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Biology OCR A level Ecosystems Summary Notes
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Biology OCR A level Ecosystems Summary Notes
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1 of 6B23- Ecosystems
6.3.1 Ecosystems
(a) Ecosystems, which range in size, are dynamic and are influenced by both biotic and abiotic
factors.
Key words:
Ecology- name given to the study of the relationships between organisms and their
environments.
Ecosystem- made up of all the living organisms that interact with one another in a defined area,
and also the physical factors present in the region. They are dynamic.
Biotic factors- interactions between organisms that are living, or have once lived. Often involve
competition over food, territory and breeding patterns.
Abiotic factors- non-living or physical factors.
• The boundaries of a particular ecosystem being studied are defined by the person or team
carrying out the study.
• Example, individual habitats may be studied such as a rock pool or a large oak tree, or small
areas of land such as a playing field or a particular stretch of a river.
Biotic factors
• Interactions between organisms that are living, or have once lived.
• Often involve competition for food, territory and breeding partners.
Light- abiotic
• Greater availability of light, greater the success of plant species.
• Plants develop strategies to cope with different light intensities.
• Example- areas of low light may have larger leaves, develop photosynthetic pigments that
require less light, reproductive systems that operate only when light availability is at an
optimum.
Temperature- abiotic
• Has a greater effect on enzymes controlling metabolic reactions.
• Plants develop quicker in warmer temperatures, as will ectothermic animals.
• Endothermic animals control internal temperature and are less affected by the external
environment.
• Changes in temperature of an ecosystem can trigger migration or hibernation in animals. Leaf
fall, dormancy and flowering in plants.
Water availability- abiotic
• Lack of water leads to stress, if severe death.
• Lack of water, plants wilt, water required to keep cells turgid and plant upright.
• Required for photosynthesis.
• Xerophytes, developed to cope with water stress.
Oxygen availability- abiotic
• Aquatic ecosystems, better to have fast-flowing cold water, contains higher conc. O2.
• Water too warm or flow rate too slow, drop in O2 conc. can lead to suffocation of aquatic
organisms.
• Waterlogged soil, airspaces between soil particles are filled with water, reduces O2 availability.
(b) Biomass transfers through ecosystems.
Key words:
Food webs- systems of interlinked food chains, are diagrams that scientists use to show the
transfer of biomass and therefore energy, trough the organisms in an ecosystem.
Trophic levels- stage in a food chain occupied by a particular group of organisms.
Food chains- sequence of transfer of matter and energy in the form of food from organism to
organism.
Consumer- organisms that eat other organisms.
Producer- organisms that produce organic molecules using sunlight energy.
Habitat- place where an organism lives.
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Decomposer- organisms that break down dead or undigested organic material.
Biomass- the mass of living material present in a particular place or in particular organisms.
Important measure of the study of food chains and webs as it can be equated to energy content.
How biomass transfers between trophic levels can be measured
• First trophic level is a producer- converts light energy into chemical energy by photosynthesis.
• Next trophic levels are consumers- obtain energy by feeding on other organisms.
• Second trophic level is occupied by a primary consumer- animal that eats a primary consumer-
continues to occur normally until quaternary consumers, as there isn’t sufficient biomass and
stored energy left to support further organisms.
• Food chains can be presented as a pyramid of numbers, each level represents the number of
organisms at each trophic level. Pyramid, producers placed at bottom.
• Calculate biomass at each trophic level, you multiply the biomass present in each organism by
the total number of organisms in that trophic level.
• Easiest way is to measure the mass of fresh material present.
• Measured in grams per square metre for areas of land, or grams per cubic metre for areas of
water.
• Food webs show how plants and animals are connected in many ways to help them all survive
whereas food chains follow just one path of energy as animals find food.
Efficiency of biomass and energy transfer between trophic levels
• Efficiency= biomass transferred/ biomass intake x100
• Biomass in each trophic level is nearly always less than the trophic level below.
• Energy available at each trophic level is measured in kilojoules per metre squared per year to
allow for changes in photosynthetic production and consumer feeding patterns throughout the
year.
• As biomass is transferred between trophic levels, the energy contained is transferred. The
efficiency with which biomass or energy is transferred from one trophic level to the next is called
the ecological efficiency.
• Amount of biomass or energy converted to new biomass by each trophic level in a food chain
can be represented by a pyramid of energy.
Net production= gross production- respiratory losses
Efficiency at producer level
• Producers only convert 1-3% of the sunlight they receive into chemical energy
• Not all of the solar energy available is used for photosynthesis.
• Other factors may limit photosynthesis, e.g. water availability.
• Proportion of the energy is ‘lost’, as it is used for photosynthetic reactions.
• Efficiency affects consumer levels
• Consumers at each trophic level convert at most 10% of the biomass in their food to their own
organic tissue as not all of the biomass of an organism is eaten, some energy is transferred to
the environment as metabolic heat, some parts of an organism are eaten but are indigestible
and some energy is lost from the animal in excretory materials.
Human activities can manipulate biomass through ecosystems
• Humans depend on agriculture, which involves manipulating the environment to favour plant
species that we can eat and to rear animals for food or their produce.
• Humans would occupy the second, third or even fourth trophic level.
• Only a tiny proportion of the energy available at the start of the food chain is turned into
biomass for consumption at these third and fourth levels.
• Agriculture creates very simple food chains.
• Farming animals or animal produce for human consumption only have three trophic levels-
producers (animal feed), primary consumers (livestock), and secondary consumers (humans).
• Cultivating plants for human consumption has only two trophic levels- producers (crops) and
primary consumers (humans).
• Means that the minimum energy is lost since there are fewer trophic levels present in the natural
ecosystem. This ensures that as much energy as possible is transferred into biomass that can
be eaten by humans.
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