Trophic Levels

Ecological Stability in Trophic Levels

B.13B

Energy enters ecosystems from the Sun. During photosynthesis, plants change sunlight into chemical energy (stored in glucose).

​

Ecosystems need energy

• Energy flows one way in ecosystems. It’s always needed from an outside source (like the Sun) because it leaves as heat during metabolism.

• Since energy leaves as heat, ecosystems need a constant energy supply to keep going.

​

Energy flows in one direction

• Matter moves through the ecosystem as organisms interact (e.g., when one eats another), these are called trophic interactions.

• Matter is conserved (it doesn’t disappear), but energy can be gained or lost.

​

​Matter also moves through ecosystems

• Primary producers (like plants) start the cycling of matter in an ecosystem. They take sunlight and carbon from the air to make glucose (a type of sugar) that stores energy.

​Matter and Energy Cycle

When organisms die, their chemical elements (like carbon and nitrogen) are recycled. Decomposition breaks them down and returns them to the soil, where plants can absorb them and use them for things like photosynthesis.

​

Matter and Energy Cycle

​Decomposers

​These organisms make their own food using light or inorganic substances (like minerals). They turn this into chemical energy. The amount of food producers make shows how stable the ecosystem is.

Autotrophs (producers)

​These organisms get their food by eating other living things.

Heterotrophs

​These organisms break down nonliving organic material, like dead animals and waste. They get nutrients from things that are no longer alive and work at all trophic levels.

Decomposers

​Trophic interactions in an ecosystem happen between three types of organisms, based on how they get their food:

​The 10% Rule explains the energy available to each trophic

level in an energy pyramid. While inexact, this rule of thumb

suggests that at each trophic level, 90% of the available energy

is used by the organisms’ metabolism or is lost as heat. This

leaves only 10% of the original energy available for the next

trophic level.

10% Rule

In an ecosystem, matter is transferred from one organism to another as food. Trophic levels are different feeding levels and represent an organism’s position in a food web and food pyramid. At the bottom of the pyramid is level one, the largest trophic level comprising producers. Producers are organisms that use sunlight or inorganic molecules to produce food.

Multiple Choice

What is the main role of producers in an ecosystem?

• They consume other organisms for energy.

• They transfer matter and energy between trophic levels.

• They produce food using sunlight or inorganic molecules.

• They break down dead organisms and recycle nutrients.

Trophic level two comprises primary consumers, which eat organisms from trophic level one. Trophic level three is composed of secondary consumers, which eat organisms in level two and soon. Food is energy. The energy flowing up the pyramid is reduced by 90% from level to level. The energy from food is used for essential chemical reactions, and some energy is lost as heat.

Multiple Choice

How does energy change as it moves up the trophic levels in a food pyramid?

• Energy increases by 90% at each level.

• Energy stays the same at each level.

• Energy is reduced by 90% from one level to the next.

• Energy is only used for chemical reactions, not lost as heat.

All participants in the food web are connected to decomposers, such as fungi and bacteria, which chemically break down dead organisms into simpler forms. Detritivores, such as worms, millipedes, and dung flies, eat this simple food source and excrete the waste as nutrients back into the environment.

Multiple Choice

After decomposers break down dead organisms, what happens to those nutrients?

• they are destroyed

• they are returned to the environment to be used by producers

• they are returned to the environment to be used by consumers

• they are used by the decomposers

Because different species are connected in food webs, changes in population sizes at one trophic level influence the cycle of matter and energy flow at other levels. The number of trophic levels an ecosystem can sustain is determined by its size, the productivity of primary producers, and the frequency of disturbances to the ecosystem.

Multiple Choice

What factors determine the number of trophic levels an ecosystem can support?

• The size of the ecosystem, the productivity of primary producers, and the frequency of disturbances.

• The number of consumers in the ecosystem and the amount of sunlight available.

• The amount of energy at the top trophic level and the species diversity.

• The number of decomposers and the size of the primary consumer population.

Disruptions in the cycling of matter and the flow of energy affect ecosystem stability. Natural disasters, such as floods, droughts, human activities, and global climate change, may cause disruptions.

Multiple Choice

Which of the following can disrupt the cycling of matter and the flow of energy in an ecosystem?

• Only human activities

• Only natural disasters like floods and droughts

• Natural disasters, human activities, and global climate change

• The movement of energy between trophic levels

Ecosystem stability is measured by the biomass of primary producers and by species diversity. Generally, ecosystems with high productivity and species diversity are better able to recover from disturbances.

Multiple Choice

Why do ecosystems with high productivity and species diversity tend to recover better from disturbances?

• High productivity leads to fewer species interactions, which in turn can benefit each species when there is less competition for limited resources.

• Species diversity increases competition, hindering recovery from disruptions like natural disasters and human interaction.

• Ecosystems with low productivity are more resilient due to not expending uneccessary energy.

• Ecosystems with high productivity and species diversity recover better from disturbances due to resource abundance and resilience from species redundancy.