Re: What food do the organisms need on the rotting log?

Hi Tan Qiu Jin!

This is a good question, as 'decomposers' as these organisms are known, are absolutely essential for life. Without them, every dead leaf, twig or fruit and every pile of animal waste would remain on the ground and we would all be walking around in several metres of dead material!

You may have heard of the Carbon and Nitrogen cycles - the series of steps which mean that Carbon and Nitrogen are passed from one organism to another before going back to the start: Carbon dioxide in the air is absorbed by plants in photosynthesis, which are eaten by animals, digested, absorbed and then respired, thus releasing the carbon dioxide back into the atmosphere. Much the same thing happens with nitrogen too.

Some of the plant material made in photosynthesis is turned into wood - the structural material of trees. All plant cell walls are made of CELLULOSE and whilst we cannot diigest cellulose, many other organisms CAN - so parts of a plant made with cellulose (fruits, petals, paper) rot down very quickly. But cellulose is white (think of cotton-wool, which is pure cellulose) and wood is darker. That is because it contains another chemical too - LIGNIN.

Now, lignin is VERY difficult to break down and very few organisms (nearly all fungi) can digest it - and that is why dead logs take many years to fully break down. Fungi are affected by moisture and temperature, so they grow best in warm, damp, places and cannot grow at all when it is cold and dry.

As well as the fungi and bacteria that live on rotting wood, there are other decomposers too. These are animals (usually insects) that feed on the fungi and bacteria and can also eat the wood and digest SOME of the chemicals in it. They burrow through the timber and weaken it (termites are famous for this!), carrying fungal spores with them on their bodies. These tunnels allow water to pentrate deep into the log, the fungi can then grow there, and eventually it is rotten right through. When the tree was alive, it produced chemicals to protect it from attack; now it can no longer do so and the decomposeres will eventually break it all down into simple molecules, which are then moved on to the next stage of the great 'Cycles of Life'.

So, when Mdm Tan took the log back to her class, she removed it from the place where the natural decomposers live. This is known as their HABITAT, and, as I explained above, their ideal habitat is dark, warm and wet. Near the window would be too hot, I suspect, and it would certainly be too dry, so the organisms that lived on the log would not be able to survive. If they were fungi or bacteria, then they would form spores and become dormant until it became damp again; the animals that lived there would be faced with a choice of moving away or dying.

So what should Mdnm Tan do? Place the log somewhere dark and damp, just like the place where she found it! In an empty aquarium is a good place in a dark corner of the room, where you can see what is going on yet the organisms cannot easily escape! Then she should spray it every now and then with water (just as though it had rained) and the organisms will be happy and multiply. Most have an egg > larva > pupa > adult lifecycle, so she may find that some of the 'maggots' have vanished and been replaced by their coccoons, before they eventually turn into beetles!

When we go into the woods or out into our gardens, we are usually only interested in the largest and most colourful or noisy animals and plants - the birds and mammals and the flowers and fruits. But there is a whole world of small insects living above our heads too, forming much of the food for those animals. We SOMETIMES notice those too - the mosquitoes, flies, bees and wasps, which annoy us when they bit or sting. But we rarely, if ever, notice the third world - that or the many, many insects (mainly beetles) that live on the forest floor and yet play such a vital role in the recycling of matter that allows us all to live together here on planet Earth!

I hope that helps you and that you will look more closely around you when you are next in the woods - there is so much to SEE - you just need to look carely, move slowly and take a magnifying-glass with you, so taht you can see these small creatures more clearly. If you lift a log to look under it, just remember to put it back when you have finished - to keep the 'mini-beasts' happy! :)

Ian