2.5: Food Webs and Ecological Pyramids
pg. 42
Key Concepts:
3. Energy passes through ecosystems, whereas matter cycles within ecosystems.
Evidence of Learning: Students can …
- make and use food chains, food webs, and trophic levels to show feeding relationships
between organisms.
- describe how living organisms lose energy to the environment, and explain why higher
trophic levels have less energy than lower trophic levels.
- use an ecological pyramid to describe energy, number, and biomass relationships.
- Different species living in a forest is influenced and limited by its surrounding and the
resources available to them.
Ecological Niches
Ecological Niche: the function a species serves in its ecosystem. Including what it eats,
what eats it and how it behaves.
- The way species interact with each other and their environment is their ecological niche.
- The role in which an organism participates in its ecosystem is its niche, what it feeds on,
what eats it, and it behaviour.
- A key feature of any ecosystem is the feeding relationships between species.
- Terms, such as; producer, consume, herbivore, carnivore, and omnivore are used to
establish feeding relations hips.
Table 1: Types of Consumers
Feeding Role
Definition
Herbivore
Animal that eats plants or other producers.
Carnivore
Animal that eats other animals.
Omnivore
Animal that eats both plants and animals.
Scavenger
Animal that feeds on the remains of another organism.
Food Chains and Food Webs
Food Chain: a sequence of organisms, each feeding on the next showing how energy is
transferred from one organism to another.
Trophic level: the level of an organism in an ecosystem depending on its feeding position
along a food chain.
Food Web: a representation of the feeding relationships within a community.
- Easiest way to present a feeding a relationship is through a linear food chain.
- A food chain identifies who eats who as energy moves up the chain.
- Food chain always starts with a producer, then herbivore, and then a carnivore.
- Arrows are found between the organisms in the direction energy is moving up the
trophic level.
Figure 3: A simple food chain in a forest ecosystem.
- Organisms use and release energy into their environment.
- Energy is continuously lost into the environment as heat, a waste energy.
- Trophic levels or feeding levels identify the position an organism processes in the food
chain.
- Producers are found in the 1st. trophic level, herbivores (first order consumer) are found
in the 2nd trophic level. Carnivores (second, third, fourth order consumers) are found in
sequential trophic levels.
- Food chains arte artificial and are not found in nature, but are used to simplify feeding
relationships being discussed.
Figure 4: Species can be divided into trophic levels depending on how they obtain their
energy.
- Food webs are used to show many different feeding relationships within an ecosystem.
There are many paths expressed at once.
- The more complex the food web, the more sustainable of the ecosystem. If one species
is lost, there are still other organisms to feed from.
- adding or subtracting organisms from a food web can either increase or decrease
organism involved directly.
Figure 5: A partial food web of the boreal forest. A complete model of the interactions in
this ecosystem food web would show thousands of species including scavengers and
decomposers which feed on dead organisms and wastes. Note that the food chain from
figure 3 is embedded in this food web. What would happen of the lynx was removed
from the food web?
Figure 6: the invasive rusty crayfish (labeled in red) competes with native crayfish for
many of the same foods. It also feeds on the eggs of bass and pickerel. Large fish feed on
native crayfish but usually avoid eating the rusty crayfish.
Ecological Pyramids
Ecological Pyramid: a representation of energy, numbers, or biomass relationships in
ecosystems.
Biomass: the mass of living organisms in a given area.
- Pyramids display relationships between trophic levels.
- There are three different types of pyramids. (Energy, numbers, and Biomass)
- Energy pyramids show energy movement up the trophic levels, transfer and loss.
- Each layer or trophic level identifies the amount of energy available.
- Although energy is passed on from one organism to the next during feeding, a lot is lost
to the environment as thermal energy.
- The 10% rules is used to describe the amount of energy taken in by individuals at each
trophic level, from the previous level.
- Species found in the upper levels of a pyramid have less energy then these found in the
lower levels.
Figure 7: Only a small proportion of the total energy at any given trophic level is passed
on to the next level. Energy is used in biological processes such as growth and
reproduction and is lost as thermal energy (red arrows).
- Biomass pyramid identifies the total mass of all organisms in a given trophic level.
Figure 8: a) A simplified pyramid of numbers and b) a pyramid of biomass for a
deciduous forest ecosystem.
- Pyramid of Numbers identifies the number of individuals at a given trophic level.
BLM: 2.5-1: Try This Weaving a Food Web
Check Your Learning
Questions 1 – 8, page 47
Summary:
- Every species occupies a unique ecological niche.
- Feeding relationships between organisms can be represented by food chains, food webs,
and trophic levels.
- Energy is continuously being lost to the environment by all living organisms.
- Higher trophic levels always have less energy available to them.
- Ecological pyramids can be used to display energy, number, and biomass relationships.