Pyramids of numbers and biomass

How do living things in a community depend on each other? Drawing pyramids To see the realtionship between two variables in a an experiment, you draw a graph. In the same way, to study the realtionship between populations in a food chain, we count the organisms and draw a diagram called a pyramid of numbers. This pyramid shows a typical shape, with lots of producers below and just a few carnivores at the top.

Drawing pyramids This pyramid is different because there is only one oak tree with lots of caterpillars living off it. The oak tree is one organism, but it is very large. To give a more realistic idea of the feeding relationship we use another pyramid called a pyramid of biomass. Ecologists draw a pyramid of biomass by weighing all the organisms at each stage of the food chain. The mass of producers is larger than the mass of consumers to give a typical pyramid shape.

Predators and prey Predators are organisms high up the food chain. They are good hunters, well adapted to finding, catching and killing their prey. Prey animals are well adapted for detecting and avoiding predators. The poison dart frog (top) has a brightly coloured toxic skin which puts off predators. The leaf frog is camouflaged to blend into the background.

Predators and prey The golden eagle is well adapted as a predator. Large forward facing eyes make it easier to spot potential prey. The beak is hooked and sharp so easily tears flesh from the body of its prey. Gliding flight is silent. Talons kill by piercing vital organs or snapping bones.

The ups and downs of the predator-prey relationship In any environment the relative number of predator and prey are constantly changing, depending on environmental factors such as the availability of green plants (producers). Grouse are birds that live in moorland environmnet such as in Scotland. Grouse eat mainly heather, and are prey for eagles. heather → grouse → eagle If there is lots of heather and therefore plenty of grouse, the eagle population will thrive. More eagle predadtors will catch lots of grouse so after a while the grouse population will fall. Then there are fewer grouse for the eagles to eat, so after a while the number of eagles falls too. This allows the number of grouse to increase again, and so the cycle continues.

Predator-prey relationship graph

A futher look at food webs Every food web contains many interconnecting food chains. If the poulation of one organism changes, many others are affected, not just those they eat or that eat them. Eagles in Scotland do not just eat grouse; they feed off hares and dead sheep as well. So if the populations of hares, grouse and sheep are high, then eagles will be plentiful. What might happen if the number of hares falls perhaps because of disease? There is more grass for sheep to eat (as fewer hare are eating it). Eagles have fewer hares to eat, so they eat more grouse. The number of grouse therefore falls. Fewer grouse eat less heather, so the heather grows and spreads. Farmers might find that hungry eagles are targeting their sheep. So a decrease of the population of just one species can have a huge effect on the populations of many organisms in a community.

A further look at food webs