OCR A2 Biology F215: Ecosystems

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·        Consist…of all the living organisms and all the non-living components in a specific habitat, and their interactions.

·        Are constantly changing – are dynamic.

·        Populations of individuals rise and fall, nutrient levels change.


·        Is…the place where an organism lives, including the non-living factors as well as the other species that live there.

·        Is occupied by a range of species which can be described as the community.

·        The species are adapted to live in a particular habitat e.g. earthworms in soil.


·        Is…all of the populations of different species who live in the same place at the same time and can interact with each other.


·        Is…the role an organism plays in a habitat.


·        Is…all of the organisms of one species that live in the same place at the same time and that can breed together.

Biotic factors

·        Are…the living components of the environment e.g. competition, disease, predation.

Abiotic factors

·        Are…the non-living components of the environment e.g. temperature, pH, light intensity.

Energy flow through an ecosystem

·        Energy enters an ecosystem in the form of sunlight – via photosynthesis.

·        Some of this energy is trapped in the form of organic carbon-based compounds e.g. glucose.

·        This energy is then transferred through via different trophic levels and food chains.


·        Are…organism that carry out photosynthesis.


·        Are…organisms that consume other organisms.

·        Primary consumers are usually herbivores, secondary/tertiary consumers are usually omnivores or carnivores.


·        Are…organisms that consume dead tissue and waste.

Productivity of ecosystems

·        Only a small percentage (less than 1%) of the solar energy falling on a plant becomes trapped in the biomass.

·        Gross Primary Productivity (GPP) is the energy trapped by producers in photosynthesis. This is the rate at which plants convert light energy into chemical energy (i.e. photosynthesis).

·        Net Primary Productivity (NPP) is the remainder of energy stored within the tissues of the plant as it grows.

GPP = NPP + Respiration

Why not all energy is used in photosynthesis

·        Some light is reflected from the surface of the leaf.

·        Only certain wavelengths of light are absorbed by chlorophyll.

·        Light may not strike a chloroplast.

·        Other limiting factor e.g. CO2.

Why consumers only trap a small proportion of their intake

·        Not all energy is absorbed – some remains in gut, some is lost in faeces e.g. cellulose cell walls.

·        Animals have a high level of activity (have high energy requirement for movement and physiology) so therefore respiration rate is high.

·        Mammals and bird are homiotherms so have to maintain a constant body temperature, so therefore respiration rate is high.

·        All of these processes need energy so a low % of energy is passed on to the next trophic level.


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