Lab 8
Ecosystem Interactions

Learning Objectives
In this lab students will:
- Describe the roles of producers and consumers in a trophic system.
- Observe and make scientific observations regarding organisms in a virtual ecosystem.
- Construct simple food webs for different ecosystems.
- Construct trophic structures for the above ecosystems.


• Include both the living (biotic) and nonliving (abiotic) aspects of the environment.
Biotic factors include the availability of nutrients or food matter, and the constant threat of predation.
Abiotic factors organisms must contend with include temperature, carbon dioxide, nitrogen, water and oxygen levels.
• Most abiotic factors cycle through an ecosystem.
• Nutrients, like carbon, nitrogen, phosphorus and water are abiotic components that cycle through an ecosystem.
Energy is another important abiotic component of an ecosystem, but energy does not cycle through an ecosystem.
Energy passes though ecosystems
– It does not cycle.
– At each step, energy is picked up from the environment and passed from organism to organism.
• Some energy is lost to the system, usually in the form of heat.
• The Biotic components of ecosystems are the biological communities.
   – Populations of many species living together in a definable space.
   – They can be organized into trophic, or energy, levels.

Trophic Structure

• All living things require energy. Moving, growing, and reproducing all require energy.
• The energy for nearly all life is provided by the sun.
• Leaves of plants and trees, and other photosynthetic organisms harness the energy of the sun and convert it into sugars, which can be consumed by other organisms.
• The transfer of energy upward from the primary plant source to other organisms is called the trophic structure.
• Within this structure, each organism plays a specific important role.

Figure 8.1 shows a simple triangular shaped tropic structure. At the base are the producers including trees and other plants. Above them are the Primary consumers including a grasshopper, a rabbit and a field mouse. Above them are the secondary consumers including a snake and a rodent. At the top us the tertiary consumer and owl.

• The number of photosynthetic organisms in an ecosystem always greatly outnumbers the top predators.
• The photosynthetic organisms also always possess the most energy in the ecosystem.
• These photosynthetic organisms are the primary producers of an ecosystem, converting sunlight energy, carbon dioxide and water into organic molecules.
• Photosynthetic organisms, like plants produce food for all the other organisms in an ecosystem.
• Photosynthetic organisms are, in turn, consumed by primary consumers (herbivores).
• Herbivores feed only on the producers in an ecosystem.
• Herbivores may be large like an elephant, medium like a rabbit, or very small lake an aphid.
• Primary consumers are found in large numbers in an ecosystem because there is usually an abundance of food available to them.
Primary consumers are prey for all the consumers above them on the trophic structure. Therefore, they must reproduce quickly.
Secondary consumers (carnivore) feed on the herbivores: coyotes feed on rabbits, frogs feed on insects.
• Secondary consumers that eat both primary consumers and producers are called omnivores; bears eat berries and salmon.
Tertiary consumers feed on other carnivores; owls feed on frogs and toads , sharks feed on big fish.
• These predators have no natural enemies, besides humans, and therefore exist at the top of the trophic structure.
• Some ecosystems have additional levels;  quaternary consumers, quinary consumers - each consuming organisms in any of the levels below them.
Figure 7.1 shows how organisms are arranged .

Energy in the trophic levels

• Energy usually enters through green plants or algae via photosynthesis. These organisms trap energy from sunlight and store it in organic compounds.
• The total amount of energy trapped by all the plants and algae in a community is called the gross primary productivity.
• Some of this energy is used by the plants as they grow, produce flowers and seeds and do other things that plants do.
• That portion of the gross primary productivity that is stored by the plants in their tissues is called the community’s net primary productivity.
• This is the energy available to move to the next trophic level.
• The energy consumed by the primary consumer (herbivores) is the community's gross secondary productivity.
• Again some is used and some stored. The stored energy is the community's net secondary productivity.
• The net secondary productivity is the energy available to secondary consumers (carnivores).
• As a rule of thumb –only about 10% of the gross productivity of one trophic level gets passed on to the next level.
• So for every 100kcal of energy trapped by plants, only 10kcal passes on to the herbivores and only 1 kcal is passes on to carnivores.

Figure 8.2 shows an example of the energy in each tropic level. The large base shows the producers have 1,000 kcal of energy. Above them, the primary consumers and 100 kcal of energy. Above them the secondary consumers have 10 kcal of energy and at the top the tertiary consumers have 1 kcal of energy.

• There is a limit to the number of trophic levels an ecosystem can have.
• This is due to there being insufficient energy to support higher levels.
• Most terrestrial communities  can support four trophic levels.
• Highly productive aquatic and marine communities can support more than four trophic levels.
• Notice in Figure 7.2 the amount of energy available for organisms in that level is 10% of the energy available in the trophic level below them.
• 90% of the energy in a trophic level is either used by the organisms or is lost to the surroundings as heat.

Food Webs

Figure 8.3 shows a simple food web. At the bottom of the food web there is a small leafy plant with arrows leading form it and pointing to a grasshopper, a rabbit and a mouse, the tree has a single arrow leading from it and pointing to the mouse and a small flowering planst has arrows leading from it and pointing to the rabbit and the mouse. Above them in the food web is he grasshopper with a single arrow leading from it and pointing to the snake, the rabbit with lines leading from it and pointing to the snake and to the owl, and a mouse with three lines leading from it and pointing to the snake, the owl and the shrew. Above that is a snake with arrows leading from it and pointing to the owl, and the shrew with a single arrow leading from it to the owl. At the top of the food web sits the owl.

• To identify the flow of energy though an ecosystem it helps to know who eats who.
• As an organism is consumed the energy from that organism moves into the organism that consumed it.
• You get energy from the food you eat!
• One way ecologists illustrate the relationships between organisms in an ecosystem to to construct a complex food web.
Food webs show the feeding relationships between organisms in a particular environment.
• Note that in the food web the arrows point from the organism being consumed to the one that consumes it.
The arrows follow the flow of energy.
 • When you eat an apple energy flows from the apple into you. Therefore we would draw an arrow from the apple (the energy source) to you (the energy recipient).
Figure 7.3 shows a simple example of a food web.

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Non-majors College Biology Lab Manual © 2021 by Marie McGovern Ph.D. is licensed under CC BY-NC 4.0