C4.2—Ecosystems as Open Systems

Open and Closed Systems in Ecosystems

Ecosystems are considered open systems because both energy and matter can enter and exit. Understanding the distinction between open and closed systems is essential for studying how ecosystems function.

• Open systems: Exchange both energy and matter with their surroundings. Ecosystems are open because sunlight (energy), water, minerals (matter), and organisms can enter or leave.

• Closed systems: Exchange energy but not matter with their surroundings. Earth is a closed system for matter but open for energy.

• Example: A terrarium is a closed system for matter but open for energy (light can enter).

C4.2.2—Sunlight as the Principal Source of Energy in Most Ecosystems

Role of Sunlight and Exceptions

Sunlight is the main energy source for most ecosystems, driving processes such as photosynthesis and supporting food webs. However, some ecosystems rely on alternative energy sources.

• Solar energy drives evaporation, transpiration, and other ecological phenomena.

• Primary producers (autotrophs) use sunlight to produce vital biomolecules.

• Energy flow in biomes is passed along the food chain by predation.

• Exceptions:

  • Deep sea hydrothermal vents: Energy comes from chemical reactions involving minerals, not sunlight. Chemosynthetic organisms (e.g., Archaebacteria) use energy from inorganic compounds.

  • Caves: Some cave ecosystems rely on high-energy minerals or organic matter washed in from outside.

C4.2.3—Flow of Chemical Energy Through Food Chains

Food Chains and Trophic Levels

Food chains represent the sequence of organisms through which nutrients and energy pass as one organism eats another. They are interconnected in food webs.

• Primary producers (1st trophic level): Use energy sources to produce biomolecules (e.g., plants, algae).

• Primary consumers (2nd trophic level): Feed on primary producers (herbivores).

• Secondary consumers (3rd trophic level): Feed on primary consumers (carnivores).

• Tertiary consumers (4th trophic level): Feed on secondary consumers.

• Example: Grass → Grasshopper → Frog → Snake → Hawk

C4.2.4—Construction of Food Chains and Food Webs

Representing Feeding Relationships

Food webs show the complex network of feeding relationships in a community. Arrows indicate the direction of energy and biomass transfer.

1. Identify the trophic level of each species.

2. Connect species with arrows showing energy flow (from prey to predator).

3. Draw food web with at least one food chain at a time.

C4.2.5—Supply of Energy to Decomposers

Role of Decomposers

Decomposers (e.g., bacteria, fungi) break down dead organisms and organic matter, recycling nutrients and providing energy for themselves and other organisms.

• Decomposers digest carbon compounds in organic matter from dead parts of organisms or feces.

• Example: Fungi decomposing fallen leaves, returning nutrients to the soil.

C4.2.6—Autotrophs and Chemotrophs

Energy Sources for Primary Producers

Autotrophs use external energy sources to synthesize carbon compounds. They are the primary producers in all ecosystems.

• Photoautotrophs: Use sunlight for photosynthesis to convert CO2 into organic molecules.

• Chemotrophs: Use inorganic compounds (e.g., H2S, FeS2) as energy sources to build complex molecules.

• Example: Sulfur bacteria at hydrothermal vents.

C4.2.7—Use of Light and Oxidation Reactions

Energy Conversion in Cells

Cells use light (in photoautotrophs) or oxidation reactions (in chemotrophs) to generate energy for life processes.

• Oxidation reactions release energy by transferring electrons from one atom to another.

• ATP is produced and used by both photoautotrophs and chemoautotrophs for carbon fixation.

• Example: Iron-oxidizing bacteria use Fe2+ as an electron donor.

C4.2.8—Heterotrophs

Energy from Organic Compounds

Heterotrophs obtain energy by consuming organic molecules produced by autotrophs. They digest and assimilate these compounds to build their own biomolecules.

• Complex carbon compounds (proteins, nucleic acids) are digested externally or internally and then assimilated.

• Example: Animals eating plants or other animals.

C4.2.9—Release of Energy in Cell Respiration

ATP Production

All organisms release energy from organic molecules through cell respiration, producing ATP to power cellular functions.

• Carbohydrates and lipids are oxidized to produce ATP.

• Energy is used for growth, movement, and other life processes.

C4.2.10—Trophic Levels

Position in Food Chains

The trophic level of an organism is its position in the food chain. Most organisms have varied diets and may occupy different trophic levels in different food chains.

• Example: An omnivore may be a primary consumer when eating plants and a secondary consumer when eating animals.

C4.2.11—Construction of Energy Pyramids

Representing Energy Transfer

Energy pyramids visually represent the flow of energy through trophic levels in an ecosystem.

• Energy pyramids: Show how much energy flows through each trophic level.

• Biomass pyramids: Show the total mass of organisms at each trophic level.

• Pyramids of numbers: Show the number of individuals at each trophic level.

C4.2.12—Reductions in Energy Availability

Energy Losses in Food Chains

Energy is lost at each trophic level due to several factors, resulting in less energy available for higher trophic levels.

1. Death without predation: Not all organisms are eaten; some die and decompose.

2. Incomplete consumption: Not all parts of organisms are consumed.

3. Metabolic heat: Energy is lost as heat during respiration and other metabolic processes.

4. Excretion: Energy is lost in waste products.

Only about 10% of energy is transferred to the next trophic level; the rest is lost as heat or waste.

C4.2.13—Heat Loss to the Environment

Energy Transfer Efficiency

Energy transfers are not 100% efficient. Heat is produced when ATP is used in cells, and this heat is lost to the environment.

• Most energy is lost as heat during metabolism.

• Energy cannot be recycled; it must be continually supplied (usually by the sun).

C4.2.15—Primary Production and Biomass Accumulation

Gross and Net Primary Productivity

Primary production is the accumulation of carbon compounds in biomass by autotrophs. It is measured as mass of carbon per unit area per unit time (e.g., g m-2 y-1).

• Gross primary productivity (GPP): Total biomass of carbon compounds synthesized per unit time.

• Net primary productivity (NPP): GPP minus the energy used for respiration by producers.

• Biomass accumulation: The process by which biomass increases as a result of growth and reproduction.

Equation:

C4.2.16—Secondary Production

Biomass Accumulation by Heterotrophs

Secondary production is the accumulation of carbon compounds in biomass by heterotrophs. It is always lower than primary production due to energy losses in the food chain.

• Most carbon compounds are converted to CO2 and water during respiration.

• Secondary production is the net gain of biomass by consumers over a period of time.

C4.2.17—The Carbon Cycle

Movement of Carbon Through the Biosphere

The carbon cycle is a biogeochemical cycle describing the movement of carbon through the atmosphere, biosphere, oceans, and geosphere.

• Plants absorb CO2 during photosynthesis; animals obtain carbon by eating plants.

• Decomposers break down dead material, releasing carbon back to the atmosphere or soil.

• Fossilization stores carbon in sediments for millions of years.

• Human activities (e.g., burning fossil fuels) release stored carbon rapidly.

C4.2.18—Ecosystems as Carbon Sinks and Carbon Sources

Balance of Carbon Uptake and Release

Ecosystems can act as carbon sinks (net uptake of CO2) or carbon sources (net release of CO2), depending on the balance between photosynthesis and respiration.

• Carbon sink: Stores more carbon than it releases (e.g., forests, oceans).

• Carbon source: Releases more carbon than it stores (e.g., animal respiration, combustion).

• Example: A growing forest is a carbon sink; a burning forest is a carbon source.