Ecosystems have the ability to recover even after experiencing a certain amount of disturbance. This is called resilience and supports ecosystem stability.
Basic Knowledge
Even when ecosystems experience external disturbances (natural: typhoons, wildfires, and floods; human-caused: logging, hunting, and pollution), they can return to their original state if the disturbance remains within a certain range. This ability is called resilience.
A representative example in rivers and oceans is natural purification. When a small amount of organic pollution enters, microorganisms decompose it and water quality recovers as the river flows downstream. The activated-sludge process used at sewage-treatment plants is an industrial strengthening of this natural-purification principle.
However, if disturbance is too great, recovery may be impossible and the ecosystem may shift to another state.
・Excess nitrogen and phosphorus entering a lake → eutrophication → algal blooms and red tides → death of fish and shellfish.
・Large-scale tropical-forest logging → conversion to grassland without forest regeneration (regime shift).
・Coral-reef bleaching → death → change to algae dominance.
Ecosystem balance is supported by biodiversity. Greater diversity tends to produce greater resilience.
Disturbance (a fluctuation imposed from outside an ecosystem, such as a typhoon, logging, or pollution)
Resilience (the ability to return to the original state after disturbance)
Natural purification (the purification of organic pollution by microbial decomposition in rivers and other environments)
Eutrophication (water-quality deterioration caused by excessive nitrogen and phosphorus)
Algal bloom (a phenomenon in which cyanobacteria proliferate in freshwater and turn the surface green)
Red tide (a phenomenon in which plankton proliferate in seawater and turn the surface reddish brown)
Biodiversity (the totality of species diversity, genetic diversity, and ecosystem diversity)
Further Exploration
When natural purification in a river is measured from the point where organic pollution enters to downstream areas, a characteristic pattern can be seen. Immediately after pollution enters, organic matter and NH4+ (ammonium) are high, while dissolved oxygen (DO) decreases sharply because it is consumed by decomposer respiration. Downstream, organic matter decomposes and changes into NO3- (nitrate), while oxygen is supplied from the atmosphere and recovers. Oxygen increases as algae (phytoplankton) grow, and fish return.
Eutrophication is caused by excessive inflow of nitrogen and phosphorus, especially phosphorus, which is often the limiting nutrient. Major causes include domestic wastewater, agricultural drainage, and livestock manure. Red tides and algal blooms in lakes cause massive deaths of fish and shellfish and severely damage fisheries. Lakes Kasumigaura, Suwa, and Biwa have experienced serious problems. Improvements have been made through the Water Pollution Control Law, enacted in 1996, and advanced treatment of domestic wastewater.
- Resilience = recovery from disturbance
- Natural purification = microbial decomposition of organic matter in rivers
- Downstream: organic matter↓, NH4+↓, NO3-↑, and DO recovery
- Eutrophication = excess nitrogen and phosphorus
- Algal blooms occur in freshwater; red tides occur in seawater
- Large-scale disturbance can exceed resilience and cause a regime shift
- Ecosystems with greater diversity tend to be more stable
Points to Note
① An algal bloom occurs in freshwater and involves cyanobacteria, while a red tide occurs in seawater and involves plankton. ② Eutrophication is caused by nitrogen and phosphorus; a small amount of phosphorus can have a particularly strong effect. ③ Natural purification has limits and cannot keep up with excessive pollution. ④ A regime shift is an irreversible change that is difficult to reverse once an ecosystem moves to another state.
Practice
- What is the ability of an ecosystem to return to its original state after disturbance called?
- What is the phenomenon in which cyanobacteria proliferate in a lake and turn the surface green called?
- Name the two elements that are the main causes of eutrophication.