pioneer community
primary communtiy: lichens are dispersed by wind → grows on rocks → send out fungal hyphae that secrets acid to break down the rocks → soil begins to form
secondary community: underground vegetative organs of plants are not burnt by the fire/ fast-growing plants with low nutrition requirements (such as ferns or grasses) grow
transitional community
death and decay of plants increase soil fertility
→ more animals are attracted to the community due to food and shelter → dead bodies and excretory products → decomposed → more organic matter
→ soil becomes suitable for plants to out-compete and overshadow each other → dominant species
Account for the upright pyramid of numbers
- Energy is lost along the food chain when energy is transferred from a lower trophic level to a higher trophic level
- The amount of energy available at higher trophic levels decrease progressively along the food chain and less energy is available to support organisms high up the food chain
- Organisms at higher trophic levels are larger in size and need more energy to maintain life
- A large number of individuals at lower trophic levels is needed to support those at higher trophic levels
Account for the inverted pyramid of numbers
- The biomass of an individual at lower trophic levels is MUCH larger than that of a higher trophic level
- Energy stored in the biomass of a smaller number of individuals at a lower trophic level is enough to support the individuals at a higher trophic level
- despite energy loss along food chains due to respiration and energy that is not consumed and not obtained due to assimilation
Explain the upright pyramid of biomass
Energy stored in biomass of organisms decrease up the trophic levels
● Some energy is lost at each trophic level as heat in respiration and energy stored in excretory wastes
● Some energy is lost between trophic levels as undigested or egested materials of __ and uneaten materials of __
Account for the inverted pyramid of biomass.
Occurs when organisms at a lower trophic level has a much higher reproduction rate and shorter lifespan (i.e. higher turnover rate) than organisms at the next trophic level
● At a particular point of time, the biomass of phytoplankton is small BUT the total amount of biomass produced by phytoplankton per year is much larger than that of zooplanktons
Account for the inverted pyramid of biomass.
Occurs when organisms at a lower trophic level has a much higher reproduction rate and shorter lifespan (i.e. higher turnover rate) than organisms at the next trophic level
● At a particular point of time, the biomass of phytoplankton is small BUT the total amount of biomass produced by phytoplankton per year is much larger than that of zooplanktons
Change in soil nitrogen content after a landslide
- Soil nitrogen content falls first
- After the landslide, the original vegetation/topsoil is removed
- Rapid growth of herbaceous plants absorb nitrogen from the soil - Soil nitrogen content rises later
- Some plant species have symbiotic nitrogen fixing bacteria in their root nodules, which enrich
the soil nitrogen content by fixing atmospheric nitrogen (1)
- As time went by, the dead mass of the new vegetation was decomposed by soil bacteria (1)
hence releasing nitrogen compounds back into the soil (1)
Change in soil nitrogen content after a landslide
- Soil nitrogen content falls first
- After the landslide, the original vegetation/topsoil is removed
- Rapid growth of herbaceous plants absorb nitrogen from the soil - Soil nitrogen content rises later
- Some plant species have symbiotic nitrogen fixing bacteria in their root nodules, which enrich
the soil nitrogen content by fixing atmospheric nitrogen (1)
- As time went by, the dead mass of the new vegetation was decomposed by soil bacteria (1)
hence releasing nitrogen compounds back into the soil (1)
Describe how symbiotic nitrogenr-fixing bacteria and the plant can cause an increase in the soil nitrogen content.
- The bacteria fixes atmospheric nitrogen into ammonium compounds
and supplies the nitrogenous compounds for the plant to synthesise proteins. - The decomposition of the plant after their death releases the fixed nitrogen to the soil.
- This increases the soil nitrogen content.
Explain why harvesting tomatoes may lead to depletion of soil nitrogen content (CE)
Tomatoes absorb nitrates in soil to synthesise proteins for growth. If they are harvested, the nitrogen in tomatoes will not be recycled to soil.
Role of producer (e.g. algae) in a simulated ecosystem
- Energy flow: Algae carries out photosynthesis to convert light energy to chemical energy which is stored in the form of organic matter. The energy stored in the algal biomass is then passed along the food chain by feeding
- Photosynthesis → absorbs CO2 and releases O2 for maintaining steady air composition
Explain why the interaction between crab (prey) and squid (predator) is on the whole beneficial to the population of crab.
Squid keeps the population size of crab in check and allows it to grow to a certain limit only. (1) The limited population size of crabs can reduce intraspecific competition for food and space among them (1) and also avoid the accumulation of waste products which will make the environment unsuitable for the survival of crab. (1)
Problems with invasive/exotic species
● They compete for food and other resources with the native species
● They may have no natural predators in the local habitat and thus may outcompete the native
species
● They may feed on other organisms and disturb the local food chains
● They may carry pathogens which spread diseases that do not exist in the habitat before
● Upset the existing ecological equilibrium in a habitat
E.g. in the long term, plants would fail to reach maturity for reproduction (1), which reduces the size of the plant community in the habitat. (1) this would reduce food availability/shelter for other animals. (1)
