1. Energy in Ecosystems

2. Definitions
Population: All of the members of the same species, living in the same ecosystem or habitat.
Community: The collection of all populations of all the species in an ecosystem or habitat.

3. Energy
All organisms need energy to live: to grow, to run, to reproduce, etc.
A major part of ecology is the study of how energy flows in ecosystems.
All energy on Earth originally comes from the Sun, but most of it is not used by organisms to create food:
30% is reflected back into space
44% heats the Earth’s surface and air
25% evaporates water
1% makes wind
0.023% is used for photosynthesis

4. Trophic Levels
Organisms can be classified into different trophic levels according to how they receive their energy.
Organisms that use the Sun’s energy to make their own food are on the 1st trophic level and are called Producers or Autotrophs (trees, algae, grass, etc.)
Organisms that eat producers are on the 2nd trophic level and are called Primary Consumers (aphids, caterpillars, rabbits, etc.).
Organisms that eat primary consumers are called Secondary Consumers and are on the 3rd trophic level (foxes, snakes, sparrows, etc.).
All consumers could also be called Heterotrophs.
The final organism in any food system is called the Top Carnivore or Apex Predator, and could be on any trophic level.

5. Energy Webs and Chains
There are 2 graphic ways to show which organism eats which other organism(s) and how they get their energy. They are the Food Chain and the Food Web.
Food Web Food Chain

6. Energy Webs and Chains Continued
A Food Chain shows one possible path for energy in an ecosystem. It starts with a producer and uses arrows to show who eats whom.
-Ex: Spruce Tree  Deer  Wolf
A Food Web shows ALL possible paths for energy. It is a more realistic representation since all food sources are shown.
Note that both graphics can be used to place different organisms on trophic levels. Stable ecosystems have complex food webs so that the removal of one organism does not cause the ecosystem to crash.

7. Trophic Levels
Trophic Level: The number of steps an organism is from the start of the food chain. Food chains start at trophic level 1 with primary producers such as plants, move to herbivores at level 2, predators at level 3 and typically finish with carnivores or apex predators at level 4 or 5.
Energy
Producer
Primary Consumer
Secondary Consumer
Tertiary Consumer
Apex Predator

8. Limits on Energy Transfer
In most ecosystems, there are usually not more than four or five trophic levels. Why? There is a limit to how many times energy can be transferred. Consider the food chain below:
Grass  Grasshopper  Frog  Heron
We know that the grass gets its energy from the Sun and the grasshopper gets its energy from the grass. However, not all the energy that the grass absorbed is available to the grasshopper. Most of the energy was used by the grass to produce important chemicals, to grow, produce heat, etc. The frog, in turn, gets even less of the original energy from that Sun.

9. Displaying Energy – Food Pyramids