Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 55.1 Physical Laws Govern Energy Flow and Chemical Cycling in Ecosystems
Ecosystems are dynamic systems in which energy flows one-way through food chains while chemical elements cycle repeatedly. These processes are governed by the laws of physics—especially the laws of thermodynamics and conservation of mass—and are mediated by organisms, particularly producers and decomposers.
1. Conservation of Energy and Mass
- First law of thermodynamics: Energy cannot be created or destroyed—only transformed (e.g., sunlight → chemical energy)
- Second law of thermodynamics: Every energy transfer increases entropy; some energy is lost as heat
- Energy flows through ecosystems and eventually exits as heat
- Mass is conserved and cycles; elements like carbon and nitrogen are recycled through ecosystems
2. Energy Flow in Ecosystems
- Energy enters as sunlight and is captured by autotrophs
- Flows through the food web: producers → consumers → decomposers
- Eventually lost as heat; ecosystems need continuous input from the sun
- Energy cannot be reused unlike matter
3. Chemical Cycling
- Elements move from abiotic reservoirs (air, soil, water) to organisms and back
- Example: carbon in CO₂ → plant → herbivore → decomposer → CO₂ again
- Decomposers recycle nutrients from all trophic levels
4. Ecosystem Structure
- Ecosystem = community + abiotic environment
- Can range from large systems (forests) to microcosms (under a log)
- Ecosystems are open systems: matter and energy flow in/out
- The biosphere is the largest ecosystem
5. Trophic Levels
- Primary producers: autotrophs (photosynthetic plants, chemosynthetic bacteria)
- Primary consumers: herbivores
- Secondary and tertiary consumers: carnivores
- Decomposers: fungi and prokaryotes that recycle nutrients from detritus
6. Net vs Gross Primary Production
- GPP (Gross Primary Production): total energy from photosynthesis
- NPP (Net Primary Production): GPP − energy used for respiration (Ra)
- NPP = GPP − Ra; this is the energy available to consumers (~50% of GPP)
- Measured in energy (J/m²·yr) or biomass (g/m²·yr)
- Forests may have high biomass but lower NPP than grasslands
In a Nutshell
Ecosystems are driven by the flow of energy and cycling of matter, both governed by physical laws. Energy enters as sunlight, flows through trophic levels, and exits as heat—requiring a constant external source. In contrast, chemical elements cycle through organisms and the environment, supported by decomposers and primary producers. Understanding these principles is fundamental to studying ecosystem productivity and sustainability.