Chapter 1. Trophic Levels

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You are part of an elite scientific unit tasked with terraforming Mars and making it hospitable for Earth-based life forms. To make life sustainable on this new planet, several different interrelated ecosystems will have to be created. Your job as Ecosystem designer will be to determine what organisms are best suited for a specific ecosystem, and how many of each are needed to make that ecosystem self-sustaining. You will set up and examine a simplified food chain first, then expand it to a much more realistic, interwoven food web. Lastly, you will examine the important role decomposers will play in the new Martian biosphere.

Are you ready to begin?

Background Info

Humans have always fantasized about moving to another planet. It is nearly impossible to imagine living on small, hot, Mercury or the gas giants like Jupiter, Saturn, Neptune, or Uranus. Venus has gravity similar to ours, but a runaway greenhouse effect has given it excruciatingly high atmospheric pressure and heat. That leaves Mars.

Mars is smaller and colder than Earth, with much lower gravity. But there are some things about it that look familiar. It has volcanoes and canyons, and it may be that water once flowed on its surface. In fact, there are some areas on Earth that are startlingly similar to Mars, particularly the permafrost of the Arctic and Antarctic. The polar ice caps of Mars seen in this photograph (left) are very similar to what we see at the poles of our own planet (right). Mars is the best candidate for human colonization.

[insert figure: This figure will be a comparison between Mars and Earth, showing the permafrost and the ice on both]

But it will take some work to make Mars habitable for us. Terraforming, or “Earth-shaping” means modifying a habitat to make it more like Earth. This term was invented by a science fiction writer in the 1940’s, but since then many scientists have debated its feasibility.

Your mission today is to contribute to the terraforming of Mars. Your role will be to design a simple ecosystem similar to one found on Earth. You will need to determine how to build a balanced, self-sustaining ecosystem from the ground up, including all the organisms—plants and animals—that live there.

What makes up an ecosystem? An ecosystem is a community of living organisms and the nonliving parts of their environment. Every ecosystem has certain essential components.

• Nonliving elements: Though not all parts of the ecosystem are alive, living organisms could not survive without all or most of these essential factors:

♦ An energy source. All living organisms need energy to live, so there must be a continuous source of energy entering the system. In almost all ecosystems, the energy source is the sun.

♦ Soil or some type of material to provide chemical nutrients

♦ Atmosphere to provide essential gases (e.g. oxygen gas for cellular respiration and carbon dioxide for photosynthesis)

• Living elements: All of the living organisms—the plants, animals, fungi, bacteria, etc.—in an area make up the community of that area. The organisms in the community can be divided into two types:

♦ Producers. The sunlight energy must be converted into chemical energy in the form of organic molecules. Organisms that can do this are called producers, because they are producing food and energy for themselves. They are also called autotrophs, which means “self-feeders.” These are the algae and land plants.

♦ Consumers. These are the organisms that must take in food to survive. The organic molecules that they consume are used for metabolism. They can also be called heterotrophs. “Hetero” means another, and “troph” is feeding, so the heterotrophs must feed on other organisms.

We can describe the consumers on the basis of what they eat.

• Primary consumers are herbivores, or plant eaters. They eat the producers in their ecosystem. Cattle, deer, and rabbits (grazing animals) are good examples.
• Secondary consumers are carnivores,or meat eaters. They eat the primary consumers. These can range from insects (that eat other insects) all the way to wolves (which eat rabbits and other primary consumers).
• Tertiary consumers eat the secondary consumers. Some of these are apex predators—nobody eats them! Orcas, lions, and eagles are in this group. Not all food chains have consumers at this level, but some do.
• Omnivores eat both plants and animals. Bears, raccoons, and humans are part of this group.
• Decomposers get their nutrients from consuming dead bodies or feces. After the consumers are finished with a meal, there are usually some body parts that they don’t eat. But these body parts contain organic molecules that could be used again. If these nutrients are not replenished, eventually the system will run out of these essential molecules. Decomposers break down dead bodies, consuming some of the molecules that they contain and releasing what they don’t need back into the ecosystem. Decomposers are the original recyclers. Some decomposers break down fecal matter because it also contains organic molecules that can be reused. These decomposers are called detritivores. The decomposers are a very diverse group, including bacteria, fungi, earthworms, and many insects.

How do the organisms interact in an ecosystem?

Let’s start with a food chain, which shows the way that energy flows through an ecosystem. Because food equals energy, a food chain is a description of who eats who in this system. No matter the ecosystem, a food chain always follows the same pattern:

External energy source → producers → primary consumers → secondary consumers

As we look at this, it really emphasizes that all life depends on the some energy source. Some environments, like the deep sea, don’t receive much light and have to look for alternative fuel sources, but for most ecosystems, the energy source is the sun. It also makes it clear that all consumers depend on producers, even the carnivores that never eat plants. Without the producers, there is nothing for the primary consumers to eat, and when the primary consumers starve, there is no food for the secondary consumers.

[insert figure: This figure will show a simple food chain including producers, primary consumers, secondary consumers, and decomposers.]

Trophic level refers to the position an organism occupies in a food chain. For example, all of the primary consumers are at the same trophic level. Each organism gets its energy from the organisms on the trophic level below them.

How much energy is actually transferred from one trophic level to the next? The answer is not much. First of all, an organism uses up some of the energy it takes in, so it can’t pass that energy to the next trophic level. Also, energy transfer is not 100% efficient. In every transfer of energy, some is lost, usually as heat. A commonly used rule of thumb is that about 10% of the energy stored in an organism is transferred to the next trophic level. This is why food chains are often diagrammed as a pyramid, with a very large number of producers at the base and a very small number of secondary consumers at the top.

This is something that you already knew. When we imagine an African landscape, for example, we picture gigantic herds of wildebeests and a small group of lions, not the other way around. But this issue of energy transfer is going to be an important one for today’s lab. In order for an ecosystem to be self-sustaining, there have to be enough producers to feed all of the consumers.

[inser figure: This figure will show an ecological pyramid.]

A food web is formed from many food chains woven together

A healthy ecosystem contains many different types of producers and consumers. Many consumers will eat more than one type of organism. For example, a rabbit will eat both grass and leaves, and a coyote will eat both mice and rabbits. Then we need to factor in the omnivores, which sometimes act as a primary consumer and sometimes as a secondary consumer. For example, a grizzly bear will eat both berries (plant, primary consumption) and salmon (animal, secondary consumption).

A food web shows how all of these different food chains interact: arrows connecting the organism and its food source show us just how complex these interactions can be. There is also less emphasis on trophic levels and more on the different paths of energy transfer. Food web diagrams rarely include every organism in the system—it would be almost impossible to make sense of such a figure. But even a simplified food web like the one you will be constructing today teaches us a lot about a particular ecosystem.

We can also use a food web to make predictions. For example, what will happen if a particular organism goes extinct? The food web shows us how interconnected all of the organisms in this ecosystem really are, and it can be surprisingly difficult to predict the exact effect of losing one species.

[insert figure: This figure will show a food web diagram. The desert ecosystem was chosen because it is not one that the students will be observing in the lab.]

Some background on today’s ecosystems

Earth is a biosphere—a planet that includes a variety of different ecosystems. We can’t reproduce all of them on the Martian biosphere, so instead we will focus on a few of them. In terrestrial systems, the temperature and the levels of precipitation are the most important nonliving components. In aquatic systems, rainfall is obviously not the most important issue. Instead, it is temperature and light levels that help to define the type of aquatic system.

1. Freshwater lake: this system is just what it sounds like. Most of the organisms live in the water. Examples are aquatic plants and algae, fish, tadpoles, and some insects. Other organisms, such as snakes, birds, and snails, are drawn to the area to feed on the animals and plants that live in the lake. In a wet environment, the decomposers flourish. Lakes are found on every continent, from the freezing cold of the Arctic and Antarctic to the heat of the tropics.
2. Savannah: this terrestrial system is a type of grassland with distinct rainy and dry seasons. Most of the producers are grasses, but there are also some trees. Many of the animals are grazers such as antelopes, giraffes, and elephants. Then they become prey for the secondary consumers like lions, leopards, and snakes. Decomposers are most active during the rainy season, slowing down during the dry season. The largest savannah is in Africa, but there are some savannahs in South America and Australia as well.
3. Tropical rainforest: this terrestrial ecosystem is warm and wet year-round. Huge trees, shrubs, and vines are abundant. Large numbers of insects, birds, monkeys, frogs, deer, snakes, and jaguars make this system their home. Because there is no really cold season, it is even possible for some consumers, like monkeys, to eat nothing but fruit year-round. And this is a great environment for decomposers, who in turn help provide a lot of nutrients to the fast-growing, large plants. While we tend to think first of the Amazon rainforest in South America, there are also rainforests in parts of Africa and Southeast Asia.
4. Temperate forest: temperate means moderate, and so this ecosystem is not as hot as a rainforest. It has four distinct seasons and plenty of rain. Northern forests may have mostly conifers like pines and redwoods, while further south there will be more hardwoods like maple and oaks. The trees provide food and shelter to many consumers. Squirrels, rabbits, and deer may be food sources for foxes, hawks, and wolves. The decomposers create a rich soil as they break down dead leaves and other plant matter as well as animal bodies. Temperate forests are common in Europe, North America, and China.
5. Marine: the seas are teeming with life, both large and small. The small phytoplankton and larger algae can sustain a great number of consumers. In warmer waters, the coral reef is home to many fish, mollusks, anemones, sea stars and sharks. Colder waters can be even more diverse, also supporting penguins, seals, and whales. Coral reefs must be in shallow water to capture sunlight, but there is a lot of life in the deep sea as well.

Now that you know more about ecosystems, it’s time to put this knowledge to work. It’s time to transform Mars!

Pre-lab Quiz

Before we can begin to design this experiment, you’ll have to understand the background material covering evolution and natural selection—how it works, what some of the possibilities are, what the supporting evidence is, etc. Make sure you've read the background material thoroughly, and then answer the following questions. You must get a score of 90% or better on these questions to proceed to the first experiment.

Figure 1.1: Desert food web

Experiment Notebook

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Lab Report

Vocabulary List

Apex predator: an animal that has no natural predators and is at the highest trophic level.

Autotroph: a self-feeder; an organism that makes its own organic molecules.

Carnivore: an animal that eats only meat.

Biosphere: All of the ecosystems that can be found on a given planet; i.e. all the livable areas and living organisms present on Earth (or any other planet).

Consumer: an organism that must take in organic molecules from an external food source; a heterotroph.

Decomposer: a bacterium or animal that uses dead bodies and feces as its nutrient source.

Ecosystem: a community of living organisms and their nonliving environment.

Food chain: a description of how energy flows through an ecosystem.

Food web: several food chains within the same ecosystem that interact.

Herbivore: an animal that eats only plants.

Heterotroph: an organism that feeds on other organisms.

Omnivore: an animal that eats both plants and meat.

Phytoplankton: microscopic producers that float near the surface in an aquatic environment.

Primary consumer: an animal that eats plants; an herbivore.

Producer: an organism that makes its own organic molecules; an autotroph.

Secondary consumer: an animal that eats primary consumers.

Terraforming: changing the surface of a planet to make it look more like Earth.

Tertiary consumer: an animal that eats secondary consumers.

Trophic levels: The divisions on a food chain—e.g. producers, primary consumers, secondary consumers, etc. Energy is transferred from one trophic level to another anytime one living organism consumers another.

Zooplankton: small consumers that float near the surface in an aquatic environment.