Using Microcosms as an Introduction to Ecology
I recently was given the opportunity to teach one of the monthly science classes for homeschoolers at my local library. Yesterday, we went over ecology, the chemistry of life, and the scientific method - and each student got to take home two of their own microcosms! Here's a rundown of the presentation and project - the information is aimed at middle-school students, but even the younger ones seemed delighted to have their own little ponds-in-a-bottle.
> Water bottles - filled with less than an inch of lake mud, lake water, and an inch or so of air left on top. Note: In all likelihood you will get muddy as you sit beside your nearest lake and start squishing mud into empty water bottles, so dress accordingly! Also, it's ok if the mud includes small rocks, twigs, decaying leaves - that's all a natural part of the ecosystem. Use a sharpie to write "C" for "control" on half of the water bottles, and "T" for "Test" on the other half.
> Plants - find a shallow part of the lake and try to find plants that are growing from the lakebed, preferably with leaves underwater. Uproot them carefully and keep them in a dish with some water added to help keep the plants fresh before the experiment.
> Baking soda, which is sodium bicarbonate, NaHCO3
> Used coffee grounds
> Ground bone meal (you can find this at your local plant nursery, in the fertilizer section; any other natural source of phosphorus will do)
> 1/2 tsp. (or similarly small) scoops for coffee grounds and bone meal
> Log Entry templates to pass out to the students - each log entry should include date, time, weather/sunlight conditions, sketches of microcosms, soil depth, air depth, water transparency, plant growth, and any other observations, such as algae growth, presence of invertebrates, bubbles, etc.
A) Introduction: What is a Microcosm?
Today we are going to be making our own microcosms. We can watch them grow over the next four weeks.
A microcosm is a miniature ecosystem. Ecosystems are places that include both living things and nonliving things. The living things might include plants, animals, mushrooms, or even tiny things we can't see called bacteria. We call these living things organisms. Our microcosms will include a lake ecosystem, so the nonliving stuff will be mud, water, and air. We call this nonliving stuff the abiotic environment of the ecosystem - abiotic since it's not biotic, or not alive.
Here I have two sets of microcosms, some entitled "C" and another "T." This is for "Control" and "Test," and we'll be going over that in a little bit. You can come up now and pick one of each so that you have two microcosms with their abiotic environments in them.
B) The Chemistry of Life
We're starting out with the abiotic parts of our microcosms. Let's move on now to life. All living things - plants, animals, mushroom caps, you and me - are made up of cells. Cells are like tiny cities with an outer wall, inner headquarters, and even different groups that work as policemen or grocery stores. These tiny cities, these cells, are the basic unit of life that make up all organisms. So then, what are cells made up of?
When you start talking about things on a cellular level, you start to talk about chemistry. There are four different types of chemical compounds that help to make up cells and keep them working. Those four compounds are carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates, or carbs, are sugars. We need sugar for energy, and so do all other organisms. Carbs are made up of lots of carbon. Where can an ecosystem get carbon?
Carbon is available to ecosystems in the form of carbon dioxide, which is part of the air we breathe. In our watery microcosms, there is also carbon dioxide dissolved in the water. Plants love carbon dioxide, and they can take it from the air (or water) and convert it into sugar. This remarkable process - which takes a gas we breathe and turns it into a food we eat - is called photosynthesis. So, carbon can enter an ecosystem through the plants and gets converted into sugars. You can then eat plants, or eat something that eats plants, and carbon cycles through the ecosystem.
Now, that's how ecosystems get carbon. If you would like to experiment with the effects of carbon on your microcosm, you can choose to add a small scoop of baking soda to it. Baking soda includes bicarbonate ions. When you put bicarbonate in water, it actually converts to carbon dioxide. So, adding baking soda will flood your microcosm with carbon dioxide that the plants will love!
The next important compound for cells is lipids, or fats. Lipids store energy very well, and organisms can get them easily if they have lots of sugar. That's because it's a relatively simple process to convert carbs into lipids, if you have extra carbs around. So we won't worry about the lipids too much.
The third important chemical for cells is protein. Proteins are the working horses of cells. They do all sorts of odd-jobs that cells need. For example, in people, our brain cells have a protein called dopamine. If you're like me, you love chocolate. Every time I eat a piece of chocolate, my brain cells release the dopamine protein, and I feel happy. Another example is rubisco. Remember how plants can use photosynthesis, and turn carbon dioxide in the air into sugar? Rubisco is the protein that helps them do that. Without the protein rubisco, life wouldn't exist.
One of the elements that proteins contain is nitrogen. Like carbon, nitrogen also exists in the air we breathe. In fact, nitrogen gas makes up 70% of the air! However, the nitrogen in the air is not a good source of nitrogen the way that carbon dioxide is a good source of carbon. That's because nitrogen gas isn't usable to most organisms. Most organisms aren't able to use nitrogen gas in the important chemical reactions they need to take up nitrogen. So then, what's the solution?
It turns out that there are a group of bacteria called nitrogen-fixing bacteria. These bacteria - too small for us to see - are one of few organisms capable of working with nitrogen gas. They take the nitrogen gas and convert it into nitrates in the soil. Plants can work with nitrates very easily, so they take them out of the soil and use them for nitrogen-rich protein. Other organisms can then eat plants, or eat an animal that eats plants, and everyone gets a little nitrogen (and thus, a little protein) in their diet. Because of this, these nitrogen-fixing bacteria are another hero of ecosystems. Without them, nobody would be able to get proteins, and life wouldn't be possible.
Now, what are some good sources of protein for us? Meat is an excellent source of protein, but so are green beans, peas, and lentils. Why are these plants so rich in protein? It's because they have those nitrogen-fixing bacteria living in their roots, giving the plant an immediate supply of nitrogen. Another plant that has those bacteria in its roots is buckbrush, which grows in many of our backyards. Deer love buckbrush because it's their primary protein source! It's the deer's equivalent of eating their green beans. [You obviously only need to include that info if you live in an area that has buckbrush. If so, it can help to actually bring some of it to class so the students can recognize it.]
If you would like to experiment with the effects of nitrogen in your microcosm, you can choose to add a scoop of coffee grounds. Coffee grounds are a good source of nitrogen that the organisms in your microcosms can use to make proteins.
Finally, the last chemical compound we will consider is nucleic acid. Who's heard of DNA? DNA, which is made up of nucleic acids, is the most important chemical compound for life because it contains the information for every living thing on the planet! It's also a very complicated compound, with carbon and nitrogen in it, but also phosphorus. Phosphorus is readily available to ecosystems through the rock cycle, so its found in a lot of soils and decaying bones. Today, we have bone meal you can add to your microcosms, if you'd like to experiment with adding phosphorus.
Now that we've considered these important compounds for life, it's time to add some nutrients to our microcosms! However, be careful to leave your microcosms labeled "C" at your desks. You will only add a nutrient to your Test microcosm. You can add more than one if you like, or [if you, the teacher, have extra microcosms left over!] you can take another microcosm and add a second or third nutrient to that one.
C) Trophic Levels
Now that we've added "vitamins," so to speak, to our test microcosms, we're ready to move on to the living stuff - the organisms. Every ecosystem has three types of organisms inside it: producers, consumers, and decomposers.
Producers are creatures that make their own food. We've talked about plants a lot so far, and how they can take carbon dioxide and turn it into sugar. That makes plants a producer. Algae are producers too, and seaweeds.
Consumers can't make their own food, so they eat it instead. The two main types of consumers are herbivores (they eat plants, like deer) and carnivores (such as hawks, which eat other animals). There are some specialty consumers - frutivores, such as fruit bats, only eat fruit, and insectivores like anteaters eat insects. We are omnivores, which means we eat both plants and animals.
Finally, there are decomposers. Decomposers eat waste and dead stuff, and they are easy to overlook. They include a lot of stuff that live close to or under the ground - fungi like mushrooms, dung beetles, worms. However, without decomposers, the plants and animals would quickly use up all the nutrients in their ecosystem and run out of materials. If I place a plant in my microcosm, and it's in there all by itself, after a while it will use up all the carbon dioxide and starve.
Decomposers stop this from happening! They take dead plants and animals and cycle the nutrients in them back into the ecosystem. Moreover, decomposers like mushrooms (and other fungi) double their work by not only recycling nutrients, but spreading the nutrients around so that everyone in the ecosystem has access to them.
Now, inspect your microcosm carefully. Which of these components do your microcosms have so far?
Each actually has all three! There are lots of tiny, tiny creatures in the mud and in the water of your microcosms. These creatures includes algae, bacteria, and animals so small you can't see them without a microscope. Some of those are producing their own food as producers. Others are eating other tiny organisms, so they're consuming. Still others are eating dead tiny organisms as decomposers. What that means is that you already have all the ingredients you need for your microcosm to succeed! Given an abiotic environment, producers, consumers, and decomposers, your microcosm has the potential to flourish over the next four weeks.
That being said, there's no way for us to tell whether the microcosms succeed or not, is there? We want to know if our microcosm survives the whole month, or if everything in it dies. So, here I have a bunch of water plants you can choose from. Pick one for each microcosm - both your Test and your Control! - and close them up again. Our plants will act as a "thermometer" of sorts - a test of whether or not the microcosm is healthy, or on the brink of a crash.
Also, there's lots of plants and lake water in here, so don't be surprised if you see any critters as you're picking a plant out! There may be freshwater snails, freshwater shrimp-like creatures, some beetle-like bugs, or even worms.
D) The Scientific Method
Now that we all have plants in both our microcosms, it's time to discuss this "Control" and "Test" stuff. A control is the part of the experiment that hasn't been exposed to the variables we're experimenting with. Here, we are experimenting with variables like coffee grounds, bone meal, or Tums. The nutrients are the variables. We need a control - a microcosm that hasn't been exposed to any nutrients - in order to determine what effect the variables have on the microcosm.
Let's say I make just one microcosm, load it with coffee grounds, put my plant inside, and let it grow. I don't open it ever again, but just place it on my windowsill to soak in the sunlight and watch to see what happens.
I then observe that my plant grows like a lightning bolt, then dies after two weeks. What can I conclude about the coffee grounds? Was adding nitrogen good or bad?
Actually, I have no idea! It's only when I bring a control into the experiment that I can figure out what's going on. Maybe my control's plant dies after one week. In that case, I can conclude that the coffee grounds helped the microcosm out. Or perhaps my control's plant lives for two weeks too, but it doesn't grow to be as tall or have as many leaves. In that case, the coffee grounds still helped - they made the plant healthier during it's lifetime. But if the plant in my control lives throughout the whole four weeks, then I know that I probably killed my plant with too much nitrogen. I gave that thing an overdose of caffeine and it just couldn't handle it.
So you see, the control will help you to understand how the nutrient you added affected your Test microcosm. As a part of understanding this, you will be keeping a scientific log. You will make your first log entry today, then again every week for this month. You should include your observations for both microcosms, and sketch what you see in each one as time goes on. After this month is up, your should have a pretty good grasp on whether your nutrient helped, hindered, or had no affect on your Test microcosm. You'll also get to watch your plant grow - and possibly see other surprises jump up in there - in the process.
As a final note, let's be sure not to shake our microcosms too much - we want the mud to settle in the water. We should also close the caps on the bottles tightly and not open them again - as a microcosm, it should have all the components it needs to survive completely on its own. When you get home, place your microcosms on a sunny windowsill and leave it there to grow.
Let's get started on our first log entry now!
You can provide your more advanced students with the following:
> Charts of the carbon cycle, nitrogen cycle, and phosphorus cycle
> Chemical configurations of a carbohydrate like glucose, a fatty acid (type of lipid), a protein fragment (preferably showing the peptide bond between two amino acids) and a nucleic acid (highlighting the carbon sugar ring, nitrogen group, and phosphate group)
> The chemical equations showing the conversion of bicarbonate ions into carbon dioxide - an excellent intro to what they'll learn in general high school chemistry
> A [slightly] more detailed look at photosynthesis
Other possible handouts:
> The water cycle
> Trophic levels/food web
> Basic diagram of a cell (plant cell may be more apt here than animal cell)