National 5 Biology Course Notes

Unit 3 : Life on Earth

Part 2 : Energy in Ecosystems

Food chain

A food chain shows how energy is passed from an organism that is eaten to the organism that eats it

e.g. GRASS Rabbit Fox

The arrows show the direction of energy flow.Energy stored in the grass passes to the rabbit when the rabbit eats the grass.Energy stored in the rabbit passes to the fox when the fox eats the rabbit.

Energy losses

Most of the energy that the rabbit gets from the grass is not passed to the fox.Only energy stored in the rabbit’s body passes to the fox.By the time the fox eats the rabbit, the energy it got from the grass has been lost by:• The rabbit producing heat• The rabbit using energy to move• Energy lost from the rabbit's body as undigested food.

In any food chain, only 10% of the energy is passed to the next level.90% of the energy is lost in these three ways:1. As heat2. In movement3. As undigested material

Producers and consumers

Producer – A green plant that is able to produce its own food – food chains begin with a producerPrimary consumer - An animal that feeds on (consumes) the producerSecondary consumer – An animal that feeds on (consumes) the primary consumerTertiary consumer – An animal that feeds on the secondary consumer

e.g. Algae Water flea Stickleback Pike (producer) (primary consumer) (secondary consumer) (tertiary consumer)

Pyramids of numbers, biomass and energy

For most food chains, the number of organisms, their biomass (total mass of all the organisms) and the energy stored in all the organisms decrease at each level of the food chain.i.e. there are more producers than primary consumers, more primary consumers than secondary consumers and more secondary consumers than tertiary consumersThe producers have a greater total biomass and more stored energy than the primary consumers, the primary consumers have a greater biomass and more energy than the secondary consumers and the secondary consumers have a greater biomass and more energy than the tertiary consumers

The decrease in these three features• Number or organisms• Biomass• Energy at each level of the food chain can be represented as a pyramid of numbers, a pyramid of biomass or a pyramid of energy:

Decreasing numbers

Producers

Primary consumers

Secondary consumers

Tertiary consumers

Pyramid of numbers

Decreasing biomass

Producers

Primary consumers

Secondary consumers

Tertiary consumers

Pyramid of biomass

Decreasing energy

Producers

Primary consumers

Secondary consumers

Tertiary consumers

Pyramid of energy

Pyramid of numbers that don’t have the usual pyramid shapes

The pyramid of numbers is shaped like this:

When the producer is a large plant like a tree that has many primary consumers feeding on it

The pyramid of numbers is shaped like this:

Producer

Primary consumer

Secondary consumer

Producer

Primary consumer

Tertiary consumer

Secondary consumer

When the food chain includes a parasite as the secondary or tertiary consumer e.g.

Grass Rabbit Fox Fleas

The pyramid is this shape since many parasites can feed on a single animal

Even when the pyramid of numbers have these unusual shapes, the pyramids of biomass and of energy for the same food chains still have the usual pyramid shape

Since the biomass and total energy stored both decrease at each higher level for all food chains including these ones.

Nitrogen cycle

Plant and animal proteins contain nitrogen.Plants get nitrogen from nitrate absorbed from the soil.This nitrogen is used to produce protein. Animals get protein by eating plants so their protein also depends on nitrate absorbed by plants.

Organisms involved in the nitrogen cycle and what they do

Decomposers

These include soil fungi and bacteria. They decompose proteins in dead organisms and nitrogen compounds in animal wastes to produce ammonium and nitrate.

Nitrifying bacteria

Nitrifying bacteria change ammonium into nitrite and nitrite into nitrate.This is called nitrification.

Nitrification by nitrifying bacteria

ammonium nitrite nitrate

Nitrogen fixing bacteria

These bacteria are able to use nitrogen gas in the air.Some nitrogen fixing bacteria live in the soil, they change nitrogen gas into nitrate. – this is called nitrogen fixation.

Other nitrogen fixing bacteria live in swellings called root nodules in the roots of a particular group of plants called legumes. (This group of plants includes clover, beans and peas)

Legume root

Root nodules

Nitrogen fixing bacteria in root nodules change nitrogen gas into a form that can be used by the plant to make protein.

Denitrifying bacteria

Denitrifying bacteria change soil nitrates into nitrogen gas which escapes into the atmosphere – this process is called denitrification.

Lightning

Nitrogen gas can also be changed to soil nitrates during lightning storms

Summary of nitrogen cycle

Nitrates

Absorbed by plant roots

Plant protein (made using nitrogen atoms

in nitrate

Animals eat plants Animal protein

Dead bodies of plants and animals broken down by decomposers,

e.g. bacteria and fungi

Ammonium

NitriteNitrification by

nitrifying bacteria

Nitrification by

nitrifying bacteria

Nitrogen gas(in the air)

Nitrogen fixation by

nitrogen fixing bacteria (free

living in soil and in root nodules

of legumes Denitrification

by denitrifying

bacteria

Process Organisms involved Reactions involved in the process

Summary of processes in the nitrogen cycle

Decomposition Decomposers, e.g. soil bacteria and fungi proteins ammonium

Nitrification Nitrifying bacteria ammonium nitritenitrite nitrate

Nitrogen fixation Nitrogen fixing bacteria nitrogen gas nitrate

Denitrification Denitrifying bacteria nitrate nitrogen gas

Competition

Competition between organisms occurs when they need the same resources, for animals they might feed on the same prey, for plants they compete for light and soil water.

Interspecific competition

This is competition between animals or plants from different species for the same resources.For example, lions and cheetahs belong to different species but they both feed on gazelle.

Intraspecific competition

Intraspecific competition is animals or plants belonging to the same species competing with each other, for example two cheetahs competing with each other.This competition is more intense because members of the same species need exactly the same resources, whereas members of different species use different resources.

Members of the same species can show behaviour that reduces competition, e.g.

Territorial behaviour – male members of the species establish territories where they do not allow other males to feed – this can be seen in gardens with male blackbirdsPecking order – for animals that live in groups, dominant individuals get first choice of resources.