Unit 4.

Ecosystem
4.1. The Sonoran Desert p126
4.2. Different Ecosystems p133
4.3 Intruders In An Ecosystem p139
4.4 Bioaccumulation p142
✓ Check your progress
4.1. The Sonoran Desert
 Ecosystems
An ecosystem is a community of
animals, plants, micro-organisms,
non-living things and their shared
environment. Find out about
energy transfer, niches and
An oak tree can be seen as an
competition in ecosystems. ecosystem in which each zone of the
tree is a habitat for a distinct
community of organisms
An ecosystem describes a natural biological unit that is made up of
both living and non-living parts. It is made up of a number of:

•habitats - the place where an organism lives

•communities - all the living organisms that live within a habitat

A community can contain a number of different species.
Species

A species is a group of organisms that can interbreed to

produce fertile offspring. For example, a horse and a
donkey can interbreed with one another but their offspring
(a mule) will be unable to reproduce as mules are infertile.
The fact that offspring cannot reproduce shows that horses
and donkeys belong to two different species.
➢ Biodiversity

✓ Biodiversity is the degree of variation that exists among all living

organisms on Earth.
When examining the biodiversity on Earth it is useful to consider
the populations of organisms in a species. These can fluctuate in an
ecosystem depending on factors such as:
•competition between organisms for resources
•predator numbers
•disease
 All living things within an ecosystem
are interdependent. A change in the size of one
population affects all other organisms within the
ecosystem. This is shown particularly clearly by the
relationship
between predator and prey populations.
 A food chain shows the linear
flow of energy between
organisms

Predator and prey populations

A predator is an animal that hunts, kills and eats other animals for
food. Prey is a term used to describe organisms that predators kill
for food.
Predator/prey relationships can be illustrated in a diagram called
a food chain or food web.
 A food web shows the energy flow
through interconnected food chains in
an ecosystem
• Producers make their own food using energy from an
abiotic source. For example plants carry out
photosynthesis to make food using light energy from the
sun.

• Consumers get food from a biotic source by eating the

biomass of producers or other consumers.
Within food chains and webs there are organisms that will only
consume particular types of food:

•A herbivore is an organism that only consumes plant material.

•A carnivore is an organism that only consumes animal material.
•An omnivore is an organism that will consume both plant and animal
material.
There is a continuous struggle between predators and their
prey:

•Predator species need to be adapted for efficient hunting if

they are to catch enough food to survive.

•Prey species must be well adapted to escape predators for their

species to continue.
If the prey population in an ecosystem grows, predator numbers
will respond to the increased food supply by increasing as well.
Growing predator numbers will eventually reduce the food
supply to the point where it can no longer sustain the predator
population ... and so on.
If an organism is removed from a food chain or web all together,
for example due to overhunting, then this can have a
catastrophic effect on the other populations in the food web.
 Energy transfer in
ecosystems

All organisms require an energy supply for cellular activities such as

protein synthesis and cell division, which enables organisms to grow.
Some of this energy is stored in the new material (for example,
cellulose and muscle protein) made by growing organisms. This is
called biomass.
Organisms release energy from their food by the process of respiration.
In most ecosystems on the planet the ultimate source of energy is
the sun.
The extreme height to which redwood
trees grow is a consequence of
competition for light.
Competition will occur between organisms
in an ecosystem when their niches overlap,
they both try to use the same resource and
the resource is in short supply.
Animals compete for food, water and space
to live. Plants compete for light, water,
minerals and root space.
 ✓ What are deserts?

Deserts are areas that receive less

than 10 inches of rainfall each
year. in hot deserts are high during
the day and reach below 0 °C at
night. The Sahara desert is the
world’s largest hot desert. It spans
11 countries in North Africa.
 ✓ What are deserts?

Cold deserts are found at higher , for

example parts of the Arctic and the
Antarctic. Hot deserts are common
between 15-30° north and south of
the . Both types of desert are found
where dry air is falling.
 Plants and animals have adapted to live in hot deserts.
Some examples can be found in the table below.
Cacti have thick, waxy skin to trap water within the
Cactus plant. Their spikes are protection from animals
that may want to eat them.
Tumbleweed dies during periods of dry weather.
The plant dries up and is then blown about by the
Tumbleweed
wind. As it moves, it scatters seed, which grows
when the rains return.
Camels have long eyelashes and nostrils that can
be closed to keep out any sand blowing in the
Camel wind. Camels store fat within their hump, which
helps them to survive for long periods of time
without eating.
Meerkats are camouflaged. Predators find it hard
to see them as they blend in with the colour of the
Meerkat
desert. They have dark rings around their eyes,
which help to reduce the glare of the sun.
 Cactus
Cacti have adapted to survive harsh hot desert conditions
 Camel
Camels have adapted to live within hot desert environments
 How do animals and plants
interact with each other in the
desert?

The fleshy roots and tubers of many

desert plants keep some desert animals
supplied with water. Jackrabbits and
cottontails seek their meals at dawn
and dusk. Lizards and snakes have scaly
skins and insects have outer skeletons
that keep water loss to a minimum.
 Adaptation
Living things are adapted to their habitats. This
means that they have special features that help
them to survive.
• An African elephant, for example, lives in a
hot habitat and has very large ears that it
flaps to keep cool.

• A polar bear, on the other hand, lives in a

cold habitat and has thick fur to keep warm.
 Adaptation
It’s not just animals that are adapted to their environment,
plants are too. A cactus is well adapted for survival in the
desert. They have long roots to collect water from a large
area and a stem that can store water for a long period of
time.
The animals and plants in one habitat are suited to live
there and may not be able to survive in other habitats.
When a habitat changes, the animals and plants that live
there are affected.
 Non- living things in the
desert

Organisms depend on each other for survival. This is called

interdependence. Both living and non-living factors will affect
the abundance and distribution of organisms in a habitat.
The abundance is the number of organisms in
an ecosystem and their distribution is affected
by abiotic factors. These are factors that are non-living. They
include:
 • Light
intensity
Some plants have evolved for optimum growth in bright
sunlight. An example of this is a cactus houseplant. Cacti
originally come from deserts where they grow in bright
sunlight. Other plants have evolved to grow in shade.
Many orchids, which are also kept as houseplants, grow on
trees in the rainforest and have evolved for optimum growth
in darker conditions. If you were to put an orchid on a bright
windowsill and a cactus in a dark corner of your room
neither plant would grow well.
 • Temperature
Both animals and plants have evolved to grow
healthily at their optimum temperatures. If you
planted either your cactus or orchid
houseplants outside in cold temperatures, they
would die. Similarly, animals that have evolved
to live at the North Pole, such as the polar bear,
could not survive in warmer conditions.
 • Soil mineral
content

Many plants require high levels of soil minerals to grow

well. An example of this is magnesium, which is
required to produce chlorophyll. Plants with
unnaturally yellow leaves may have a
magnesium deficiency. Carnivorous plants, such as
pitcher plants, have evolved to catch insects to
supplement the low levels of minerals found in the soils
in which they grow.
• Carbon dioxide levels for plants

Carbon dioxide is a reactant in photosynthesis which means plants

need it to survive. Areas with higher levels of carbon dioxide are
more likely to have healthy plants growing. Farmers often release
carbon dioxide within their greenhouses to maximise their crop
yield. Woodlands often have higher carbon dioxide levels than open
grassland, so many plants living in open areas have evolved
mechanisms to overcome a shortage of carbon dioxide.
• Water

All organisms need water to keep their cells alive.

Rain, when it comes, allows them to become more
active and to reproduce.
✓Glossary
Adaptations: features of organisms that help them to live and reproduce in their
habitat
Nectar: a sugary liquid made by flowers to attract insects for pollination
Pollen: tiny grains made by flowers, which contain male gametes
Pollinating: moving pollen from an anther where it is made, to a stigma
Interact: affect one another
Environment: every thing around an organism that affect it
Nocturnal: active at night
✓Glossary

Ecosystem: a network of interactions between all the living organisms in a habitat,

and the non-living things around them

Food webs: a diagram showing many interconnected food chains

Ecology: the study of organisms in their natural environment

Habitat: the place where organism lives

4.2. Different ecosystems

In this topic you will:

• Learn about some of the many different kinds of ecosystem on Earth
• Describe some of the different habitats in an ecosystem
✓ More about ecosystems

The ecosystem in the Arizona Sonoran Desert is just

one of many different ecosystems on Earth.
Here are three more examples of ecosystems:
mangrove forest, sea ice in the Arctic
Ocean, and a rice paddy.
 ▪ Mangrove forest

• Mangroves are trees that can grow their roots in sea water. They form forests along the
coasts of many tropical countries.
• Young fish live among the mangrove roots, safe from larger fish that might eat them.
Mud skippers climb out onto the mud when the tide is out, feeding on what ever they
can find.
• As the mangrove leaves fall onto the mud, they are decomposed by bacteria. Prawns
and crabs eat the partly decomposed leaves.
• Crab-eating macaques , a type of monkey, climb through the trees and catch crabs on
the tree roots and mud.
 Sea ice in the
Arctic Ocean
•During the winter in the Arctic Ocean, it so cold that some of the sea water freezes.
•Seals hunt for in the water, but have to come to the surface to breathe air.
•Polar bears patrol the ice, looking for seals to kill and eat. Polar bears are good swimmers,
and can move from one ice floe to another.
•Arctic foxes also look for food on the ice.
•Enough light passes through the ice to allow tiny algae (simple-celled plants) to grow on the
underside of the ice floes.
•Tiny shrimps-like organisms eat the algae. Fish eat the shrimp-like organisms.
 ▪ Rice paddy
• Not all ecosystems are natural. This area of rice paddies in Malaysia is formed by people.
• At some times of year, the paddy fields are flooded with water. Algae grow in this shallow
water, and on the mud at the sides of the flooded areas.
• Fish swim into the flooded paddies from the irrigation canals. Frogs and dragonflies
breed in the water.
• Because the water is shallow, it heats up quickly during the day, and cools down quickly
at night.
• Farmers often add fertilizer to the paddy fields, making not only the rice but also the
algae grow faster, providing more food for the animals.
• Many birds feed in and around the paddy fields.
4.3. Intruders in an ecosystem

In this topic you will:

• Learn about how new or invasive species can affect an ecosystem
 Introduced
species

Introduced species are those that have been

moved intentionally or unintentionally by humans
into a new geographic location where they are not
naturally found.
Some may become established in this new
location and are then termed naturalised species.
 Introduced
species

Some of these naturalised species can

spread rapidly and outcompete or prey
on native species - these are termed
invasive species.

Invasive species can eliminate native

species therefore reducing species
diversity.
 Introduced
species
They may spread rapidly because this
new location is free of predators,
parasites and competitors that would
normally limit their population in their
natural environment.
Invasive species can have a severe
negative effect on the biodiversity in
an ecosystem.
 ✓ Glossary
Native species: types of organism that are

Extinct: no longer existing

Eradicate: get rid of; totally destroyed

Invasive species: a species that has been introduced into an ecosystem where it does not
belong
 4.4 Bioaccumulation

In this topic you will:

• Find out about DDT
• Use model to explain what happens to DDT in a food chain
• Learn what bioaccumulation is, and why it happens
❖ DDT
DDT is an insecticide that can pass
up the food chain from insects to
small birds, and then from the small
birds to birds of prey, like hawks. It
bioconcentrates in the birds of prey
because it cannot be excreted.
A high concentration of DDT in birds
causes weakness in the shells of
their eggs, meaning that they are
crushed accidentally when being
incubated by their parents.

DDT is now banned because of this.

Bioaccumulation takes place in a single organism over the span of its
life, resulting in a higher concentration in older individuals.
 Biomagnification takes place as chemicals transfer from
lower trophic levels to higher trophic levels within a food
web, resulting in a higher concentration in apex predators.
 Glossary
Insecticide: a chemical used to kill insects

Persistent: a persistent substance stays in the environment for a long time

Toxic: poisonous

Accumulates: gradually increase in quantity

Bioaccumulation: gradual increase of a substance in an organism’s body; it happens when the

substance cannot be broken down inside the body cells

Biomagnification: increase of the concentration of a substance as you go up a food chain