2.2

The definition of energy in ecosystems

The ability to do work. In ecosystems, most organisms rely on light or chemical energy. Heat is another form of energy but has a limited capacity to do work.

How energy moves and changes in an ecosystem

Energy flows from one location to another (e.g., food chains) and often transforms between forms.

• Example: Light energy to chemical energy (photosynthesis)

• Example: Light energy to heat (greenhouse effect)

The greenhouse effect

Solar energy warms Earth's surface, and greenhouse gases (CO₂, CH₄, water vapor) trap some re-emitted heat. Without this, Earth's temperature would be about –18°C instead of +15°C.

The two types of matter in an ecosystem

• Organic: Carbon-based nutrients that are the building blocks of cells

• Inorganic: Water, carbon dioxide, nitrate, ammonia, etc.

How matter cycles and transforms in an ecosystem

Continuously, converted into new forms through chemical reactions.

• Example: Photosynthesis converts CO₂ and water into glucose and oxygen.

• Example: Cellular respiration breaks down glucose to release ATP, CO₂, and water.

The 1st law of thermodynamics in ecosystems

Energy can be transformed from one form to another but cannot be created or destroyed.

The organisms that perform photosynthesis

Autotrophs, such as:

• Plants

• Algae

• Photosynthetic bacteria

How light energy is converted into chemical energy

Through photosynthesis, using sunlight to convert CO₂ and water into glucose and oxygen.

The role of glucose in photosynthesis

To be, in part, stored as biomass, while some is converted into proteins, fatty acids, waxes, and nucleic acids.

The definition of biomass

The mass of dry organic matter of an organism/population in a given area, usually measured in g m⁻².

• Only the dry mass is measured because water is inorganic.

How biomass is converted into energy value

1g of the substance can be burned and then, based on how much heat was released, be converted to its value in kilojoules (KJ).

• Example: Burning plant material releases heat, which can be measured.

The process of cellular respiration

Occurs in all organisms and releases energy from glucose to produce ATP for cellular processes.

Energy transformations are inefficient, with most lost as heat, especially in cellular respiration.

This heat cannot be converted back into usable energy, making energy transfer less efficient at each trophic level.

The biggest losses of energy in ecosystems

• Only some energy in glucose is converted into ATP.

• Most energy is lost as heat or waste (e.g., feces).

• Example: In aerobic respiration, about 40% of glucose energy is stored as ATP, while 60% is lost as heat.

The function of food chains

To show the flow of matter and energy between organisms.

The structure of a food chain

Each trophic level represents an organism's position in the chain.

Example:

• Grass (Producer)

• Rabbit (Primary Consumer)

• Fox (Secondary Consumer)

The primary level of food chains

Producers (autotrophs) always occupy the first trophic level and transform sunlight into chemical energy.

The function of consumers in ecosystems

They transfer energy through trophic levels, regulate populations, and help cycle nutrients in ecosystems. They are heterotrophs (not autotrophs that can produce their own energy), obtaining energy by CONSUMING other organisms.

Herbivores

Animals that consume plants to obtain energy.

• Example: Deer eating grass

Detritivores

Animals that consume dead organic material and break it down into smaller particles.

• Example: Earthworms decomposing organic matter

Predators

Animals that hunt, kill, and consume other animals for energy.

• Example: Lions hunting zebras

Parasites

Organisms that live on or inside a host and derive nutrients at the host’s expense.

• Example: Tapeworms in mammals

Saprotrophs

Organisms that break down dead organic material by secreting enzymes and absorbing nutrients.

• Example: Fungi decomposing material

Scavengers

Animals that consume the remains of dead organisms but do not kill them.

• Example: Vultures feeding on carcasses

Decomposers

Microorganisms that break down dead organic material into simpler substances, recycling nutrients.

• Example: Bacteria breaking down waste

The measurement of productivity in ecosystems

The amount of biomass gained per unit area over time, measured in kJ m⁻² year⁻¹ or g m⁻² year⁻¹. The higher this amount, the more energy is available to support trophic levels.

How productivity varies by ecosystem

• High Productivity: Tropical rainforests (due to sunlight and moisture)

• Low Productivity: Deserts (due to water limitations)

The difference between gross and net productivity

• Gross productivity (GP): Total biomass gained.

• Net productivity (NP): Biomass remaining after respiration (NP = GP - R).

• Example: A plant produces 100 units of biomass; after using 20 units for respiration, 80 units remain as NP.

The definition of maximum sustainable yield (MSY)

The largest yield that can be taken from a population without reducing future availability and sustainability. Exceeding the largest yield can lead to population decline.

An example of MSY in fisheries

• A fish population produces 100 biomass units per year.

• Harvesting 80 units leaves 120 next year → Sustainable

• Harvesting 100 units leaves 100 next year → Sustainable

• Harvesting 120 units leaves <80 next year → Unsustainable

The function of food webs

To show complex trophic interactions, making ecosystems more resilient.

An example of a marine food chain

Phytoplankton → Zooplankton → Small Fish → Large Fish → Seabirds/Marine Mammals

The function of a pyramid of numbers

A diagram showing the number of organisms at each trophic level.

• Example: 1 tree (low count, high biomass) vs. many insects (high count, low biomass).

The function of a pyramid of biomass

Shows the biomass (total dry mass) at each trophic level. More accurate than a pyramid of numbers.

The function of a pyramid of energy

Shows the rate of energy flow at each trophic level over time.

• Example: Solar input can be included.