3. MICROBES

This topic will expand on what has been said so far about microbes, i.e. that microbes are very small living things that complete the ecosystem cycle by converting plant and animal waste and dead bodies back into soil nutrients. In order to grasp the concept of microscopic organisms, it is useful to look at microscope photographs (micrographs), and to convey some appreciation for their size.

3.1 Smaller than you ever believed

To lead the children from the scale of familiar animals, to the world of the microscopic, discuss progressively smaller life forms that they have seen: Horses, foxes, squirrels, birds, hedgehogs, worms, bees, ants.

Introduce the idea that there are even smaller creatures that can't be seen with the naked eye, but require a microscope. This could be backed up by hands-on microscope work, though lack of detail from basic equipment probably warrants the use of professional micrographs instead.

On the "Mites and Microbes" worksheet (W2) is a micrograph of a mite:

The mite's body in the picture (magnification x200) is 10cm long. Ask the group:

Q. If you increased the size of a real mite to the size of the picture, how long would the picture become?

A. 10cm x 200 = 2000cm = 20 metres

That is the length of two double-decker buses. Compare it with the length of the classroom.

Mites are very small, but microbes are very much smaller. Look at the bacterium's picture on the worksheet. This time, the micrograph's magnification is x20,000.

Q. If a real bacterium was increased to the picture's size (10cm), how long would the picture be this time?

A. 2km long!

That's one and a half times the height of Britain's tallest mountain. Compare it with a location 2km from the classroom.

An alternative approach is to imagine how big a human would appear to a microbe: If you increased the size of the microbe to that of a human, how large would the human become?

If the children are familiar with cells being the "building blocks of life," they can be told that microbes are usually made of just one cell.

Q. How many cells do you think a human is made of?

A. Ten million million (10 000 000 000 000) cells.

For comparison, there are:

Four hundred thousand (400 000) kilometres to the moon.

Fifty million (50 000 000) drops of water in a standard swimming pool.

There are thirty million (30 000 000) seconds in a year.

150 million (150 000 000) kilometres to the sun.

[Around 5000 bacteria will fit into that tiny dot, on the worksheet.]

4. CO-OPERATION BETWEEN DIFFERENT LIVING THINGS

Having talked about the most obvious interactions between species: one organism eating another (feeding relationships), progress to non-feeding relationships. In particular, co-operation between species [technically, mutualism] is a fascinating topic, as it provides a greater awareness about the interdependence of all life forms within ecosystems. There are examples everywhere of one species benefiting from, but not harming, another (see below) [when both depend upon each other, the relationship is symbiosis]. These interactions are vital for the existence of ecosystems and a diversity of species (Rose, 1997).

When studying something like ecosystems, it is very important to give the children plenty of direct experience with their surrounding environment (Cornell, 1994). Only when they observe Nature itself, will the ideas be fully assimilated.

Take the group outside, to the leafiest area of school grounds, or to a local park. Ask them to look, alone, for examples of one living thing benefiting another. To give an idea of what to look for, say a couple of the "easily-observed" examples below. They should aim to find at least three examples and to draw their most favourite. After a suitable amount of time, re-group and discuss what was found. Emphasise the point that all living things in an ecosystem are helping each other all the time.

If the children understood the "easy" examples well, they can consider some "complicated" ones (below). These cannot be directly observed, but perhaps the children can suggest experiments to investigate them.

4.1 Some easily-observed examples of co-operation in ecosystems:

Mosses and lichens benefit from trees, by growing on their bark.

Plants' flowers depend on bees (and other insects) for pollination. The bees benefit from the flowers' nectar and pollen.

Birds benefit from trees. Trees supply twigs for nest-building and nesting sites that provide shelter from wind, rain and predators and are sufficiently high for chicks learning to fly. Holes in tree trunks allow nesting sites for other birds, e.g. owls and woodpeckers.

Squirrels benefit from trees.

Ivy benefits from trees. When it has something to climb, it can reach more light and avoid being "shaded out".

Spiders depend on plants for places to build their webs, in order to catch prey.

Small creatures ("minibeasts"), such as beetles, ants and woodlice, depend on litter (dead debris) from plants, to provide shelter from wind, rain and predators.

4.2 Some more complicated examples:

Some plants depend on animals to disperse their seeds. For example, birds that eat berries will travel far from the parent plants, before excreting the berries' undigested seeds, along with a healthy dose of fertilising manure. Plants such as burdock and cleavers (also called "goosegrass," "sticky bud" or "sticky Jack") have seeds that hook on to animal fur (and human clothes) and get carried some distance before falling on new ground.

Plants depend on animals and animals depend on plants, for gas exchange. Animals require oxygen and produce carbon dioxide. Plants require carbon dioxide and produce oxygen.

Plants depend on burrowing soil animals, especially worms, to move through and consume soil. This mixing provides aeration, water drainage and nutrient distribution. (This will be demonstrated in 6. Plants and worms in the soil).

Plants with shallow roots depend on plants with deep roots (e.g. thistle and trees) to draw deep nutrients to the soil surface. When the deep-rooted plant drops leaves or dies, these nutrients will return to shallow soil.

Humans (and other animals) depend on "friendly" microbes living in their bodies (particularly the guts) to prevent the growth of harmful microbes.

Bacteria that live in the guts of humans, earthworms and other animals benefit from a warm, moist place to live and some of the animal's nutrients. They digest parts of the animal's food that could not be digested otherwise, providing the animal with further nutrients.

Plant roots release nutrients [exudates] into the soil. These benefit soil microbes, which break down food in the soil that the plant wouldn't otherwise be able to use.

THE "WEBBING" GAME

Materials

Ball of string

This is a modification of a game that appears in Cornell (1994). It portrays the interrelationships within ecosystems, emphasising how all things work together in a balanced web of life.

The children form a circle:

"Who can name a plant that grows in this area?"

..."Dandelion."

"OK, you can hold the end of this string. Wrap it round your hand, so you don't let go...

Now, who can name a living thing that benefits from the dandelion?"

..."a slug."

"Good. You take hold of the string here. You are connected to the dandelion, by your dependence on her leaves. Wrap it round your hand and pull it tight, but don't let go...

Now, what kind of living thing might benefit from this slug?"

..."a blackbird."

...This sequence could continue through, say, a holly bush [the bird spreads its seeds], microbes [benefit from exudates from the plant roots (see 4. Co-operation between different living things)], an earthworm [has his food softened by the microbes], etc. You could allow other elements in the environment, such as soil, water or wind.

In this way, the entire circle of children is strung together like an ecosystem. To demonstrate that each individual is important to the whole community, take away by some plausible means one member of the web, for example a fire or a logger kills a tree. When the child representing the tree falls down, he will tug on the string he's holding. Anyone who feels the tug is in some way affected by the tree's death. Now everyone who felt a tug from the tree gives a tug. This continues until every individual is shown to be affected by the tree.