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OCR A2 Biology
Module 3
Ecosystems & Sustainability
Energy Transfer…read more

Slide 2

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With a few exceptions, all ecosystems depend on solar energy as a primary energy source.
Primary productivity is the result of energy captured by producers. Some of this energy is lost when
producers, such as plants, use energy for respiration. Only a portion of the energy captured by
producers is passed on to consumers. Consumers also lose energy due to respiration. Note that the
energy flow through an ecosystem is one way; it is not recycled. All the energy taken in by producers is
ultimately lost as heat through respiration. Autotrophs must continue to capture the sun's energy for
ecosystems to persist.…read more

Slide 3

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The primary productivity in ecosystems is dependent upon sunlight, nutrients, and
water availability. Tropical rain forests have high productivity because of
abundant rainfall and sunlight. Estuaries and marshes have high productivity
because of high nutrient influx from rivers and streams.…read more

Slide 4

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The complicated feeding relationships that exist in ecosystems are called food webs. The concept of the food
web incorporates the fact that some species in the ecosystem eat more than one species or feed on both
plants and animals (omnivore). These diagrams can be misleading in that they emphasize herbivore and
carnivore species. In this forest ecosystem, only 1% of the producer biomass is consumed by herbivores.
99% ends up as dead material and goes into the detritus food web where it is broken down by a host of
detritivore and decomposer species.…read more

Slide 5

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All the energy used by living things comes ultimately from the sun. Energy enters living systems as a result
of photosynthesis by plants and some bacteria and protists. Less than 4% of the incident sunlight is
captured. More than half of the energy captured by plants is used in respiration for maintenance. Energy
used in respiration is lost as heat and therefore unavailable to other organisms. The other half is converted to
plant tissues. There are 2 types of organisms that have direct access to the energy in plant tissues,
herbivores, that feed on the plant while it is alive, and decomposers, that feed on the plant after it is dead. In
most ecosystems, the majority of the energy goes to the decomposers. In a grassland, for example, only
10% of the energy in plants is taken by grazing animals such as buffalo. Herbivores use almost all of their
energy intake on respiration for body maintenance; the rest goes to herbivore biomass. Much of the energy
in herbivore biomass is taken by carnivores, such as wolves, while some goes to decomposers. Almost all of
the energy taken in by carnivores goes to maintenance. The decomposers, which receive most of the plant
energy, use up over half of it in maintenance. The rest may be locked up in soil organic material or taken by
organisms that feed on decomposers. Ultimately, all of the energy captured by plants is transformed and lost
as heat; energy is not recycled.…read more

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Another way of examining the energy flow through an ecosystem is to construct an energy
pyramid. An energy pyramid shows the amount of energy contained in each trophic level
from the lowest, the producers, to the highest, the top carnivores. The amount of energy in
each level is represented by a volume and the volumes are stacked from the lowest trophic
level (producers) to the highest. As a general rule, only 10% of the energy in one level makes
it to the next highest level. The rest is lost as heat through respiration.…read more

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