Following Energy Movement in Ecosystems
If you ran blindfolded through a forest in Ontario, you would not likely run into a moose or trip over a
raccoon. Instead, you would quickly hit a tree or trip over the plants tangled around your feet.
Why are there so many large trees and other plants and so few large animals? Why are you surrounded
by hundreds of hungry black flies and mosquitoes but not hundreds of birds or frogs ready to eat them?
The answers lie in the relationships between species in ecosystems. Each species is influenced and
limited by its surroundings and by the resources it requires. Think about which plants and animals are
abundant and which are rare in the ecosystem you live in.
Ecological Niches
 The function a species serves in its ecosystem, including what it eats, what eats it, and how it
behaves.
 No two species occupy identical niches.
 Ie. The niche of a black bear is as follows: black bears feed on tender plant parts such as nuts
and berries. They supplement their diet with insects and other small animals. Bears may carry
seeds long distances in their digestive systems before the seeds are expelled and germinate.
Bears go into hibernation during the winter. While they have few predators other than human
hunters, black bears are themselves fed on by many blood-feeding insects and are hosts to a
variety of parasites.
Food Chains and Food Webs
Food Chain
 The most common interactions between species are through feeding relationships.
 The easiest way to display this is through food chains
 A food chain is a sequence of organisms each feeding on the next, showing how energy is
transferred from one organism to another.
 In the example below the pine cone is the producer, eaten by the squirrel, who is eaten by the
weasel, who is eaten by the goshawk.
 All organisms continually use and release energy to their environment and therefore means energy
is continuously lost from all levels.
Food Web
 A more accurate, but still incomplete, way to illustrate interactions
 A representation of the feeding relationships within a community
Trophic level
 A way of categorizing living things according to how they gain their energy; the first trophic level
contains producers or autotrophs, and each higher level contains heterotrophs.
First trophic level
 Organisms can make their own food from basic nutrients and sunlight or some other non-living
energy source. (ie. Plants, algae, and some types of bacteria.)
Second trophic level
 Contains organisms that feed on the autotrophs.
 Referred to as primary consumers.
 Ie. mice, deer
Third trophic level
 Relies on primary consumers for their source of energy
 Still dependant on the autotrophs.
 Referred to as the secondary consumers
 Ie. wolves, eagles
Fourth trophic level
 secondary consumers rely on other secondary consumers.
 referred to as the tertiary consumer.
 ie. birds, owls
Heterotrophs
 Cannot make their own food, and must obtain food and energy from autotrophs and other
heterotrophs. ie human beings
Every organism within an ecosystem provides energy for other organisms.
Ecological Pyramids
 A representation of energy, numbers, or biomass relationships in ecosystems.
Pyramid of Energy
Pyramid of Numbers
Example
1. deer grazing on spruce tree
2. deer only eats the buds
3. not all of the bud is digested
4. some eliminated in the deer’s wastes (feces)
5. energy lost to heat during the chemical transformation
of digestion.
6. some energy is used to fuel the deer’s cells
7. energy is used to maintain body temperature
8. only 10% of the energy from the plant becomes
available for the wolf.
9. Same thing happens to the wolf as did to the deer with
the plant.
Pyramid of Biomass
How have humans changed the energy in ecosystems?

By increasing the food supply, improving health, and increasing the lifespan for humans.
Agriculture
 trees were cut down to make room for cultivation
 lumber used for homes.
 planting of crops
 domestication of animals
 allowed humans to stay in one place
Industry
 invention of technological devices to perform work
 requires more energy to run devices
 devices being made increased food production and improved health of humans.
 an increasing human population places greater demands on the ecosystem.
Questions
1. What is meant by ecological niche? Describe the ecological niche of humans and of three other
species.
2. Explain the differences between food chains and food webs.
3. Describe some possible impacts of adding or removing a species from an ecosystem.
4. As you go up from one trophic level to the next, the amount of available energy decreases. Explain
where the energy has gone.
5. What trophic level contains the greatest biomass in most ecosystems? Explain why this occurs.
6. What trophic levels are occupied by carnivores? Provide examples.
7. Bison, zebra, and kangaroos are three large mammals. Explain why they have similar ecological niches
but cannot be shown in the same food web.
8. a) How would the food web below change if the red fox were killed off by rabies?
b) What species would benefit?
c) What species might decline?