In ecosystems, the basic principles are that energy flows in one direction and is lost as heat, while matter (carbon, nitrogen, water, and so on) cycles.
Basic Knowledge
Energy flow:
Producers convert solar-light energy into chemical energy (organic matter) through photosynthesis. This is transferred to consumers through food chains. However, at each stage, it is released as thermal energy through respiration and movement and is not reused. Energy flows in one direction through an ecosystem and leaves it.
Energy efficiency, the proportion transferred to the next trophic level, is considered to be about 10%, so usable energy sharply decreases at higher levels.
Matter cycling:
Carbon, nitrogen, oxygen, water, and other substances cycle between the environment and organisms in inorganic forms.
・Carbon cycle: atmospheric CO2 → photosynthesis → organic matter → returned to CO2 through respiration. Remains are decomposed by decomposers and returned to CO2. Atmospheric CO2 is rapidly increasing because of fossil-fuel combustion.
・Nitrogen cycle: atmospheric N2 → nitrogen-fixing bacteria (rhizobia, Azotobacter, and Nostoc) → ammonium ions → nitrate → plants → animals → decomposers → returned to N2 by denitrifying bacteria.
・Water cycle: evaporation, precipitation, and runoff are repeated.
Energy flow (energy flows in one direction through an ecosystem and is lost as heat)
Energy efficiency (the proportion of energy transferred to the next trophic level; about 10%)
Matter cycling (carbon, nitrogen, water, and other substances cycle between the environment and organisms)
Carbon cycle (CO2 cycles through photosynthesis and respiration)
Nitrogen fixation (the reaction that converts atmospheric N2 into ammonium ions and other substances)
Nitrogen-fixing bacteria (rhizobia, Azotobacter, Nostoc, and others)
Denitrifying bacteria (bacteria that return nitrate ions to N2)
Further Exploration
The law that energy efficiency is about 10% (Elton's 10% law) means that usable energy decreases sharply to 1/10, 1/100, and 1/1000 as trophic levels rise. Therefore, higher-level predators have fewer individuals, and conserving ecosystems requires large areas. This is why protecting habitats for whales and lions is meaningless unless it covers a broad region.
The nitrogen cycle is greatly affected by human activity. The invention of chemical nitrogen fixation through the Haber–Bosch process (developed in 1909) dramatically increased food production, but excessive fertilizer application causes eutrophication of rivers and oceans (including red tides and algal blooms discussed later). Plants absorb nitrogen through their roots as NO3- (nitrate ions) and NH4+ (ammonium ions), and use it to synthesize amino acids, nucleic acids, proteins, and other substances.
- Energy flows in one direction and is lost as heat
- Matter (carbon, nitrogen, and water) cycles
- Energy efficiency is about 10%
- Carbon cycling involves CO2 entering and leaving through photosynthesis and respiration
- Nitrogen-fixing bacteria include rhizobia, Azotobacter, and Nostoc
- Denitrifying bacteria return nitrogen to N2
- The Haber–Bosch process is artificial nitrogen fixation
Points to Note
① Energy does not cycle; it flows in one direction. Matter cycles. This contrast is especially important. ② Nitrogen fixation (N2→NH4+) and denitrification (NO3-→N2) are opposite reactions. ③ Rhizobia are nitrogen-fixing bacteria that live symbiotically in the roots of legumes. ④ The approximately 10% energy efficiency is an estimate and varies among ecosystems.
Practice
- Does energy cycle within an ecosystem, or does it flow in one direction?
- What bacteria live symbiotically in the roots of legumes and carry out nitrogen fixation?
- What is the industrial method of chemically fixing atmospheric nitrogen called?