Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 55.2 Energy and Other Limiting Factors Control Primary Production in Ecosystems
Primary production—the foundation of energy flow in ecosystems—is controlled by the amount of light energy captured by autotrophs and the availability of limiting nutrients. These factors vary across aquatic and terrestrial systems, with climate change now playing an increasing role in altering productivity and carbon dynamics.
1. Energy Budget and Production
- Primary production: energy converted into organic compounds by autotrophs
- Gross primary production (GPP): total energy fixed by photosynthesis
- Net primary production (NPP): GPP − autotrophic respiration (Ra)
- NPP = energy available to consumers ("ecosystem take-home pay")
- Net ecosystem production (NEP) = GPP − total ecosystem respiration (RT)
- Includes respiration by decomposers and heterotrophs
- NEP > 0 = carbon sink; NEP < 0 = carbon source
2. Aquatic Ecosystems: Light and Nutrients
- Light penetration limits photosynthesis at depth (only 5–10% at 75 m)
- Nutrients (especially nitrogen and phosphorus) are main limiting factors
- Example: Long Island phytoplankton limited by nitrogen
- Eutrophication: nutrient overload → algal blooms → oxygen depletion → dead zones
- Iron is a limiting micronutrient in open oceans
- Upwelling zones bring nutrients → highly productive (e.g., Peru, California)
3. Freshwater and Eutrophication
- Phosphorus often limits lake productivity
- Fertilizer and sewage runoff → cyanobacteria blooms → oxygen loss → fish kills
- Mitigation: use phosphate-free detergents and reduce nutrient runoff
4. Terrestrial Ecosystems: Climate and Nutrients
- Temperature and moisture control global productivity patterns
- Tropical forests: warm, wet → high NPP; deserts/tundra → low NPP
- Solar energy drives evaporation and transpiration → fuels productivity
- NPP increases with precipitation
5. Nutrient Limitation on Land
- Nitrogen is the most common limiting nutrient globally
- Phosphorus limits growth in leached tropical soils
- Example: Arctic foxes disrupted seabird guano input → grasslands converted to tundra
- Fertilizer restored original vegetation
6. Plant Adaptations for Nutrient Uptake
- Nitrogen-fixing bacteria in symbiosis with plants
- Mycorrhizal fungi enhance phosphorus uptake
- Root hairs and enzymes (e.g., phosphatases) improve nutrient absorption
- Enable survival in nutrient-poor environments
7. Climate Change and NPP
- 1982–1999: Global NPP rose 6% (esp. tropics)
- Since 2000: Droughts decreased productivity
- Hotter droughts → wildfires, pests → tree mortality ↑, NPP ↓
- Carbon sink → carbon source: e.g., Alaska now emits more CO₂ than it absorbs
In a Nutshell
Primary production is shaped by energy input and nutrient availability, with strong variation between aquatic and terrestrial ecosystems. Climate, nutrient cycles, and biological interactions all influence how much energy is stored in biomass. As climate change alters these patterns, ecosystems may flip from storing carbon to releasing it—accelerating global warming in a dangerous feedback loop.