Craters Life Web
Students learn about the complexity of life and interrelationships between living things by constructing a Craters food web.
- Students will be able to describe several relationships between living things at Craters.
- Students will be able to trace the flow of energy through a food web.
John Muir said, "When we try to pick anything out by itself, we find it hitched to everything else in the universe." For example, lichens accelerate the erosion of rock and tree trunks on which they grow, enriching nearby soil and improving growing conditions for syringa. Syringa creates shade and further enriches the soil when it dies and is decomposed by wind-dispersed fungi and bacteria. A host of insects colonize the crevice where the syringa grows. A spider eats the insects. A Clark's nutcracker hides a limber pine nut in the cool syringa crevice but is later caught by a Cooper's hawk. The pine nut germinates and finds the crevice to its liking and begins to grow, owing its future to all that came before it.
The story of life is about interrelationships, something of which we are largely ignorant. Our lack of ecological knowledge towers over what little we know of nature's ways. For example, the first step in understanding life is identifying the species and we don't know how many there are-not even to the nearest order of magnitude. The number could be could 10 or 100 million.
We do know that nothing lives in isolation. However, we often act as though life on Earth were a massive tool box, with many unnecessary minor nuts and bolts. In North America we once used the wonder pesticide DDT, but were ignorant of its effect on the rest of the food chain, where it caused egg shell thinning in predatory birds and tainted mothers' milk. Today, third world countries use DDT widely. How will long-term use of this and other pesticides affect the global environment? We condone the liquidation of ancient forests around the world by doing little to stop it. Is it mere poetry to refer to them as the lungs of the world and a treasure trove of biodiversity, or will their loss have catastrophic effects on Earth? How did the species that become extinct every day around the world fit into their ecosystem? Will their loss have far-reaching ecological effects or will it simply be an aesthetic loss? We don't know the answers to these questions - perhaps we can't know them, until it's too late.
To get a grasp of the complex interactions between living things, we can start by looking at a simple linear food chain: sun, dandelion, rabbit, and hawk. Sunlight is converted into chemical energy by green plants. Part of that energy is captured by herbivorous insects and vertebrates when they eat the plant. Carnivores then eat herbivores. In reality, however, the flow of energy through an ecosystem is more like a web. Species share energy back and forth in subtle ways. If we consider other relationships (i.e., plants that provide cover and nesting habitat to animals; insects, birds, and bats that pollinate flowers; rodents that disperse seeds; animals that require shade created by plants; and so on), our web approaches a symbol of what nature is really like.
- Ball of string 300-400 feet long
- Enclosed Craters species cards
What is the origin of all energy, the source of all food? The sun. Give the ball of string to the student who has the sun card. Now ask who needs the sun for its energy? There will be several kids with plant cards who should raise their hands. Plants use sunlight to make chemical energy (carbohydrates or simple sugars) through the process of photosynthesis. The sun holds onto the end of the string and passes the ball to the plant. Now who gets energy from the plant? It may be a rabbit or a deer that eats the plant, a hummingbird that sips its nectar, or a bee that collects its pollen. The plant holds onto the taught string that goes to the sun and passes the ball to the next person in the web. A predator might get its energy from the rabbit or hummingbird so the ball of string is passed to him or her. Note: To prevent tangling the string, make sure each student passes the ball over the top of the web.
When something dies it is reduced to its elemental form by decomposers like bacteria, fungi, and some insects. These decomposers help to enrich soil, which is critical to the needs of plants. The plants thrive and are then grazed and browsed by animals. When the animals and plants die, the decomposers complete the cycle.
If at some point you get stuck in making your food web-there's no clear relationship or your choices are limited-simply give the ball to someone that has already had it. They will hold two strands. Don't limit yourselves to what it says for each of the species; if they can think of some other relationship, use it. You could call it a "life web" and start talking about relationships other than food.
How do plants need insects? For pollination. How do swallows need dead trees? As places to nest. The ball should go in the direction where it is needed. Challenge your students to know what their species requires and speak up when the ball of string is at a species they need.
When everyone has at least one loop of string have the students carefully pull the web taught. While students are holding the web tell them that there are over 300 plant species and more than 175 amphibian, reptile, mammal, and bird species at Craters. There are thousands of species of insects, worms, and microscopic organisms. How many are represented here? Have we talked about all the possible relationships these organisms share? What if we had all the species with links representing all the relationships? It would require lots of string. The web might get so thick we couldn't see through it. Do we know about the many relationships between all the species? No way. What if we tug on one of the strings. In theory, each of you'd be able to feel it. Have one species become extinct by letting go of its string. What happens to the web? Have a few other species go extinct and see how the web is affected.
Before ending the activity you might want to read the following quote, attributed to Chief Seattle:
"This we know, the earth does not belong to man, man belongs to the earth. This we know, all things are connected like the blood that unites one family ... Man did not weave the web of life, he is merely a strand in it. Whatever he does to the web, he does to himself."
To avoid tangles, have everyone lean forward and gently place their strands on the ground. The person with the end of the line can wind up the string as you wrap up the activity.