Plankton to fish: a food cycle

[Music] oh in the waters of the lake there are many different kinds of living things each kind depending on the others for survival this isn't an ordinary fishing trip the equipment is designed to catch the smallest of the living things in the lake plants and animals of microscopic size our catch has been transferred to a beaker in the laboratory we'll use these small plants and animals as part of a miniature community of living things first a closer look at what we've caught we use the term Plankton to refer to many different kinds of plants and animals that live in water and Float with the current most of them are very small as we increase the magnification we can see smaller and smaller Plankton these diatoms like other plants are able to use the energy of sunlight to produce their own food from carbon dioxide and water Plankton service food for larger animals such as these water fleas this is the beginning of a food chain the beating motion of the antenna creates a small current which sweeps Plankton into the food groove or mouth of the water flea to demonstrate something about the food chain we're removing the food tube or digestive tract of a water flea here's the food tube and its contents it takes a great deal of Plankton to nourish a single water flea the body is transparent so you can see young water fases developing inside The Brood pouch of their mother water fleas produce a large number of Offspring in a very short time that means they can maintain their own population while also serving as a source of food for larger animals what can we expect to find in the intestine of a fish it takes a large number of water FAS to perish a single fish in a lake or river these fish would become food for larger fish but we've made them part of a miniature World in an aquarium there are no larger animals so these fish are at the top of our food chain the chain begins with the microscopic plants which produce food that benefits the water fleas which in turn benefits the fish is that the end of the chain or do the fish and water fleas benefit the plankton in some way well we know that the body of a dead fish will decompose and living fish release waste products into the water will this have an effect on the microscopic platin after a short while we can see that the Plankton have multiplied that's because the wastes and decomposed matter provide nutrients for the Plankton to benefit the Plankton the waste products must be acted on by bacteria so the food chain is really a continuous cycle each type of living thing depends on the others now that we have a food cycle operating let's try a few experiments three tanks each containing Plankton water fleas and fish a complete food cycle along with the bacteria that help make the food cycle operate and in each tank the food cycle is balanced there are just enough Plankton to feed the water fleas and just enough water fleas to to feed the fish if we add a few more fish to one of the tanks will that have an effect on the food cycle there are more fish competing for the same food supply after several days you notice any change compare this tank where we added some fish to this one which we left as it was here are some fish from the a tank and some from the B tank any difference growing thinner and some of them dying the population of fish is being reduced and this will restore the balance between the number of fish and the available food supply there are other ways in which a food cycle can be thrown out of balance we'll use the B tank for a second experiment what will happen if all the fish are removed how will this change of balance affect the water fleas after a few days the water fleas have multiplied many times quite a difference from the tank where fish are feeding on water fleas does that mean the water fleas here are better off a few days later the water is clearer because all the Plankton have been eaten and the water fleas are dead or dying the water fleas multiply too fast for their food supply and now they've used up that food supply removing the fish has caused the food cycle to break down completely so balance is important to the operation of a food cycle when we added too many fish some of them died reducing their number and restoring the balance but if a link in the food cycle is broken completely as it was when we removed all the fish it may be impossible for other living things to survive Sur in a balanced food cycle each kind of living thing depends on the others for survival in our miniature food cycle water fleas feed on Plankton and in turn become a source of food for fish and the waste products of the fish acted on by bacteria help to provide nutrients for the microscopic plankton the food cycle we've seen as a simple one involving only three kinds of living things in the larger world the food cycle is much more complicated in fact the food cycle here is more like a web in which many different kinds of plants and animals depend on each other in various ways and in this food web there must be a balance among living things so that all of them including man can continue to [Music] survive