A2/A-level Biology - Ecosystem (1)

• Ecosystems are dynamic systems because the population rises and falls due to the interactions of living organisms between each other and with the physical environment.

• Any small changes in one thing can affect the others.

• For example, if a predator's population size goes up, the population size of the prey will go down.

• The effects of living organisms, e.g. food supply, predation, disease, competition.

• The effects of non-living components, e.g. temperature, pH of soil, soil type, light intensity, oxygen concentrations, carbon dioxide concentrations.

• An organism that converts light energy to chemical energy /converts inorganic molecules to organic molecules - autotroph.

• An organism that receives energy by feeding on other organisms - heterotroph.

• An organism that feeds on dead organic material releasing molecules, minerals and energy for other living organisms.

• The position at which an organism is at in a food chain or web.

• Food chains show how energy is transferred from one organism to another.
• Different food chains join together to make a food web, which helps us understand how energy flows through the whole ecosystem.
• The arrows in a food chain show the direction of energy transfer.
• Energy is lost at each trophic level and is unavailable to the next trophic level.
• Energy is used for respiration which is lost through heat energy.
• The energy is stored in dead organisms and waste material which can only be accessed by decomposers.
• Because of this, there is less living tissue (biomass) at higher levels of a food chain.

When it comes to A2/A-level Biology, do you know how energy transfers between trophic levels can be measured?

• Each bar within the pyramid is proportional to the dry mass of all organisms at that trophic level.
• All the organisms are collected within that trophic level and are put into an oven at 80oC until all the water is evaporated out of the organisms.
• This method is very destructive so we tend to measure organisms' wet mass and work out its dry mass from that value.

• Different organisms may release different amounts of energy per unit mass.
• Ecologists may prefer using pyramids of energy which involves burning the organisms in a calorimeter and working out how much energy is released per gram, through calculating the temperature rise of a known mass of water.
• This is also destructive and time consuming, so ecologists revert to using pyramids of biomass instead.
• They also only take a snapshot of an ecosystem at one moment in time.
• Since population size fluctuate over time, this provide a distorted idea of the efficiency of energy transfer.

• Productivity is the rate at which energy passes through each trophic level.
• It gives an idea of how much energy is available to the organisms at a particular trophic level, per unit area (m2) in a given amount of time (1 year).
• Primary productivity is the name for the productivity of the plants.
• Gross primary productivity is the rate that plants convert light energy into chemical energy.
• However, as energy is lost when plants respires, less energy is available to the primary consumer.
• Net primary productivity (NPP) is the remaining energy after a plant has respired.

Do you how human activities can manipulate the flow of energy through ecosystems in A2/A-level Biology?

• Plant crops earlier in the season to allow them to harvest more light = longer growing season.
• Producing drought-resistant strains of crops and irrigating crops in countries where lack of water is common.
• Greenhouses are used to grow plants in warmer temperatures, increasing NPP.
• Crop rotation (growing a different crop in each field on a rotational cycle) ensures that there is not a lack of available nutrients - nitrogen-fixing crops, like peas or beans, replenishes levels of nitrates.
• Using pesticides to protect against pests - pests remove biomass and stored energy from the food chain, and lower the yield.
• Using fungicides to protect against fungal infection - root rot (reducing water absorption), damage xylem vessels (interfere with water transport), damage foliage through wilt, blight or spotting (interfere with photosynthesis directly), damage phloem tubes (interfere with translocation of sugars), or damage flowers and fruits (interfere with reproduction).
• Using herbicides to protect against weeds, which would compete for light, water and nutrients.

• Harvesting animals just before adulthood minimises loss of energy from the food chain.
• Treating animals with steroids make them grow quicker, increasing the proportion of energy allocated to growth.
• Selective breeding has been used to produce breeds with faster growth rates, increased egg production and increased milk production.
• Treating animals with antibiotics to avoid unnecessary loss of energy to pathogens and parasites.
• Zero grazing for pig and cattle farming maximise energy allocated to muscle (meat) production by stopping the animal from moving about, by supplying food for them and keeping the temperature constant.

In part 2, you will learn more about succession and the role of decomposers.

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Drafted by Bonnie (Biology)