Ecosystems (6.3.1)
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A habitat is the environment in which an organism lives characterised by biotic (living) and abiotic (non-living) components that influence its survival and reproduction.
A population is all of the organisms of one species, living in the same place at the same time.
A community consists of all the populations of different species living and interacting in the same habitat or ecosystem.
An ecosystem is a self-contained dynamic system influenced by the biotic (living) and abiotic (non-living) factors in an area.
Ecosystems range in size and constantly change over time due to shifts in environmental conditions, species interactions, and disturbances. The types of change that affect a population include:
- Cyclic changes: repeating changes in a rhythm, e.g., predator-prey relationships.
- Directional changes: last longer than an organism’s lifetime and go in one direction, e.g., coastline erosion.
- Erratic changes: no rhythm or constant direction, e.g., hurricane.
Biotic factors are all the living organisms within an ecosystem.
Biotic factors include plants (producers), animals (consumers), bacteria, fungi, and microorganisms (including decomposers).
Biotic components interact with each other through predation, competition, and symbioses/mutualism (mutually beneficial relationships).
Abiotic factors are the non-living components of an ecosystem that influence living organisms.
Abiotic factors include light, temperature, water, oxygen, , and nutrient levels.
Abiotic components affect the availability of resources and the conditions for survival, influencing the distribution and abundance of organisms.
Biomass (the total mass of living material in an area) decreases at each trophic level of a food chain. As a result, less biomass is available to support organisms at higher trophic levels. This is why most food chains only have four or five trophic levels.
Biomass is lost between trophic levels through:
- Respiration: energy is released and lost as heat.
- Excretion: waste products are not passed to the next trophic level.
- Heat loss: energy is lost to the surroundings during metabolic processes.
- Uneaten or indigestible material (bones, roots, etc.).
The efficiency of the energy transfer between trophic levels can be measured using the following equation:
This can also be written as:
Gross Primary Productivity is the total rate at which plants (producers) convert light energy into chemical energy through photosynthesis.
Photosynthesis is relatively inefficient as only a small percentage of the Sun’s light energy is absorbed and converted into chemical energy. Some of the chemical energy produced is used in respiration by the plant. The remainder is converted into biomass (e.g., proteins, lipids, cellulose).
Net Primary Productivity is the rate at which energy is stored as biomass and is available to the next trophic level in the food chain. This is a small percentage (8), as a lot of energy from the Sun has not been used to produce biomass.
Humans can increase primary productivity by modifying environmental factors to improve plant growth.
Examples include:
- Irrigation and the use of drought-resistant crop varieties.
- Planting crops earlier or using artificial lighting to increase the duration of photosynthesis.
- Using pesticides and fungicides.
- Crop rotation and the application of fertilisers to maintain soil fertility and provide essential mineral ions.
These practices increase the rate of photosynthesis and reduce losses resulting in more biomass being produced and a higher primary productivity.
As the efficiency of biomass between trophic levels is poor, humans can make changes to increase this efficiency.
These include:
- Selective breeding can increase the animals’ growth rate and feed conversion efficiency.
- Providing animals with food reduces the energy expended on grazing and finding food.
- Preventing and treating disease reduces energy loss from fighting infections.
These changes will reduce the energy loss between trophic levels and increase biomass production, which can be transferred to the next trophic level in the food chain.
Ecosystems recycle essential nutrients because they are finite and must be reused for life to continue.
Saprotrophs (fungi and some bacteria) obtain nutrients by secreting extracellular enzymes to break down dead organisms and organic waste. The products are then absorbed and used for respiration,growth, or biomass production.
Nutrient cycles such as the carbon cycle and the nitrogen cycle, rely on microorganisms to recycle carbon and nitrogen in ecosystems. This makes these nutrients available for reuse by other organisms.
The nitrogen cycle recycles nitrogen within an ecosystem. Nitrogen is essential for the synthesis of amino acids, proteins, nucleic acids and chlorophyll.
The nitrogen cycle involves several different types of bacteria and includes the following stages:
- Nitrogen fixation
- Ammonification
- Nitrification
- Denitrification.
During nitrogen fixation, atmospheric nitrogen gas () is converted to ammonium ions () by nitrogen-fixing bacteria such as Azotobacter or Rhizobium.
During ammonification, decomposers break down the organic nitrogen in dead organisms and waste releasing ammonium ions () into the soil.
Nitrification is when ammonium ions are oxidised to nitrite () by chemoautotrophic bacteria such as Nitrosomonas. Other bacteria, such as Nitrobacter, oxidise nitrite ions into nitrate ions (). These reactions require oxygen, so nitrification occurs in well-aerated soil.
Denitrification is carried out by denitrifying bacteria which convert nitrate ions back into nitrogen gas under anaerobic conditions, returning to the atmosphere.
The carbon cycle shows how carbon is recycled between the abiotic and biotic components of an ecosystem.
Carbon dioxide () in the atmosphere is taken up by plants and algae during photosynthesis to form biomass.
Carbon is transferred through food chains when organisms feed on plants or other animals. Respiration by plants, animals and microorganisms releases back into the atmosphere. Decomposers break down dead organisms and waste, also releasing via respiration.
Under certain conditions, dead organic matter can form fossil fuels. Carbon is also stored long-term in carbon sinks such as forests, oceans and carbonate rocks.
Primary succession occurs on newly exposed or bare land with no existing soil and no previous community.
Primary succession involves:
- Colonisation by pioneer species – the first organisms to colonise the barren environment are typically lichen and algae.
- Soil formation – pioneer species die and decompose, forming basic soil (humus). The weathering of rock adds minerals – plants like ferns and moss begin to populate the area, replacing the lichen and algae.
- Climax community – eventually, a stable, self-sustaining community develops, often including trees (depending on the climate). The species composition remains relatively constant until disturbed.
Deflected succession occurs when human activity prevents an ecosystem from reaching its natural climax community.
This is when a regular disturbance (e.g., grazing, mowing, burning, ploughing) interrupts succession and maintains the ecosystem at an earlier stage. This results in a stable but artificial community, maintained only by continued human management. If human intervention stops, succession will continue toward the natural climax community.
Examples include grassland maintained by grazing and heathland maintained by burning or grazing.
Ecologists take measurements and samples to estimate the abundance and the distribution of organisms within an ecosystem.
It is usually impractical to count every individual in a population, so ecologists use sampling techniques to obtain representative data.
The information collected can be used to investigate how biotic and abiotic factors, such as light intensity, temperature, or soil affect the distribution and abundance of a species.
Randomly placed quadrats are used to estimate the abundance of slow-moving or non-motile organisms (e.g., plants and some invertebrates). This method is most appropriate when organisms have a uniform or random distribution.
Quadrats are placed using random sampling (e.g., by generating random coordinates). The number of organisms in each quadrat is counted, and the mean number of organisms per quadrat is calculated. This value can then be scaled up to estimate the population size for the entire area.
Quadrats can be placed at regular intervals along a belt transect. This technique is used to investigate changes in species abundance across an environmental gradient (e.g., across a seashore or changes in light intensity or moisture).
Alternatively, a line transect can be used, taking note of organisms touching the line is recorded at regular intervals. This shows changes in distribution across an environmental gradient.
