Producers, Consumers and Decomposers

In autumn, many Virginia trees drop their leaves onto the forest floor. Turn over the top layer and the older leaves below are often dark, soft and full of holes.

Leaves fall every year. If nothing happened to them, the layers would stack higher and higher. Under the leaves you may find white threads of fungus, mushrooms, worms and tiny animals. So where does all that leaf material go? First, decide what you think now.

Explore: run a forest floor for five years

This simulation shows a patch of forest floor over five years. You choose how many decomposers live there (none, few or many) and how warm and wet the seasons are. A switch lets people spray a soil poison or add compost. Bars show the height of the leaf pile, the nutrients in the soil and how much the plants grow each year. The rates are made up, and all decomposers are shown as one group.

Start here: choose how many decomposers live in the soil, then change the season or the human action.

Decomposers

None Few Many

Seasons

Cold and dry Mild Warm and wet

People

Do nothing Spray soil poison Add compost Reset

Bars use relative units, not to scale. The dashed line marks where the pile and the soil nutrients started.

In this sketch, the leaf pile and the plants change over five years.

Made up for this sketch: one kind of leaf; decomposers drawn as one group; none removes all of the breakdown and few is half of many; warm and wet is fastest, mild in between and cold and dry slowest, as an invented setting (real results can differ); the poison leaves a tenth; compost adds a little extra food once, in year 1; plants take up a fixed share of the soil nutrients each year and drop it as leaves; no carbon leaves as carbon dioxide. All rates and sizes are invented, in relative units.

Decomposers break down dead leaves and return nutrients to the soil, where plants use them again; with few decomposers, leaves pile up and plants may grow less. Warm, damp conditions often help decomposers, but it is not a simple rule.

Try this.

1. Predict what the leaf pile and the plants will look like after five years with no decomposers. Then test it.
2. Set decomposers to many. Predict how a cold, dry season will change the leaf pile compared with a warm, wet one. Then test it.
3. Spray the soil poison, then add compost instead. Predict which choice gives more plant growth by year five, and check.

Explain: three jobs in an ecosystem

Every living thing in a forest does one of three jobs. These are jobs, not kinds of living things.

A producer is an organism that captures energy from sunlight and stores it in sugar (glucose). Trees, grasses, mosses and algae are producers. They build sugar from carbon dioxide and water. You saw how in Photosynthesis Feeds the World.

A consumer is an organism that gets energy by eating other organisms. A deer eating acorns is a first-order consumer, because it eats producers. A fox eating a mouse is a consumer that eats other consumers. Producers are eaten by consumers.

A decomposer is an organism that breaks down dead plants, dead animals and wastes. The main decomposers are fungi and bacteria. Fungi include mushrooms and molds. Most of a fungus is a mass of thin threads hidden in the leaves or soil. Bacteria are single cells, far too small to see without a microscope.

Job	Gets energy from	Forest example
Producer	sunlight	oak tree, moss
Consumer	eating living organisms	deer, fox, hawk
Decomposer	dead matter and wastes	fungi, bacteria

Some animals help too. Earthworms and millipedes eat dead leaves and tear them into smaller bits. Some textbooks call them decomposers, and some call them detritivores. Either way, smaller bits give fungi and bacteria more surface to work on.

Explain: decomposers send matter around again

A fungus does not have a mouth. It releases chemicals that break down the leaf outside its body. Then it takes in the small pieces. Bacteria work in a similar way. People often say decomposers “eat” dead leaves. That word is fine if you remember how it works.

A nutrient is a substance, such as nitrogen, that living things need to grow. As decomposers break down a leaf, nutrients go back into the soil. In a stream, they go into the water. Plant roots take up those nutrients and use them again. Some of the leaf’s carbon goes into the air as carbon dioxide, because decomposers release it in respiration as they use food for energy. You will follow that carbon in The Carbon and Water Cycles.

Matter goes around and around. Energy does not. Energy enters as sunlight and leaves as heat, as you learned in Food Webs and Energy.

Abiotic factors, or nonliving conditions such as temperature and moisture, change how fast decomposers work. In many places, warm and damp conditions help fungi and bacteria break leaves down faster. But it is not a simple rule. Too dry or too cold slows them, and one experiment found less breakdown in warmer chambers.

Explain: when one group changes

Producers, consumers and decomposers are linked. A change in one group reaches the others.

• Fewer producers: plant eaters have less food. Animals that eat the plant eaters may then have less food too.
• Fewer decomposers: dead leaves pile up. Fewer nutrients return to the soil, so plants may grow less over time.
• More decomposers: leaves break down faster, but only while there are dead leaves to break down.

People change these groups too. Some actions harm them. Pesticides and pollution can kill soil life. Clearing a forest removes producers and the leaf litter that feeds decomposers. Raking away every leaf removes a source of nutrients. Other actions help. Planting trees adds producers. Leaving leaves in a corner of a yard, or making compost, feeds decomposers and gives small animals shelter.

Elaborate: two years in a bag of leaves

Researchers sealed dried yellow birch leaves in mesh bags. In October 2011 they set the bags on the ground at three heights in the Weimer Run watershed, near Davis, West Virginia. The table shows the lowest site and the highest site. They brought back bags five times and weighed the dried leaves.

Time after setting out (months)	Lower site: mass left (%)	Higher site: mass left (%)
0 (October 2011)	100	100
1 (November 2011)	68.9	72.0
8 (June 2012)	70.9	68.9
12 (October 2012)	54.4	59.7
20 (June 2013)	51.6	55.8
24 (October 2013)	41.2	47.8

Source: Atkins, Epstein and Welsch, Ecosphere, 2017, Weimer Run, West Virginia, 2011 to 2013. Each value is a mean of 8 or 9 bags, from the authors’ shared data, rounded to one decimal. Different bags were weighed each time. The 100 percent at month 0 is the starting value, not a weighing of recovered bags. This is the closest real data we found; it is not from Virginia.

1. Describe the trend at the lower site from month 0 to month 24.
2. At month 24, how many percentage points more mass was left at the higher site than at the lower site?
3. The lower site went from 68.9 to 70.9 percent between months 1 and 8. Why might that happen when different bags are weighed each time?

Design problem

Plan an investigation to test whether moist soil makes leaves break down faster than dry soil. Use leaves, cups of soil and a balance.

One way to do it

• Question: Do leaves lose mass faster in moist soil than in dry soil?
• Independent variable: moist soil or dry soil.
• Dependent variable: the dry leaf mass left after 4 weeks, in grams, and the percent of the starting mass.
• Constants: the same kind of leaf, 2.0 grams of air-dried leaves per cup, the same soil, cup size and room temperature.
• Control: cups of dry soil with no water added.
• Trials: five cups of each. Find the mean for each group.
• Measuring: after 4 weeks, rinse the leaves gently, let them air-dry for a week, then weigh them.
• Safety: wear gloves, keep cups covered with a lid that has small holes, do not sniff moldy cups, and wash your hands. Students with mold allergies should not handle the cups.

If the moist cups keep less leaf mass, moisture likely helped decomposers in this setup.

Evaluate: check your thinking

The leaves that fall each autumn do not vanish. Fungi, bacteria, worms and other small living things break them down, and the nutrients go back into the soil, where trees take them up again. Without decomposers, dead leaves would pile up and the forest would run short of nutrients.

Science practices you used: predicting and testing with a model, reading a data table and finding a difference, making a claim from evidence, and planning a fair test.