Feeding Relationships
Organisms in an ecosystem are connected in numerous ways. Some organisms may benefit another, compete with each other for resources and of course, eat one another. Feeding on another organism passes energy from one organism to the next. The flow of energy through an ecosystem is represented in a food chain.
Food chains
We can use food chains to show which organisms feed on other organisms within an ecosystem and therefore the flow of energy from one organism to another. Energy always starts from the Sun and is converted into glucose by photosynthesis in green plants. Glucose is used in respiration to generate energy which is used to help the organism to grow (increase in biomass). Organisms which produce biomass by capturing the energy from sunlight are called producers and are at the beginning of every food chain. An organism which feeds on a producer are called primary consumers and will also be herbivores, since producers tend to be mostly green plants. The next organism in the food chain is the secondary consumer which will be eaten by the tertiary consumer. A predator is an animal which hunts and kills other animals and prey are the animals which are hunted and eaten.
Food chains are always written starting with the producer and followed by the primary, secondary and tertiary consumers. The arrows always point from the organism being eaten to the organism which feeds on it.
A food chain showing how green leaves (the producer) harness the energy from sunlight and convert it into biomass which passes along the rest of the food chain. The beetle eats leaves so is a primary consumer and is itself eaten by birds which are the secondary consumer. The snake is the tertiary consumer since they eat birds.
Food webs
Food webs show the interaction of multiple interconnected food chains within an ecosystem. For example, in the marine ecosystem pictured below there are a number of food chains which are all connected together. You can see that if the population size of any of these organisms changes, it will affect all the other organisms in the food web. This is known as interdependence. For instance, if the number of squid in the ocean decreased, this would boost mackerel populations (as there are fewer squid to eat them) and reduce the amount of seaweed and red algae, since there are more mackerel consuming these plants.
Decomposers
Decomposers also play important roles in food chains by breaking down dead matter. When organisms die, decomposers such as bacteria and fungi secrete digestive enzymes onto the dead material. The enzymes break down the molecules in the organism, converting it into simpler substances which can be absorbed by plants, allowing nutrients to be recycled within an ecosystem.
For example, decomposers will feed on fallen leaves, excreted faeces and even whole organisms, such as a dead woodlouse, causing them to decay. The protein, carbohydrate and lipid molecules which make up the organism will be broken down into simple molecules which are small enough to be absorbed by plant roots. In this way, the molecules which used to be part of the woodlouse may be used by the tree to grow more leaves, then the molecules are incorporated into the next organism when those leaves are eaten by a hungry insect.
Decomposers such as bacteria and fungi break down dead or undigested material (faeces) and enable nutrients to be recycled in an ecosystem. Decomposers feed by a process called saprotrophic nutrition, which involves released digestive enzymes onto the dead material and absorbing the digested molecules.
Pyramids of numbers show the number of organisms at each trophic level. They can look like a regular or an irregular pyramid, depending on the size of the producer.
Pyramids of numbers
Pyramids of numbers show the number of organisms (the population size) at each trophic level. The producer is represented at the bottom of the pyramid, with the primary, secondary and tertiary consumers represented on the levels above. Energy is lost at each trophic level of the food chain, which is why the population size usually decreases as you go up the pyramid.
These are not always a pyramid shape - for instance if we have a very larger producer such as a tree, then the number of producers (trees) will be much smaller than the primary consumers which feed on it (such as insects).
Pyramids of biomass
Biomass is the dry mass of a plant or animal species within a food chain. Pyramids of biomass represent the amount of biomass at each tropic level and will have a regular pyramid shape, since biomass is always lost at each step of the food chain.
A pyramid of biomass showing the dry mass of the organisms of different trophic levels in an ecosystem. When a lizard eats its prey, only a proportion of the energy available in the dragonfly is used for growth (biomass). The rest of the energy is used in metabolic reactions, for example to maintain body temperature.
Only a small proportion of biomass (around 10%) is passed onto the next trophic level, which reduces the total biomass after only a few stages in the food chain. This is why food chains rarely exceed four or five trophic levels. Biomass is lost at each stage in the following ways:
Used for respiration to generate energy for processes such as active transport and to maintain body temperature
Not all of the organism is eaten (i.e. the bones)
Not all of the organism is digested - some will be excreted as faeces
Did you know..
Sometimes predator-prey relationships can be complicated. This lioness was spotted in the wild looking after a baby antelope after killing and eating its mother. The lioness was spotted picking up the antelope by the scruff of its neck and caring for it as she would her own cubs.
Download worksheet: Feeding Relationships / Answers Next Page: Cycles within Ecosystems
