School Science Lessons
2024-10-27
(UNBiol1a)
Communities, food chains, populations
Please send comments to: j.elfick@uq.edu.au
Contents
9.1.1 Ecology
9.1.2 Desert community
9.1.3 Community of aquatic organisms
6.1 Forest food chains (Primary)
9.1.4 Forest floor community
9.1.5 Levels of organization
9.1.6 Meadow community
9.1.7 Pond community
9.1.8 Rotting log community
9.1.9 Succession in a pond community - hay infusion culture
9.1.10 Human population growth
9.1.1 Ecology
Ecology is the study of organisms, the environment and how the organisms interact with each other and their environment. There are many definitions of ecosystem and some ecologists have rejected the term "ecosystem" entirely - in the same way as they have discredited "balance of nature" and "stability of nature", because these terms do not describe reality.
They say that there is no such thing as "self regulating ecosystems" - and that the numbers of species in a community just fluctuate with chaotic instability.
9.1.2 Desert community
See diagram 9.3.37: Desert community.
Get sand from a beach or garden supply store.
Some kinds of desert animals - including horned lizards - can be found in pet shops.
The lizards will eat small insects - e.g. ants and meal worms - available from pet shops.
Get small cacti and other succulents
s that hold water in their fleshy leaves.
Put rocks in the terrarium - making cliffs or overhangs near the edges.
Put a small dish of water in one corner.
Leave an open area of sand in the centre - especially if you have a horned lizard.
Keep the temperature of the desert terrarium between 20oC and 27oC.
9.1.3 Community of aquatic organisms
See diagram 9.39.1d: Filamentous algae.
An aquarium with fish - snails and Elodea
s - can manipulate the oxygen - CO2 content through adding / taking out organisms - multiplying them - changing water temperature - light levels / illumination periods - and other variables.
1. Study communities
A grouping of populations in a particular location is called a community.
Typically - communities consist of
s and animal populations that have certain roles.
Some populations are the producers.
They are so called, because they can trap energy from sunlight and producing food.
Populations that feed on other living populations are called consumers.
hose populations that feed on dead material are called reducers - since they disorganize organic matter to yield simpler chemical substances.
2. Establish natural communities
Use a closed plastic container or a fish tank with a glass lid so that only light can enter.
Seal the lid with melted wax.
Submerge the container in water to show that the system is not open to air.
Try to create a balanced community so that the different kinds of organisms survive for a long time.
Select a community to enclose - e.g. a square spade width of your garden or lawn - a forest floor community - ferns and liverworts - a dead animal - a rotting log - water from a pond.
3. Study living things
Study them both in the classroom or laboratory - especially aquatic
s and animals by making an aquarium for aquatic organisms.
Make it ready in advance - so that you may put samples taken from a visit to a pond or stream in it upon return.
4. Simple aquarium
Use a large glass tank for a simple aquarium if it is well stocked with submerged water
s to aerate the water - e.g. Elodea or Myriophyllum.
Use a jam container for keeping caddis larvae - pond snails - small crustaceans and
s.
The pond life will remain balanced if carefully stocked.
Feed Dytiscus beetles or other predacious larva on tadpoles and keep in a separate tank.
Use 3 cm clean sand to provide hibernating quarters for the caddis flies at the bottom of the container - and attach a muslin cover to ensure that the caddis flies do not escape.
Record egg laying - other changes - and habits.
Use a strainer or net to collect aquatic specimens.
Do not put an aquarium in direct sunlight, because excessive light produces a heavy growth of algae on the glass walls that obscures the contents of the aquarium.
Wipe off algae growths with an abrasive dish cloth.
5. Large aquarium
Find fine silt from the bottom of a clear stream or pond and wash it carefully in running water.
Use it to cover the floor of the aquarium to a depth of 3 cm.
Plant water
s and weigh down the roots with stones.
Add coarse sand - gravel and stones for hiding places.
To reduce cloudiness - fill with a slow stream of water falling on a sheet of cardboard and leave to stand for a day or two until clear.
Then washed sand and aquarium plant, e.g. Elodea, - and weigh down the roots with stones.
If many water weeds are present aerating by pumps is not needed.
Add live food - e.g. Daphnia - and add snails to keep the glass clean.
Very little feeding will be necessary.
Fish will eat the snails' eggs and small water organisms introduced with the water
If worms are used as food - add them only once a week.
Cut them in pieces small enough to eat.
Remove food not consumed immediately or fungi will grow and infect the fish.
Cover the aquarium with a glass plate to keep out dust.
If frogs or newts are kept - put in a floating piece of cork to sit on.
Do not put the fish tank in direct sunlight to avoid heavy growth of algae on the glass walls.
9.1.4 Forest floor community
See diagram 9.3.39: Forest floor community.
This is the kind of habitat most often modelled in a terrarium.
For
s - obtain small ferns - tree seedlings - wildflowers - and especially evergreen
s - e.g. partridge berry or wintergreen.
Put a few of these
s into the soil and cover the rest of the surface with mosses - attractive stones - and perhaps a small limb.
For animals - look for small toads - frogs - e.g. cricket frogs or tree frogs - and red newts - small salamanders.
These animals and the
s of the forest floor all need moisture - so keep the terrarium watered and make a small woodland pool in one corner.
9.1.5 Levels of organization
Structure and function at cell - organ and body system levels
Life can be understood as a natural order of living things - groups of living things - and parts of living things.
Organisms are individual life forms - e.g. a dog - tree - fish - earthworm - mushroom - or yeast cell.
At both the upper level of organization - the biosphere - and the lower level - the possibility of another level of organization is uncertain.
Students will study life most frequently at the central levels of organization - near the level occupied by organisms.
Conceptual scheme:
Group of organisms:
1. Biosphere
2. Biome
3. Community
4. Population
5. Organism
6. Organelle
Parts of organisms
7. Macromolecule - e.g. chlorophyll
8. Molecule
9. Atom
10. Atomic particle.
Higher levels of organization
1. Population
A group of organisms comprising all of a particular kind is called a population.
A sub population refers to the space that it occupies.
For example - one may refer to the snail population in a classroom aquarium - or the population of that kind of snail in a pond.
If no space is mentioned - it is assumed that the population consists of all snails of that type in the world.
2. Community
Populations do not exist in isolation.
They are commonly found in an environment that they share with other populations.
All the populations within a defined space form a community.
A lake community consists of all the
and animal populations found in the lake.
The populations found in school grounds would be a community.
3. Biome
Certain large areas of the earth contain communities that are similar.
This collection of similar communities is called a biome.
A biome may occupy a large portion of a continent.
For example - a grassland biome is found in the central portion of North America or inland Australia.
Climate and topography are uniform across a biome.
4. Biosphere
Life on the earth is normally found within a few metres of the surface.
This hollow spherical space is the biosphere.
It contains all life on the planet.
Lower levels of organization
5. Organ systems
Animal organisms contain systems of organs that do vital functions - e.g. the circulatory system.
6. Organ
Most
s and animals contain basic structures called organs that in turn are composed of tissues - e.g. heart - leaf - lung - root.
Simple
s and animals may not have distinct organ systems.
7. Tissue
A tissue is a group of similar cells that do a single function - e.g. muscle tissues are composed of cells that can contract and produce the "pull" of the muscle.
Some organisms are composed of tissues - but do not have organs.
8. Cell
Tissues consist of individual units called cells.
The cell is the fundamental unit in most organisms.
Cells vary considerably in size from the largest - an ostrich egg - to one of the smallest micro-organisms.
Cells vary in their function and degree of specialization.
Organisms composed of a single cell are called unicellular organisms.
9. Organelle
Cells contain parts called organelles that you can easily see with a light microscope - e.g. the nucleus.
The electron microscope allows study of the structure of organelles.
10. Macromolecule
Organelles are composed of large molecules - macromolecules - e.g. proteins - lipids (fats and oils) and nucleic acids (DNA and RNA).
11. Molecule
Macromolecules are long chains of linked individual molecules.
A molecule is the smallest possible piece of a substance that retains the properties of the substance.
Molecules are composed of atoms joined or bonded together.
12. Atom
An atom is the smallest part of an element.
13. Atomic particle
Atoms are composed of fundamental particles - e.g. protons - neutrons - and electrons.
This is the present limit of understanding of organization at the lower level.
9.1.6 Meadow community
See diagram 9.3.38: Meadow community.
Use only few of the grasses - weeds - seedling trees - and other
s that grow in meadows.
Choose from the many animals.
Orb spiders need lots of room to make their webs - e.g. a 50 litre aquarium tank.
Find
s with insect eggs or cocoons on them and watch them to see what hatches.
A small snake will eat earthworms and large insects, but keep the terrarium dry, because snakes often get skin diseases if kept in damp surroundings.
9.1.7 Pond community
See diagram 9.3.40: Pond community.
An ecosystem is the living community plus the non-living surroundings.
An ecosystem is studied by observing and measuring relationships between its various subsystems.
For example - a pond community contains a great variety of
s (producers) - animals (consumers) - and decomposing micro-organisms (reducers).
Observe the feeding habits and dissect organisms' stomach contents to understand the food chain in the ecosystem without destroying the ecosystem being studied.
Beware of using inference instead of direct observations.
The presence of a frog and a bee in the pond ecosystem may to the conclusion that a link on the food chain is bee to frog.
However - the bee may not be eaten by frogs and would never appear in the frog stomach contents.
9.1.8 Rotting log community
See diagram 9.3.36: Rotting log community.
Break open a rotting log with a trowel - put two or three chunks into a plastic bag - and take them back to put in the terrarium.
Construct a terrarium from an aquarium with a cloth cover.
No soil is needed.
If the log was in a damp place - add water to the terrarium from time to time.
Many creatures may live in the log including ants - termites - spiders and horned beetles.
If the log contains ants - provide a few crumbs and sugar water on a piece of sponge for them.
To keep the ants from crawling out of the terrarium - spread a layer of Vaseline along the upper edge.
Water to see what kinds of insects and other animals come from the log.
Some may be eggs when you collect the log and may develop into adults while in the terrarium.
9.1.9 Succession in a pond community - hay infusion culture
Phylum Ciliophora - ciliates (have cilia) - Paramecium - Vorticella - Colpoda - Tetrahymena - Balantidium
1. Put dry grass in boiled water in two sealed containers.
Keep one container in the light and the other in the dark.
Examine the container daily with the eye - with a magnifying glass and examine a water sample with a microscope.
At first see bacteria - later ciliated protozoa and later rotifers - nematodes and crustaceans.
Note the disappearance of populations and the appearance of new populations.
Compare gross changes seen with the eye to the changes seen with the microscope.
2. Use the hanging drop technique.
Dip the open end of a test-tube in petroleum jelly to make a ring on the centre of a microscope slide - slightly smaller than the size of a coverslip.
Put the sample drop of water on the centre of the coverslip.
Pick up the coverslip and invert it so that the drop hangs down.
Lower the coverslip over the microscope slide so that the petroleum jelly supports the coverslip.
Examine the contents of the hanging drop with low power.
3. To culture pond organisms - dissolve 1/2 teaspoon of bakers' yeast in 1 litre of boiling water and add some vegetable - e.g. peas.
Inoculate the solution at room temperature and keep in indirect sunlight.
4. Combine or average the data derived from a ten day population growth study and graph the results for the entire class.
(Remember that the two-day-old culture was started on the eighth day!).
Compare the results obtained with yeast populations with a curve of human population growth.
If a microscope is not available for yeast cell counting - compare daily counts of fruit flies or some other available population that grows rapidly.
5. Put dead grass or fresh hay in a jar - cover with water and leave it uncovered to stand at room temperature for some days.
Do not use mouldy hay, because it may contain pathogenic fungi.
After a few days - a skin forms on the surface of the water - the mould pellicle.
After eight days - use a glass rod to transfer a part of the pellicle and some fluid to a microscope slide.
Spread out the pellicle evenly and apply a coverslip.
Examine the pellicle under the microscope.
Observe rod-shaped or spherical unicellular bacteria.
Observe unicellular organisms moving very rapidly through the field of view - e.g. Paramecium.
9.1.10 Human population growth
Compare the results obtained with yeast populations with a curve of human population growth.
If a microscope is not available for yeast cell counting - compare daily counts of fruit flies or another available population that grows rapidly.
Let b = birth rate - d = death rate - and r = rate of natural increase.
So if birth rate is 14 per 1000 per year and death rate is 8 per 1000 per year - the rate of natural increase is 6 per thousand - 0.6%.
In February 2008 - the total human population was estimated at almost 7 billion - 7 000 000 000.
However - the rate of increase has declined since the 1963 peak of 2.2% per year.
In 1798 - the Rev. T. R. Malthus (1766-1843) published a famous "Essay on population" - which included the idea that population tends to outrun the means of subsistence.
He advocated late marriage and sexual continence to control the increase of population.
However - he may not have realized that the apparent increase in population was influenced by the decrease in death rate.
Nowadays - an important factor in population growth is that people in developing countries are living longer.