising domesticated animals. This is not the only example of such a conflict. You may know of one in your region that could be used instead, in which case you would need to find a few resources that outline the arguments of opposing stakeholder groups.

To what extent should the state support limiting the wolf numbers to protect the economic interests of animal farmers?

If you are using a different local example, the question should be adjusted accordingly.

Ideas for longer activities, deeper engagement, and projects are listed in Subtopic 1.5 Taking action

Checking for understanding

Further exploration


Daly, H., Farley, J. (2011). Ecological Economics (2nd ed.). Washington, D.C.: Island Press.

IUCN (n.d.). The benefits and risks of rewilding. https://www.iucn.org/resources/issues-brief/benefits-and-risks-rewilding.

Preshoff, K. (2015). Why is biodiversity so important? https://ed.ted.com/lessons/why-is-biodiversity-so-important-kim-preshoff.

Rutherford, J., Williams, G. (2015). Environmental Systems and Societies. Oxford: Oxford University Press.

Sustainable Human (2014, February 13). How wolves change rivers [Video]. YouTube. https://youtu.be/ysa5OBhXz-Q.

Terminology (in order of appearance)

Link to Quizlet interactive flashcards and terminology games for Section 1.2.5 Ecosystems: interactions, energy and stability

ecosystem: the interaction of groups of organisms with each other and their physical environment

extinction: the complete disappearance from the Earth of a species, a group of organisms that have similar characteristics and breed to produce fertile offspring

rewilding: protecting an environment and returning it to its natural state, passively by leaving it alone or actively by reintroducing native organisms that might have disappeared

regenerate: the process of restoring and revitalising something

organism: a living thing, such as an animal, a plant, a bacterium, or fungus

ecologist: a person who studies the interactions between organisms and their physical environment

biotic: living

abiotic: nonliving

energy: the ability to do work or cause change

matter: anything that takes up space and has mass

transfer: to move something from one place to another

transform: a change in the state, energy or chemical nature of something

predator-prey: an interaction where one organism (predator) eats another organism (prey)

mutualism: an interaction between organisms where both organisms benefit

competition: an interaction between organisms where they are each trying to get the same thing and only one may be successful

parasitism: an interaction between organisms where one organism benefits and another organism is harmed

autotroph (primary producer): an organism that makes its own food, like a plant

radiant energy: energy from electromagnetic waves

chemical energy: energy stored in the bonds of chemical compounds and released during chemical reactions

photosynthesis: the process by which green plants and some other organisms use sunlight to transform carbon dioxide and water into food

trophic level: organisms that share the sharing the same function in the food chain

food chain: a series of organisms, each one dependent on the one before it as food; shows the transfer and transformation of energy and matter through living organisms in an ecosystem

herbivore (primary consumer): an animal that eats plants or other autotrophs

secondary consumer: an animal that eats primary consumers

tertiary consumers: an animal that eats secondary consumers

quaternary consumer: an animal that eats tertiary consumers

carnivore: animals that eat other animals

omnivore: animals that eat both plants and animals

food web: a complex set of feeding relationships between organisms, with multiple connections between them; shows the transfer and transformation of energy and matter through living organisms in an ecosystem

trophic cascade: changes to an ecosystem when an organism is removed from the food web

tipping point: a condition where even a small further change can push a system into a different state

deforestation: removing a wide area of trees, often for farming, mining, or urbanisation

transpiration: the transformation of water from a plant's leaves, stem, or flowers to a gas

resilient: able to recover after a disturbance

carrying capacity: the maximum population size of organisms that an ecosystem can support

limiting factor: an abiotic or biotic factor that limits the size of a population of organisms

balancing feedback: a situation where feedback produces change in the opposite direction

J-curve: a situation where populations grow rapidly when resources are abundant, resembling a "J”; if the growth exceeds carrying capacity it can lead to abrupt population declines

disease: any harmful change from the normal structure or function of an organism, often caused by harmful bacteria or viruses

S-curve: a situation where populations grow rapidly, then growth slows and stabilises, forming an "S" shape as resources become more scarce, indicating a balance within the ecosystem

fossil fuel: a non renewable energy source including coal, oil, and natural gas, formed over millions of years in the Earth's crust from decomposed plants and animals

biodiversity: the variety of living organisms on Earth