S3 Biology Revision Notes
Habitats
Local habitats
Biology starts at home. Around the school there are several distinctly different types of habitats. Each
habitat supports its own particular community – a group of populations of animals and plants who are
adapted to live in that particular habitat. You looked a number of different local habitats – what
conditions were like and which living things you would expect to find there.
Each different habitat is defined by a set of environmental conditions which are found there, and which
affect each other. Environmental conditions are not living things – the term used for this is an abiotic
factor (a=not, biotic=caused by a living thing). Some of the particularly important abiotic factors in local
habitats are –





Light – which particularly affects which plants are most successful
Temperature – the more sunlight there is, the warmer it is likely to be. However, the more
exposed it is, the colder it is likely to be at night.
Moisture – which is often linked to light; the brighter the habitat is, the faster the soil dries out
Soil – the soil structure can affect how quickly soil drains water away, and how much nutrient is
available to plant species.
Shelter – this may be to do with how exposed they are to the wind, how much shelter or cover
is available, or how easy it is for animals to construct ‘homes’ – burrows or nests.
Some of the local habitats we looked at were –
Grassland.
This is very exposed, and gets a lot of light. The soil dries out quite quickly. Though it gets a lot of light
during the day, it also loses heat quickly at night. Though grass is the most common plant, there are a
number of other low-growing plant species too – dandelions, daisies, plantain and clover, for example. A
variety of ground living insects can be found, and larger animals such as rabbits often feed in this
habitat.
Woodland.
Woodland has a dense, high canopy formed by broad leaved trees. This means that the light reaching
the ground is restricted, so the soil often stays quite damp, and the temperature is cool. This habitat is
seasonal, as the trees lose their leaves in the Autumn. This creates rich soil as the leaves decompose and
return their nutrients to the soil. It also means that in the spring there is more light available, so many
plants grow and flower early in the spring before the tree canopy shots out most of the light again.
There is much more biodiversity in woodlands, as there is a huge variety of food sources. Many insects
and small mammals live in the leaf litter (slugs, beetles, hedgehogs), while others use the trees as their
habitat (greenfly, ladybirds, woodpeckers, squirrels).
Stream
The stream provides a habitat for a particular set of water dwelling insects, who feed on the pond weed
found in the water. In turn, the insects are food for small birds, small fish and amphibians such as frogs
and toads. The stream also provides a very rich, moist soil. Some plants are very suited to this type of
soil. Willow trees, for example, grow best with their roots actually in water, so are often found along the
banks of streams.
Coniferous woodlands
Coniferous trees are those which make cones. They are usually evergreen trees, with needle like leaves.
Most coniferous woodlands are planted by man, rather than natural. They trees grow very close
together, so very little light gets to the ground. The trees do not drop their leaves, so no nutrients are
returned to the soil, which tends to be poor in quality. These abiotic factors mean that very few other
plants grow in coniferous forests. The coniferous trees are a poor source of food for animals, so there is
relatively little biodiversity in coniferous woodland.
Summary For each habitat, the principles are that –





Each habitat has its own set of environmental (abiotic) conditions
These abiotic conditions determine which plants and animal will be successful in the habitat
The animals and plants affect each other. The more diversity there is in plant life, the more
different kinds of animals will be able to feed on them. The more different kinds of animals
there are, the more different kinds of predators there will be.
The range of different of animals and plants present is a measure of the biodiversity supported
by the habitat.
The animals and plants found in a particular habitat have adaptations which help them survive
there. These adaptations may be physical features or behaviours.
Word list –
Word
Meaning
S3 Biology Revision Notes
Habitats
Sampling habitats
Biologists often look at habitats which are very big – this could be a woodland, a rugby pitch or even the
Sahara desert!
If the Biologist wants to know about the different populations of animals and plants present, it would be
far too much work to try to count every single living thing. The solution is sampling – measuring a small
area within the habitat, and assuming that the rest is much the same.
The rules of sampling help to make the results you get reliable. This means if someone repeated your
measurements, you would expect them to get the same results.



Sampling should be done at random. It is not a reliable sample if you choose a sample site
because you like the particular results you get there.
Sampling should be representative. This means that samples should be spread throughout the
habitat so that the full range of microhabitats is sampled.
Sampling should be repeated. The more samples you take, the more reliable your results are.
Usually, you take an average of your results so that you can compare more easily with other
data.
Sampling abiotic factors
You often try to find how abiotic factors affect the distribution of living things. There are a number of
abiotic factors which you have measured –



Light – measured with a light meter. Make sure that you hold the window on the meter
upwards and that you do not cast a shadow over it.
Soil moisture – measured with a moisture meter. You need to make sure that you push the
probe on the meter into the soil to the same depth each time. You also need to wipe the probe
each time you use it, so that moisture is not carried over from one reading to the next.
Temperature – measured using a thermometer
There are a range of other measurements which would also show abiotic factors – flow rate of a river,
depth of a pond, pH (acidity) of a stream, wind speed and so on, depending on the habitat.
Sampling populations
Plant populations are usually sampled using a quadrat.
Quadrats are dropped at random across the whole area to be sampled. An abundance score is
measured for each plant species by counting how many squares it appears in. The maximum score is 25.
Animal populations are sampled in a variety of ways, depending on the habitat.
Tree beating
Bushes and shrubs
Sticky tape traps
Tree trunks
Sweep nets
long grass
Pitfall traps
surface level insects
Mammal traps
small mammals in woodland areas
Usually, the abundance of the animal is related to how many of each type are found. There are sources
of error in some of these methods. Animals may eat each other in pitfall traps for example, or small
birds may feed on the insects stuck on sticky tape traps.
Transect studies
Sometimes the aim of sampling is not simply to look at a particular habitat, but to see how one abiotic
condition affects the distribution of living things. In this case, a transect study is carried out. This breaks
the normal sampling rules, as sampling is done at regular intervals, rather than being done at random.
An example is how the shade created by a tree affects the living things that you find –
Every metre, you take a light reading, a moisture reading and a quadrat score for the various plant
populations you find. You then try and link the changes in the abiotic factors to the changes in
distribution of the populations.
A transect study can be carried out anywhere that abiotic conditions show a distinct change – across a
stream, away from a pond, up a slope, away from a power station etc.
Food chains and food webs
Animals and plants rely on each other as part of the energy flow in an ecosystem.
Plants use the sun’s energy to build sugar. This sugar is the energy for the plant, but becomes food for
animals who feed on them. The energy from the plant’s sugar is used by the animals, which then
become food for other animals. The energy is therefore passed from one living thing to another, forming
a food chain.
The arrow symbol in the food chain shows the direction in which energy is being passed.
Lettuce
caterpillar
sparrow
hawk
The different organisms in the food chain can be described by the food their position.
The lettuce is a producer, as it uses the sun’s energy to produce the chemical energy for all the other
organisms in the food chain. Producers are always green plants, as only green plants can
photosynthesise.
The animals are all consumers. The caterpillar is the primary consumer and the sparrow the secondary
consumer.
Animals can also be described by what they eat.



Animals which eat only plant materials are herbivores
Animals which eat only animal material are carnivores.
Animals which eat both animal and plant material (e.g. seeds and worms) are omnivores.
It is very rare for animals to only have one source of food. This means that lots of food chains can
interconnect to make a more complicated diagram called a food web.
In a food web, you should be able to identify producers, primary and secondary consumers, herbivores,
carnivores and omnivores.
The second task you may be asked to do is to predict what would happen to various populations in the
food web if one was removed. These changes can be far reaching.
If the Chubb died out due to over-fishing –




There would be more shrimp, as there are fewer predators feeding on them
There would be fewer osprey, as they have lost their food source
There would be more sticklebacks, as there are fewer predators feeding on them
This is turn would affect the insects and the heron
Food chains and pyramids
There is a general rule which says that there must always be more prey than there are predators. If you
had a situation where there were 10 rabbits and 50 foxes, this would not create a stable situation. A
pyramid of numbers is often shown as a diagram to show the relative masses or the numbers at
different stages in the food chain.
Energy and food chains
At each stage in the food chain, the organism uses most of the energy that it has for its own living
processes. This means that when a cat eats a mouse, not all the mouse’s energy is being passed on.
Some has been used as movement, some as heat and same lost when the mouse produced its waste
products. This means that the cat gets as little as 10% of the energy which the mouse had.
If 90% of the energy is lost and not passed on at each stage, then the longer the food chain is, the more
energy is being lost along the way.
It is more efficient to have short food chains, rather than long food chains – so what should we do with
cereal?
Cereal
human
100%
10%
Cereal
pig
human
100%
10%
1%
More of the energy from the cereal reaches the human if the human eats the cereal directly. Since
energy is lost at each stage of the food chain, it is inefficient to eat animals which have been fed the
cereal. Much more energy is lost. This thinking could help solve the world’s food shortages.
The problem with food chains...
Food chains allow energy to be passed on from one organism to the next. Unfortunately, this is not all
that can be passed on. In the 1940s and 50s, biologists noticed a sharp drop in the numbers of birds of
prey. Investigations suggested that the birds were laying eggs which had very thin shells. This led to the
eggs breaking before the young were ready to hatch. Chemical tests of the birds showed very high levels
of DDT, a pesticide sprayed on crops to kill insects.
The birds of prey were carnivores – they had never eaten any crops sprayed with DDT. The DDT had
accumulated along the food chain, because at each level the consumer had eaten large numbers of the
previous organisms. The DDT then accumulated to toxic levels in the birds of prey
Huge numbers of producers,
each with a small dose of
DDT
Large numbers of primary
consumers (e.g. mice), each
with a large dose of DDT
small numbers of secondary
consumers (e.g. eagle), each
with a huge dose of DDT
Biodiversity
From your knowledge about habitats and food webs, you know that


Habitats usually support a variety of animals and plants
The numbers of each animal or plant in food webs are affected by changes to other populations
These two facts are the key to the concept of biodiversity. Biodiversity is a measure of the number of
different kinds of living things found in a habitat. The more different kinds of living things you can find,
the greater the biodiversity is.
Biodiversity gives stability to the community in an ecosystem. If there is a rich biodiversity, any
consumer will have a choice of several different types of food. If one dies out, the consumer can still
survive by using its alternative sources of food. This is a stable situation. A change in one population only
has a very limited effect on others.
If the ecosystem has low biodiversity, there will not be as great a choice of food for consumers. If one
food source dies out, there may not be sufficient numbers of other food sources to allow the consumer
to survive. This situation is much less stable – a change in one population has a much greater effect on
others.
In the natural situation, harsh environments generally show low biodiversity. This would be the case in
deserts or the polar regions, for example, where few types of living things can survive. Richer habitats
can sustain a much greater variety of types of living things. Rainforests or woodlands would be
examples. In addition to these natural factors, humans are also affecting biodiversity by changing the
environment.
Human effects on biodiversity
Some examples of how human activity has affected biodiversity are Over-fishing – reduces biodiversity as some fish species are selectively removed from the food web.
Deforestation – reduces biodiversity as rich forest habitats are removed. This can also disrupt the water
cycle, meaning less rain, and the removal of plant roots which bind the soil can contribute to landslides
and mud slips.
Desertification – reduces biodiversity as the habitat becomes less able to support living things. This is
caused by over-use of water for industry, irrigation of crops or intensive farming. The water table gets
lower, so plants are unable to reach the water, further underground
Habitat destruction – can be of different types, but tend to reduce biodiversity as richer habitats are
replaced by poorer ones. Examples are deforestation and removal of hedgerows.
Intensive farming – reduces biodiversity as the farmer concentrates on a limited number of cash crops.
Other plant and animal species are actively targeted using pesticides and herbicides (weedkillers).
Intensive farming also removes nutrients from the soil, meaning plants find it harder to grow without
help from artificial fertilisers.
Pollution – most forms of pollution make the environment harder to survive in, so they reduce
biodiversity. Examples are herbicides, pesticides and fertilisers being washed off fields into rivers;
sulphur dioxide from burning fuels killing plants; oil spills damaging rock pools and killing seabirds
Biomes
Biomes are different world habitats. They are defined by the environmental
conditions that you find in them. These conditions then affect what living things are
able to survive in that particular world habitat.
The biomes are found in different parts of the plant, often determined by how close to
the equator or the polar regions they are.
The main biomes are 




deserts
forests - tropical, temperate and boreal
tundra
grasslands
aquatic (including the marine environment)
Deserts
Deserts cover about one fifth of the
Earth’s surface and occur where rainfall is
less than 50 cm/year. Most deserts, such
as the Sahara of North Africa and the
deserts of the southwestern U.S., Mexico,
and Australia, occur fairly close to the
equator. Most deserts have many
specialized plants, as well as specialized
animals. Soils often have plenty nutrients
and need only water to become very productive. Disturbances are common in the form
of sandstorms and intense rains that cause flooding.
There are relatively few large mammals in deserts because most are not capable of
storing sufficient water and withstanding the heat. Deserts often provide little shelter
from the sun for large animals. The dominant animals of warm deserts are vertebrates,
such as reptiles. Mammals are usually small, like the kangaroo mice of North American
deserts.
Desert surfaces receive a little more than twice the solar radiation received by humid
regions and lose almost twice as much heat at night. The extreme maximum ranges
from 43 - 49° C. Minimum temperatures sometimes drop to -18° C.
Rainfall is usually very low and/or concentrated in short bursts between long rainless
periods. The rain that does fall evaporates very quickly in the heat.
Soils are course-textured, shallow, rocky or gravely with good drainage and have no
surface water. They are coarse because there is less chemical weathering. The finer
dust and sand particles are blown elsewhere, leaving heavier pieces behind.
Coverage by plants in most deserts is very rare. In the cacti, the leaves are muchreduced (to spines) and photosynthesis is restricted to the stems.
The animals include small nocturnal (active at night) carnivores. The dominant animals
are burrowers and kangaroo rats. There are also insects, spiders, reptiles (liards and
snakes) and birds. The animals stay inactive in protected hideaways during the hot day
and come out to forage at dusk, dawn or at night, when the desert is cooler.
Forests
There are three major types of forests, classed according
to how close to the equator or polar regions the forest is:
 tropical (close to equator)
 temperate (between polar and equator)
 boreal forests (close to the polar regions)
Tropical forests
Tropical forests are characterized by the greatest
diversity of species. They occur near the equator. Only
two seasons are present (rainy and dry). The length of
daylight is 12 hours and varies little.
Temperature is on average 20-25° C and varies little throughout the year: the
average temperatures of the three warmest and three coldest months do not
differ by more than 5 degrees.
Rainfalln is evenly distributed throughout the year, with annual rainfall exceeding
2000 mm.
Soil is nutrient-poor and acidic. Decomposition is rapid and most nutrients are
washed out of the soil very quickly.
Canopy in tropical forests made of many overlapping layers, allowing little light to
get down to the forest floor.
Plants are present in a huge variety of types.: one square kilometre may contain
as many as 100 different tree species. Trees are 25-35 m tall, with thick trunks
and shallow roots, mostly evergreen, with large dark green leaves. Plants such
as orchids, bromeliads, vines (lianas), ferns, mosses, and palms are present in
tropical forests.
Animals include numerous birds, bats, small mammals, and insects.
More than ½ of tropical forests have already been destroyed by clearing and
deforestation.
Temperate forests
Temperate forests occur in eastern North America, northeastern Asia, and western and
central Europe. There are well-defined seasons with a distinct summers and winters..
Temperature varies from -30° C to 30° C.
Rainfall (75-150 cm) is distributed evenly throughout the year.
Soil is fertile, enriched with decaying litter.
Canopy is moderately dense and allows light to penetrate, resulting in welldeveloped and lots of biodiversity on the forest floor.
Plants show 3-4 tree species per square kilometer. Trees have broad leaves that
are lost annually and include such species as oak, hickory, beech, hemlock,
maple, basswood, cottonwood, elm, willow, and spring-flowering herbs.
Animals such as squirrels, rabbits, skunk, birds, deer, mountain lion, bobcat,
timber wolf, fox, and black bear.
Only scattered remnants of original temperate forests remain.
Boreal forests
Boreal forests, or taiga, represent the largest biome on Earth. They are found half way
between the equator and the poles. Seasons are divided into short, moist, and
moderately warm summers and long, cold, and dry winters.
Temperatures are very low.
Rainfall is usually in the form of snow, 40-100 cm annually.
Soil is thin, nutrient-poor, and acidic.
Canopy permits low light penetration, and as a result, understory is limited.
Plants consist mostly of cold-tolerant evergreen conifers with needle-like leaves,
such as pine, fir, and spruce.
Animals include woodpeckers, hawks, moose, bear, weasel, lynx, fox, wolf,
deer, hares, chipmunks, shrews, and bats.
Boreal forests are under threat from logging where trees are felled as a cash crop.
Grasslands
Grasslands are dominated by grasses rather
than large shrubs or trees. There are two
main types of grasslands:
(1) tropical grasslands, called savannas
(2) temperate grasslands.
Savanna
Savanna is grassland with scattered individual trees. Savannas of one sort or another
cover almost half the surface of Africa (about five million square miles, generally central
Africa) and large areas of Australia, South America, and India. Climate is the most
important factor in creating a savanna. Savannas are always found in warm or hot
climates where the annual rainfall is from about 50.8 to 127 cm (20-50 inches) per year.
They tend to be found quite near the equator.
In Africa, a heavy concentration of elephants in protected parkland have created a
savanna by eating leaves and twigs and breaking off the branches, smashing the trunks
and stripping the bark of trees. Elephants can convert a dense woodland into an open
grassland in a short period of time. Annual fires then maintain the area as a savanna.
The soil of the savanna shows rapid drainage of water. It has only a thin layer which
provides vegetation with nutrients. The main vegetation is grass. Savannas receive an
average annual rainfall of 76.2-101.6 cm (30-40 inches). However, certain savannas
can receive as little as 15.24 cm (6 inches) or as much as 25.4 cm (10 inches) of rain a
year.
Savanna has both a dry and a rainy season. Seasonal fires play a vital role in the
savanna’s biodiversity. In October, a series of violent thunderstorms, followed by a
strong drying wind, signals the beginning of the dry season. Fire is prevalent around
January, at the height of the dry season. Fires in savannas are often caused by
poachers who want to clear away dead grass to make it easier to see their prey. The
fires do not devastate the community. Most of the animals killed by the fires are insects
with short life spans. A fire is a feast for some animals, such as birds that come to sites
of fires to eat grasshoppers, stick insects, beetles, mice, and lizards that are killed or
driven out by the fire. Underground holes and crevices provide a safe refuge for small
creatures. Larger animals are usually able to run fast enough to escape the fire.
Although the dry stems and leaves of grasses are consumed by fire, the grasses’ deep
roots remain unharmed. These roots, with all their food reserves, are ready to send up
new growth when the soil becomes more moist. The scattered shrubs can also survive
on food reserves in their roots while they await the time to grow above the soil again.
During March, violent thunderstorms occur again, this time signalling the rainy season.
When the rains come, savanna grasses grow very quickly. Some of the larger grasses
grow an inch or more in 24 hours. The savannas experiences a surge of new life at this
time. For example, many antelope calves are born. With so much grass to feed on,
mothers have plenty of milk. Calves die if the rains fail to come.
Other animals include giraffes, zebras, buffaloes, kangaroos, mice, moles, gophers,
ground squirrels, snakes, worms, termites, beetles, lions, leopards, hyenas, and
elephants.
There are also some environmental concerns regarding savannas such as poaching,
overgrazing, and clearing of the land for crops.
Temperate Grassland
Temperate grasslands also have grasses as the dominant plants. Trees and large
shrubs are absent. Temperatures vary more from summer to winter, and the amount of
rainfall is less in temperate grasslands than in savannas. the plains and prairies of
central North America. Temperate grasslands have hot summers and cold winters.
Rainfall is moderate. The amount of annual rainfall influences the height of grassland
vegetation, with taller grasses in wetter regions. Temperate grasslands are found mod
way between the equator and polar regions.
Rainfall in the temperate grasslands usually occurs in the late spring and early summer.
The annual average is about 50.8 to 88.9 cm (20-35 inches). The temperature range is
very large over the course of the year. Summer temperatures can be well over 38° C
(100 degrees Fahrenheit), while winter temperatures can be as low as -40° C (-40
degrees Fahrenheit).
The animal populations include gazelles, zebras, rhinoceroses, wild horses, lions,
wolves, prairie dogs, jack rabbits, deer, mice, coyotes, foxes, skunks, badgers,
blackbirds, grouses, meadowlarks, quails, sparrows, hawks, owls, snakes,
grasshoppers, leafhoppers, and spiders.
There are also environmental concerns regarding the temperate grasslands. Few
natural prairie regions remain because most have been turned into farms or grazing
land. This is because they are flat, treeless, covered with grass, and have rich soil.
Tundra
Tundra is the coldest of all the biomes.
Tundra is a treeless plain. It has extremely
low temperatures, little precipitation, poor
nutrients, and short growing seasons.
Characteristics of Tundra
1.
2.
3.
4.
5.
6.
Extremely cold climate
Low biodiversity
Simple vegetation structure
Limited drainage
Short season of growth and reproduction
Large population swings
Arctic
Arctic tundra is located in the north, surrounding the north pole and extending south to
the coniferous forests of the taiga. The arctic is known for its cold, desert-like conditions.
The growing season is only from 50 to 60 days. The average winter temperature is -34°
C (-30° F), but the average summer temperature is 3-12° C (37-54° F) which enables
this biome to sustain life. Rainfall may vary in different regions of the arctic. Yearly
rainfall, including melting snow, is 15 to 25 cm (6 to 10 inches). A layer of permanently
frozen subsoil called permafrost exists, consisting mostly of gravel and finer material.
This layer of permanently frozen soil means there are no deep root systems possible,
so there are no trees or shrubs. There are about 1,700 kinds of plants in the arctic and
subarctic, and these include:



reindeer mosses, liverworts, and grasses
400 varieties of flowers
lichen
All of the plants are adapted to sweeping winds. Plants are short and group together to
resist the cold temperatures and are protected by the snow during the winter. The
growing seasons are short. The animal life in the arctic tundra is quite varied:





herbivores: lemmings, voles, caribou, arctic hares and squirrels
carnivores mammals: arctic foxes, wolves, and polar bears
Migratory birds: ravens, snow buntings, falcons, loons, ravens, sandpipers, terns,
snow birds, and various species of gulls
Insects: mosquitoes, flies, moths, grasshoppers, blackflies and arctic bumble
bees
Fish: cod, flatfish, salmon, and trout
Animals are adapted to handle long, cold winters and to breed and raise young quickly
in the summer. Animals such as mammals and birds also have additional insulation
from fat. Many animals hibernate during the winter because food is not abundant.
Another alternative is to migrate south in the winter, like birds do. Reptiles and
amphibians are few or absent because of the extremely cold temperatures. Because of
constant immigration and emigration, the population is very different in the summer and
the winter.
Aquatic Biomes
Water is the common link among the five biomes and it makes up the largest part of the
biosphere, covering nearly 75% of the Earth’s surface.
The aquatic biome can be broken down into two basic regions, freshwater (i.e, ponds
and rivers) and marine (i.e, oceans and estuaries).
Freshwater Regions
Freshwater is defined as having a low salt concentration
Ponds and Lakes
These regions range in size from just a few square meters to thousands of
square kilometers.
The surface zone is the warmest since it is shallow and can absorb more of the
Sun’s heat. It sustains of algae (like diatoms), rooted and floating aquatic plants,
grazing snails, clams, insects, crustaceans, fishes, and amphibians. In the case
of the insects, such as dragonflies and midges, only the egg and larvae stages
are found in this zone. The vegetation and animals living in this zone are food for
other creatures such as turtles, snakes, and ducks.
Temperature varies in ponds and lakes seasonally. During the summer, the
temperature can range from 4° C near the bottom to 22° C at the top. During the
winter, the temperature at the bottom can be 4° C while the top is 0° C (ice).
Streams and Rivers
These are bodies of flowing water moving in one direction. The characteristics of
a river or stream change during the journey from the source to the mouth. The
temperature is cooler at the source than it is at the mouth. The water is also
clearer, has higher oxygen levels, and freshwater fish such as trout can be found
there. Towards the middle part of the stream/river, the width increases, as does
biodiversity—many water plants and algae can be found. Toward the mouth of
the river/stream, the water becomes murky from all the mud that it has picked up
upstream. Since there is less light, there is less plant life, and because of the
lower oxygen levels, fish that require less oxygen, such as catfish and carp, can
be found.
Marine Regions
Marine regions cover about three-quarters of the Earth’s surface and include
oceans, coral reefs, and estuaries. Marine algae supply much of the world’s
oxygen supply and take in a huge amount of atmospheric carbon dioxide. The
evaporation of the seawater provides rainwater for the land.
Oceans
The largest of all the ecosystems, oceans are very large bodies of water that
dominate the Earth’s surface. Like ponds and lakes, the ocean regions are
separated into separate zones.
The intertidal zone is where the ocean meets the land—sometimes it is
submerged and at other times exposed, as waves and tides come in and out.
Because of this, the communities are constantly changing.
The water near the surface is generally cold. The plants in this zone include
surface seaweeds. The fauna include many species of fish and some mammals,
such as whales and dolphins. Many feed on the abundant plankton.
The deep water zone consists of sand, dead animals and plants and the
scavengers who feed on them. As you get deeper, the water gets colder, less
light can penetrate and the pressure increases.
Coral Reefs
Coral reefs are widely distributed in warm shallow waters. They can be found as
barriers along continents (e.g., the Great Barrier Reef off Australia), fringing
islands, and atolls. Naturally, the dominant organisms in coral reefs are corals.
Corals are interesting since they consist of both algae (zooanthellae) and tissues
of animal polyps. Besides corals, the fauna include several species of
microorganisms, invertebrates, fishes, sea urchins, octopuses, and sea stars.
Estuaries
Estuaries are areas where freshwater streams or rivers meet the ocean. This
mixing of waters with such different salt concentrations creates a very interesting
and unique ecosystem. Estuaries support a variety of animals , including a
variety of worms, oysters, crabs, and waterfowl. Many animals, such as wading
birds, can feed here because of the rich nutrients brought down to the estuary by
the rivers.
Population growth
The size of a population is worked out by two numbers – the death rate and the birth rate. As long as
the birth rate is greater than the death rate, the population will be growing. If they are the same, the
population will stay exactly the same.
The way that a population grows always produces a graph of the same shape.
A small number of rabbits are washed
up on a desert island. There is plenty
of grass, fresh water and soft sand for
digging burrows.
numbers
The population stays the same at the
start, as it takes 10 weeks to produce
a baby rabbit.
time
The population begins to grow. There
are only a small number of rabbits so
they can only produce a few babies at
a time. As the population grows, they
can produce more and more babies
numbers
time
Now we are in trouble. The
population is so large that they are
running short of food and water.
Waste products are building up at an
alarming rate, and this spreads
disease. News of their success has
attracted predators , such as foxes,
who have now come to the island to
numbers
enjoy their new food supply. The
death rate now increase and the birth
rate decreases until the population
time
levels off.
From the story, you should be able to identify four reasons why populations cannot show unlimited
growth. These are the factors which limit the size of a population.
The population has now reached a size where it is stable for its habitat. Changes in the habitat (e.g.
drought) can change the size of the populations that it can support.
The Nitrogen cycle
The nitrogen cycle is an example of how chemical elements are re-cycled within an
ecosystem. This is important to the ecosystem, as many essential chemical elements are
in short supply.
The most common form of nitrogen is nitrogen gas. This takes up roughly 80% of the
atmosphere. This form of nitrogen cannot be used by animals or plants. You breathe in
nitorgen from the air, but breathe it straight back out again.
The main form of nitrogen found in the soil is nitrate. This is formed by nitrogen being
joined to oxygen. Nitrates can be taken in by green plants as it dissolves in soil water,
so is taken in through the roots. The plant uses this nitrate to build proteins in its body.
Some plants also store protein in their seeds.
The plants are then eaten by animals, which change the plant protein in to animal
protein. This can, in turn, be eaten by other animals.
Animals and plants eventually die, and animals produce quantities of waste products.
The dead and waste materials decay in the soil, and are converted first to ammonia,
which is harmful to most living things. This breakdown is the job of decomposers;
bacteria and fungi which feed on the dead and decaying material, and break it down.
Other bacteria in the soil make use of the ammonia and convert it into nitrates. These
are making nitrogen more available to living things, so are called nitrifying bacteria.
The nitrates can, again, be taken in by plants, completing the cycle.
Unfortunately, a different group of bacteria in the soil do the opposite – the denitrifying
bacteria. These take useable nitrates in the soil and convert it to nitrogen gas in the
atmosphere, which animals and plants cannot make use of.
The final group of bacteria involved in the nitrogen cycle are the nitrogen fixing
bacteria. These very specialised bacteria live in the roots of legumes – plants with
pods, such as peas and beans. They form root nodules on the root. These bacteria are
the only living things which can make use of nitrogen in the air, which they convert to
nitrates. These can then be used by the plant.
Growing legumes can be a natural way for farmers to return nitrogen to the soil, making
it better for other crops in the following years. The farmer can also increase the quantity
of dead and decaying material in the soil by spreading manure. This gives a new
source of nitrogen for the decomposers in the soil, which eventually leads to more
nitrate being made available to plants.
In the exam, you would be expected to complete labelling on a nitrogen cycle diagram
and to explain the role of the different types of bacteria and fungi involved.
Nitrogen gas
Animal protein
Legume (pods)
Nitrogen fixing
bacteria in
root nodules
Dead and decaying
material
Plant protein
(Decomposers)
Denitrifying
bacteria
Nitrates
Nitrifying
bacteria
Fertilisers
Farmers can add manure to soil in order to increase the quantities of dead and decaying
material for decomposers in the soil. Nutrients can also be supplied to plants in fertilisers. This
can be on a large scale – fields of crops, or on a small scale – pot plants in home.
There are three essential nutrients that fertilisers supply –



Nitrogen (as nitrates) – for leaf growth
Phosphorus (as phosphates) – for root growth
Potassium (chemical symbol K) – for flower and fruit growth
Different fertilisers contain different ratios of Nitrogen, Phosphorus and K(potassium), and this
is always shown on the label of the fertiliser.
A high Nitrogen fertiliser 25:10:5 might be used for a lawn, where the strong green leaf is the
most important thing.
A high potassium fertiliser 5:5:15 might be used for tomatoes, where the flowers, and the fruits
which develop from them, are the most important part of the plant.
By selecting a fertiliser with the best N:P:K ratio for their particular plant, gardeners can target
specific parts of the plant to grow best.
The over-use of fertilisers can lead to problems in the environment. If farmers rely on putting
fertiliser on crops every year,the soil becomes exhausted with all its natural elements used up.
Unless the farmer puts fertiliser on the soil, it will be too poor for anything to grow.
If large quantities of fertilisers are applied to fields, the fertiliser can we washed out of the soil by
rain and make its way into rivers and lakes, where it builds up. This can make the plants in the
lake grow much faster than normal and can cause algal blooms, where microscopic algae in
the water grow out of control, making the water a thick green colour. These algae absorb other
nutrients from the water, making it harder for other plants to grow, and can produce chemicals
that are toxic to animals. This drastically reduces the biodiversity of these lakes.
Natural disasters
Natural disasters are disasters caused by the structure of the Earth, or its climate.
Weather related disasters tornadoes can include – hurricanes, and. Weather can also contribute to
wildfires, drought and flooding.
Hurricanes are huge weather storms, hundreds of miles across. They are generated by air currents and
pressure changes around the equator, and the hurricanes then move either north or south towards to
poles. Moving North makes them move over far more populated areas than when they head south.
Hurricanes can cause rotating winds of up to 140 km per hour, which can damage forest and woodland
habitats. As they pass over oceans, the storms also pick up large quantities of moisture, so they bring
lots of rainfall. This, along with large waves picked up by the wind, can cause considerable flooding near
the coasts. This was the case with hurricane Katrina which struck the south-east of America in 2011.
Tornadoes are very localised areas of rotating winds. The tornado is typically only a mile or two across,
but the winds can be up to 300 km per hour. The winds are very damaging to forests and property.
Tornadoes usually form over land, from storm systems. They are so common in a strip across North
America that it is known as tornado Alley. Tornadoes are very unpredictable, both in terms of when they
form and the path that they take.
High temperatures, low rainfall and high winds can combine to increase the chances of wild fires. Wild
fires are often the result of careless human activity, but can also be caused naturally – by lightning, for
example. Wildfires can damage huge areas of grass land or forests, causing habitat loss.
Flooding is also caused by combinations of factors. High rainfall, snow melt and tidal surges can all cause
flooding. Flooding is often made worse by human activity – blocking natural water courses and diverting
rivers for irrigation, for example. Flooding can cause damage to property, and also has a devastating
effect on agriculture, rotting crops while they are still in the soil.
The structure of the Earth itself can also contribute to natural disasters. The surface of the earth is made
up of a number of tectonic plates. Slight movements of these colossal plates can lead to earthquakes. If
the earthquake takes place under the sea, the movement of the plates can generate huge tidal waves
called tsunamis, which can cause huge amounts of damage and widespread folding in coastal regions.
Another feature of where plates meet is the occurrence of volcanoes. The joins between the plates
create weak spots where red hot magma from deep under the surface can break through, causing a
volcano. Volcanic activity can damage environments by the action of lava, but also due to the huge
clouds of volcanic ash which can affect photosynthesis in plants. The ash is, however, useful in some
ways. It is very rich in minerals, so can help to make the soil more fertile in the long run.
Areas of particular volcanic and earthquake activity are around the ‘ring of fire’, which runs through Asia
and down the East coast of America.
Animal behaviour
Animal behaviour can be classified as two types.
Innate behaviour is in-built behaviour, which the animal is born with. Innate behaviour tends to have a
very high survival value. It is the main type of behaviour shown in fish and amphibians. Examples would
be reactions to light, heat or moisture by meal worms, woodlice or worms. More complicated animals
such as mammals also show innate behaviour – the sucking reflex in babies, for example.
Learned behaviour is behaviour which is learned either by experience or by imitating parents or
members of social groups. These behaviours also have a survival advantage, but are not necessarily
things an animal will need straight away when it is born. Examples would be hunting strategies in
mammals or foraging behaviour in monkeys, for example.
A characteristic of learned behaviour is that it allows tasks to be completed more efficiently or quickly.
This can be demonstrated in humans using simple jigsaw tasks or mirror writing.
Group behaviour is to do with animals being able to operate more effectively or efficiently as a group
than individually. The behaviour may simply be to form a group, or the group may work together, with
different animals having or sharing different roles.
Animals may swarm as a feeding response. This is the case with locusts, where large quantities of food
becomes available in a very short growing season.
Some animals swarm to form groups for migration. This is the case for birds which gather to fly south
for the winter, or the migration of wildebeest in Africa to follow the rains. This is the biggest migration
on the planet.
Many animals form groups as a defence against predators. Large numbers of moving animals makes it
harder for a predator to single out prey. This can be compounded by animal colours or patterns. Group
formation can be very organised. Musk ox form a defensive circle with the young protected in the centre
if they are threatened by wolves. The preys’ survival chances increase substantially if they are able to
stay together as a group. The predators’ tactics are to do the opposite. They try to panic the group to
break it up. They then have a better chance of selecting an individual prey – particularly a young or sick
animal. The predators themselves stand a better chance if the work as a team. Wolves hunt as packs,
with different animals taking turns in the lead position, to conserve energy. Lions hunt as a team with
some chasing animals and others lying in ambush. Working as a team lets predators take prey much
larger than themselves with an improved chance of success.
Social organisation
If animals exist in social groups, they work for the good of the group as well as for their own benefit.
Social animals will be able to recognise members of their own group and will defend their group against
outsiders. One factor which contributes to the success of social groups is the establishment of a social
order (hierarchy) to establish who is dominant over who. This establishes who has the right to choose
mates within the group and who gets first turn at feeding. This means that the strongest and fittest in
the group survive best and breed most, making the production of healthy offspring increasingly likely.
In such well-organised social groups animals often take on different roles within the group, working for
the good of the group as a whole. Meerkats, for example, will take turns acting as sentries so that the
rest of the group can forage in safety. Females take it in turn to stay below ground with the young for
protection, giving all a turn at foraging and feeding. Similar shared upbringing can be seen in lions,
primates and elephants.
Communication
In social groups, communication can come in a variety of forms –
Visual displays – breeding colours and displays in birds and fish
Sound – whales and dolphins
Smell – animals often signal readiness to breed by releasing scents. Ants leave scent trails for others to
follow to find food sources.
A simple example of a visual display as a form of communication is the waggle dance in bees.
The bee is trying to communicate where it has found flowers with a good source of nectar. Bees have no
sound communication and are notoriously bad at using sat nav.
The bee performs a dance to show distance and direction to direct others in the hive to the flowers.
The bee waggles its body as it follows a line inside the hive. The strength of the waggle indicates the
distance to the flowers. The more the bee vibrates its body, the further the flower is. The direction that
the bee takes is shows the angle that the bees need to fly relative to the sun. This is the direction to the
flowers.
After other bees have visited the flowers, they return to the hive and repeat the dance. This reinforces
the information for other worker bees to be able to find the flowers too.