25 August 2026

Momentum on an Air Track — Watching Conservation Laws Happen in Front of You

There are some experiments in physics where the result is mathematically satisfying but visually rather unimpressive.

Momentum on an air track is not one of them.

Two gliders move towards one another, collide, and separate. Sometimes one stops while the other carries on. Sometimes both rebound. Sometimes they stick together and continue as a single object.

It happens in a fraction of a second.

Yet hidden inside that brief collision is one of the most important ideas in physics:

momentum is conserved.

What makes the air track particularly powerful is that we do not have to accept this merely because a textbook tells us it is true. We can measure the velocities immediately before and after a collision, calculate the momentum, and test the law ourselves.

Add electronic data capture, and the experiment becomes even more compelling. Instead of spending most of the lesson recording numbers, students can see the motion displayed almost immediately and begin asking the more interesting question:

Where did everything go during the collision?

Why Use an Air Track?

The great enemy of almost every school mechanics experiment is friction.

If I push a trolley across an ordinary bench, it begins slowing almost immediately. The wheels have friction, the bearings have friction and the surface itself may not be perfectly level.

An air track attempts to remove much of that problem.

A series of small holes produces a cushion of air underneath the glider. The glider effectively floats just above the track.

Friction is not literally zero, but it can become small enough for us to investigate motion that is much closer to the idealised situations used in physics calculations.

That makes it particularly useful for studying collisions.

First: What Is Momentum?

Momentum is defined as:

p = mv

where:

p = momentum
m = mass
v = velocity

The unit of momentum is:

kg m/s

The important word here is velocity, rather than simply speed.

Momentum has direction.

If we decide that motion towards the right is positive, then:

0.40 m/s to the right = +0.40 m/s

while:

0.40 m/s to the left = -0.40 m/s

Those signs become extremely important when gliders collide and rebound.

The Conservation of Momentum

For an isolated system:

total momentum before a collision = total momentum after the collision

For two objects:

m1u1 + m2u2 = m1v1 + m2v2

where:

u = velocity before the collision
v = velocity after the collision

The equation looks straightforward.

The experiment behind it is much more interesting.

Instead of simply inserting numbers into the formula, students can create the collision themselves and ask whether the equation actually works.

Experiment 1: The Classic Equal-Mass Collision

Start with two gliders of approximately equal mass.

Place one at rest.

Send the other towards it.

Suppose:

Mass of glider A = 0.25 kg

Velocity of A before collision = +0.80 m/s

Mass of glider B = 0.25 kg

Velocity of B before collision = 0 m/s

The initial momentum is therefore:

p = mv

p = 0.25 x 0.80

p = 0.20 kg m/s

Now allow them to collide using a system designed to give a reasonably elastic collision.

With two equal masses, something very satisfying may happen.

The first glider almost stops.

The second glider moves away at approximately the original velocity of the first.

It appears that the motion has been passed from one glider to the other.

The result will never be absolutely perfect in a real laboratory, but it can come remarkably close.

And that immediately raises a question:

Has the momentum been transferred from one object to another?

Yes — but the total momentum of the complete system has remained approximately constant.

PASCO Data Makes the Experiment Much More Powerful

This is where modern data collection can transform what would otherwise be a fairly traditional mechanics practical.

Velocity sensors, photogates or suitable electronic motion measurements can record what happens immediately before and after the collision.

Instead of simply seeing:

Glider A hits Glider B.

students can examine data showing the change.

Imagine seeing one velocity trace suddenly fall while the other suddenly rises.

The collision may have lasted only a tiny fraction of a second, but the computer has captured what happened.

That changes the conversation.

Rather than asking:

"What number did you measure?"

we can begin asking:

Why did the velocity change so rapidly?
Was momentum really conserved?
How close was the experiment to an ideal collision?
Where might the missing energy have gone?
How much difference did friction make?
What happens if the masses are changed?
What happens if both gliders are moving?

That is where a practical becomes an investigation.

Momentum Is Not the Same as Kinetic Energy

This distinction is one of the most important parts of studying collisions.

For an isolated system, total momentum is conserved.

But kinetic energy does not necessarily remain constant.

Kinetic energy is:

KE = 1/2 mv^2

Notice the squared velocity.

This produces very different behaviour from momentum.

And it allows us to divide collisions into different types.

Elastic Collisions

In an ideal elastic collision:

momentum is conserved

and

kinetic energy is conserved

Perfectly elastic collisions are idealisations, although some physical collisions can get reasonably close.

Air-track gliders with suitable spring or magnetic collision systems can provide a good approximation.

Suppose our 0.25 kg glider is travelling at 0.80 m/s.

Its kinetic energy is:

KE = 1/2 x 0.25 x 0.80^2

KE = 0.08 J

If that energy is effectively transferred to another identical glider, the kinetic energy afterwards should also be close to:

0.08 J

But real experiments rarely produce precisely the theoretical result.

That is not a failure.

That is where the science becomes interesting.

Inelastic Collisions

Now change the collision mechanism.

Instead of allowing the gliders to bounce apart, arrange for them to attach to one another.

Velcro is an obvious simple method.

The two gliders collide and continue moving together.

This is an example of a perfectly inelastic collision.

Momentum is still conserved.

Kinetic energy is not.

A Surprisingly Good Numerical Example

Return to our original experiment.

Glider A:

m = 0.25 kg
u = +0.80 m/s

Glider B:

m = 0.25 kg
u = 0 m/s

Initial momentum:

p = 0.25 x 0.80

p = 0.20 kg m/s

Now suppose they stick together.

Their combined mass becomes:

0.25 + 0.25 = 0.50 kg

Using conservation of momentum:

0.20 = 0.50v

Therefore:

v = 0.40 m/s

The two gliders should move away together at approximately 0.40 m/s.

That prediction can now be tested experimentally.

But What Happened to the Energy?

This is where the experiment becomes much more interesting.

Before the collision:

KE = 1/2 x 0.25 x 0.80^2

KE = 0.08 J

Afterwards:

KE = 1/2 x 0.50 x 0.40^2

KE = 0.04 J

We appear to have "lost":

0.08 - 0.04 = 0.04 J

Half the kinetic energy has disappeared.

Except, of course, it has not actually disappeared.

Energy is still conserved overall.

Some of that kinetic energy has been transferred into other forms.

It may become:

sound;
vibration;
thermal energy;
deformation of the collision surfaces;
internal movement within the gliders.

This is an important distinction:

Momentum can be conserved even when kinetic energy is not conserved.

Students sometimes find that surprising.

Where the Air Track Becomes an Investigation

The basic demonstration is excellent.

But I think the air track becomes much more educational when we stop giving students one collision to perform and instead allow them to change the conditions.

There are dozens of possibilities.

Investigation 1: Change the Mass

Add masses to one glider.

What happens when:

a light glider hits a heavy stationary glider?
a heavy glider hits a light stationary glider?
two unequal gliders move towards one another?

Students can make predictions before releasing anything.

Sometimes the results are counter-intuitive.

A very light object bouncing from a much heavier one behaves quite differently from a heavy object striking something light.

Investigation 2: Make Both Objects Move

Instead of having one stationary object, start both gliders moving.

Try:

both travelling in the same direction;
moving towards one another;
one travelling much faster than the other;
equal masses moving at equal speeds in opposite directions.

That final case is particularly interesting.

Suppose:

Glider A momentum = +0.20 kg m/s

Glider B momentum = -0.20 kg m/s

Total momentum is:

0 kg m/s

There is plenty of movement.

There is plenty of kinetic energy.

Yet the total momentum is zero.

That is a wonderful reminder that zero momentum does not mean nothing is moving.

Investigation 3: Elastic Versus Inelastic

Perform approximately the same collision twice.

First allow the gliders to rebound.

Then arrange for them to stick together.

Measure:

total momentum before;
total momentum after;
total kinetic energy before;
total kinetic energy after.

Students should discover something fundamental.

Momentum behaves similarly in both experiments.

Kinetic energy does not.

That is a much stronger way of learning the distinction than memorising a definition.

Investigation 4: How Elastic Is "Elastic"?

Real collisions lie on a spectrum.

We often describe a collision using the coefficient of restitution.

For a one-dimensional collision:

e = relative speed of separation / relative speed of approach

A perfectly elastic collision has:

e = 1

A perfectly inelastic collision in which the objects stick together has:

e = 0

Real collisions normally lie somewhere between these extremes.

This makes an excellent extension for stronger GCSE students or A-level students.

Rather than simply saying a collision is "elastic", ask:

How elastic is it?

Students can compare different collision materials.

For example:

spring bumpers;
magnets;
rubber;
foam;
Velcro.

Suddenly the experiment has become quantitative.

Try Predicting the Result Before the Collision

One improvement I particularly like is not showing students the experimental answer immediately.

Give them:

the two masses;
their starting velocities;
the collision type.

Then ask them to predict what will happen.

Will one glider stop?

Will both continue forwards?

Will one rebound?

If they stick together, what will their final velocity be?

Only then perform the collision.

Physics becomes much more satisfying when the apparatus appears to answer a question that has already been asked.

The Graph Is Often More Interesting Than the Final Number

With data capture it is tempting to look only at the values immediately before and after the collision.

But the shape of the graph can reveal much more.

Look at the velocity against time trace.

Before impact there should be a fairly steady region.

Then comes the collision.

The velocity changes dramatically.

Afterwards another relatively steady region appears.

That invites questions about the collision itself.

How long did it last?

Was the acceleration enormous?

Could we investigate force?

And that leads directly towards impulse.

From Momentum to Impulse

The change in momentum is called impulse.

Impulse = change in momentum

So:

J = delta p

It is also related to force:

J = F delta t

or, more accurately when force varies:

Impulse = area under a force-time graph

Now our collision experiment connects several important topics:

force → acceleration → impulse → momentum

If suitable force sensors are available, an even more sophisticated experiment becomes possible.

Measure the force during the collision.

Then compare:

area under the force-time graph

with:

change in momentum

Two apparently different measurements should give approximately the same answer.

That is a particularly elegant experiment.

Newton's Third Law Appears Too

Imagine placing force sensors so that the interaction forces between the two objects can be measured.

During the collision, glider A pushes glider B.

But glider B simultaneously pushes glider A.

Newton's Third Law predicts that these forces are equal in magnitude and opposite in direction.

If the data are displayed together, students can see the two force curves.

One positive.

One negative.

Almost mirror images of each other.

So a single air-track collision can connect:

momentum;
conservation laws;
kinetic energy;
impulse;
Newton's laws;
velocity;
acceleration;
experimental uncertainty.

That is remarkable for an event that may last only a fraction of a second.

Why Real Results Never Match Perfectly

One of the great benefits of doing this experimentally is discovering that physics does not produce perfectly neat numbers simply because the equation is correct.

Suppose you calculate:

Momentum before = 0.200 kg m/s

Momentum after = 0.193 kg m/s

Has the conservation of momentum failed?

Almost certainly not.

Instead we need to consider the experiment.

Possible sources of discrepancy include:

residual friction;
the track not being perfectly level;
uncertainty in velocity measurement;
uncertainty in mass;
glider rotation;
air resistance;
vibrations;
imperfect collision alignment;
external forces acting during the measurement period.

The more interesting question therefore becomes:

Is the difference greater than we would reasonably expect from experimental uncertainty?

That is much closer to the way real experimental science works.

Calculate the Percentage Difference

A simple comparison might be:

percentage difference = |p_after - p_before| / |p_before| x 100

If:

p_before = 0.200 kg m/s

and:

p_after = 0.193 kg m/s

then:

percentage difference = 0.007 / 0.200 x 100

percentage difference = 3.5%

Students can then ask whether 3.5% is reasonable for the apparatus being used.

Repeat the experiment several times and the discussion becomes even better.

Do Not Hide the Imperfections

It is tempting in school practical work to want results that reproduce the textbook exactly.

I think that misses something important.

If momentum before is 0.200 kg m/s and momentum after is 0.200 kg m/s every single time, I would actually become suspicious.

Real measurements contain uncertainty.

The educational value lies partly in discovering that the laws of physics emerge through experimental uncertainty rather than because the uncertainty somehow disappears.

Students should become comfortable saying:

"Our measurements support conservation of momentum within the uncertainty of the experiment."

That is a much more scientifically mature conclusion than:

"They were nearly the same, so momentum is conserved."

Take It Further: Can You Identify the Collision?

Give students only the velocity data.

Do not tell them how the collision was arranged.

Ask them to determine whether it was:

approximately elastic;
partially inelastic;
perfectly inelastic.

They must calculate both momentum and kinetic energy.

If momentum remains approximately constant but kinetic energy decreases, the collision is inelastic.

If momentum and kinetic energy are both approximately conserved, it is close to elastic.

If the two objects have the same final velocity, they have probably coupled together.

Now students are analysing evidence rather than following instructions.

A Collision Competition

Another enjoyable version is to turn the experiment into a prediction challenge.

Give students a target.

For example:

Can you arrange a collision so that Glider A stops after the collision?

Or:

Can you make the two gliders move away with equal speeds?

Or:

Can you produce a collision where the total momentum is zero before and after, despite both gliders moving?

Or:

Can you arrange the masses so that the lighter glider rebounds?

Students have to use the theory to design the experiment.

That reverses the usual practical lesson.

Instead of:

apparatus → measurements → equation

we get:

equation → prediction → apparatus → test

That is much closer to genuine scientific thinking.

From Air Tracks to the Real World

The apparatus may look artificial, but the principle certainly is not.

Momentum is enormously important in understanding:

vehicle collisions;
rockets;
recoil;
sports;
ball games;
particle physics;
spacecraft manoeuvres;
explosions;
railway wagons;
collisions between astronomical bodies.

Even catching a ball involves momentum.

A fast-moving ball has momentum. Your hands must change that momentum to zero.

If you move your hands backwards while catching it, you increase the stopping time.

Since:

F = delta p / delta t

increasing the stopping time reduces the average force.

The same idea lies behind:

airbags;
crumple zones;
crash mats;
helmets;
padded surfaces.

The small gliders on an air track are demonstrating physics that matters far beyond the laboratory.

One Apparatus, Many Levels of Physics

That is perhaps why I like the air track so much.

At GCSE level, the experiment can simply demonstrate:

p = mv

and:

total momentum before = total momentum after

At A-level, the same apparatus can investigate:

elastic and inelastic collisions;
kinetic energy changes;
impulse;
force-time graphs;
coefficients of restitution;
experimental uncertainty;
mathematical modelling.

And students who want to go further can begin exploring centre-of-mass frames and more sophisticated collision mechanics.

The apparatus has not changed.

Only the depth of the question has.

Conclusion: Conservation Laws You Can Actually See

Conservation of momentum can easily become another formula students memorise for an examination.

But an air track makes it physical.

Two objects move.

They collide.

Their individual momenta change dramatically.

Yet when we consider the complete system, something remarkable emerges.

The total momentum remains.

Sometimes the gliders bounce apart.

Sometimes they continue together.

Sometimes kinetic energy remains almost unchanged.

Sometimes a substantial fraction is transformed into other forms of energy.

And with electronic data capture, all of this can be turned into measurements and graphs almost immediately.

That is what makes this such a satisfying experiment.

We are not merely telling students that momentum is conserved.

We are giving them the opportunity to try to prove us wrong.

And when repeated collisions, different masses and different collision types continue to reveal the same underlying conservation law, the equation:

m1u1 + m2u2 = m1v1 + m2v2

stops being something written on a formula sheet.

It becomes a description of something they have actually watched happen.

24 August 2026

Flies and Wasps: The Creatures We Love to Hate — But Perhaps Cannot Afford to Lose

If I had to nominate one of the most irritating creatures in the natural world, the fly would probably be fairly high on my list.

There I am trying to work, perhaps writing, teaching or setting up an experiment, and a fly decides that the most interesting place in the entire room is somewhere within a few centimetres of my face.

Buzz.

It disappears.

Buzz.

It comes back.

Then it lands on the computer screen.

For other people, the equivalent creature is the wasp. A peaceful summer lunch in the garden can suddenly become an exercise in plate-covering, arm-waving and keeping a suspicious eye on something yellow and black that appears intensely interested in your drink.

It is very easy to conclude that both creatures are simply pests.

But biologically, that would be a serious mistake.

In fact, flies and wasps provide an excellent A-level Biology lesson because they demonstrate one of ecology's most important principles:

An organism does not need to be attractive, popular or convenient to be ecologically important.

Take away some of the creatures we regard as irritating and the effects could spread through an ecosystem in ways that are surprisingly profound.

Our View of Nature Is Often Very Selective

We tend to divide wildlife into rather human categories.

Butterflies are beautiful.

Bees are useful.

Ladybirds are welcome.

Dragonflies are impressive.

Flies are annoying.

Wasps are dangerous.

But ecosystems do not work according to our aesthetic preferences.

Evolution has not produced organisms because humans find them pleasant. Organisms occupy ecological niches because they have survived and reproduced successfully within complex networks of interactions.

That is a useful starting point for an A-level Biology student.

Rather than asking:

"Do I like this organism?"

biology asks:

"What does this organism do?"

And once we ask that question about flies and wasps, they become considerably more interesting.

First: What Exactly Is a Fly?

When most people say "fly", they are probably imagining the familiar housefly.

But biologically, flies are an enormous group of insects belonging to the order Diptera.

The name is useful.

"Di" means two and "ptera" relates to wings.

Unlike most flying insects, true flies have only one functional pair of wings. Their second pair has evolved into small balancing structures called halteres, which help them control their flight.

That partly explains one of the things that makes flies so irritating.

They are exceptionally manoeuvrable.

Trying to swat one is effectively trying to outmanoeuvre an animal equipped with a sophisticated biological flight-control system refined by millions of years of natural selection.

Suddenly missing it with the newspaper feels slightly less embarrassing.

The Diptera include:

houseflies;
hoverflies;
blowflies;
crane flies;
mosquitoes;
midges;
fruit flies;
horseflies;
numerous other groups.

So saying "flies are useless" is rather like saying "mammals are useless" because you dislike rats.

The group is enormously diverse.

Flies as Pollinators

When we think of pollination, bees receive most of the publicity.

Quite rightly, bees are extremely important pollinators.

But they are not alone.

Many flies visit flowers to feed on nectar and pollen, and as they move from flower to flower they can transfer pollen between plants.

Hoverflies are particularly noticeable examples.

Some are remarkably bee-like in appearance, an example that can lead into another excellent biological topic: mimicry.

A harmless hoverfly with yellow and black markings may receive some protection from predators because it resembles a stinging insect.

But hoverflies are more than convincing impersonators.

They can also be useful pollinators.

That gives us an immediate ecological interaction:

Plant produces nectar -> fly visits plant -> pollen is transferred -> plant reproduction is increased.

The fly receives food.

The plant receives a reproductive advantage.

That is a form of mutualistic interaction.

Pollination Is Really About Plant Sex

This is where an apparently simple discussion about irritating insects suddenly becomes very relevant to A-level Biology.

For sexual reproduction in flowering plants, pollen must ultimately reach the appropriate female reproductive structures.

Animals visiting flowers can act as biological transport systems.

Pollen attaches to their bodies.

When they visit another flower, some may be deposited.

That can result in fertilisation, seed production and eventually the next generation of plants.

Remove enough pollinating insects and reproduction may decline.

And if those plants form part of a food web, the consequences do not necessarily stop with the plant.

Flies Are Also Nature's Recycling Department

Perhaps the most important ecological role of many flies is less attractive.

They eat dead things.

Or, more accurately in many cases, their larvae do.

That may not make for an appealing poster campaign, but decomposition is absolutely fundamental to ecosystems.

Imagine a woodland in which organisms died but nothing decomposed them.

Dead leaves would accumulate.

Dead animals would remain.

Faecal material would build up.

Most importantly, valuable nutrients would remain locked inside this organic matter.

Decomposers and detritivores help return those nutrients to ecological cycles.

The Maggot Has a Job

The word "maggot" is not particularly good public relations.

But maggots are simply the larval stage of certain flies.

For some species, adult flies locate decomposing organic material and lay eggs upon it.

The larvae hatch into an environment rich in food.

They feed rapidly.

Their activity contributes to the breakdown of the material.

Other decomposers, including bacteria and fungi, are also involved.

Eventually nutrients that were locked inside tissues can become available again.

A simplified sequence might be:

Dead organism -> decomposers and detritivores -> mineral ions -> plant uptake -> new biomass

That is nutrient cycling in action.

Without decomposition, ecosystems could not continue functioning indefinitely.

A Dead Animal Is Actually an Ecosystem

This is something I think students sometimes overlook.

Finding a dead animal may simply look unpleasant.

Biologically, however, it is a dramatic transfer of energy and nutrients.

Almost immediately, organisms begin exploiting that resource.

Different insects may arrive at different stages of decomposition.

Microorganisms multiply.

Predators arrive to eat the insects.

Nutrients eventually enter the soil.

Plants may absorb them.

Those plants are eaten by herbivores.

The atoms that once formed one organism can eventually become part of completely different organisms.

This is one of the wonderful ideas in biology.

Matter is continually recycled.

The carbon atom in your body today may once have been part of a tree, a bacterium, another animal or the atmosphere.

Flies are part of that recycling machinery.

Flies Can Even Help Solve Crimes

The predictable colonisation of dead organisms by insects has given rise to forensic entomology.

Certain fly species tend to arrive at carcasses during particular stages of decomposition.

Their eggs and larvae develop at rates influenced by factors such as temperature.

By identifying insect species and estimating their developmental stage, forensic scientists can sometimes obtain information relevant to estimating the period since death.

For an A-level Biology student, this creates links between several topics:

life cycles;
development;
enzymes;
temperature;
ecosystems;
succession;
classification.

Something as seemingly unpleasant as a blowfly larva therefore connects ecology with forensic science.

Then There Are Wasps

Wasps have an even worse public-relations problem.

A bee approaching a flower is usually tolerated.

A wasp approaching a glass of lemonade frequently causes an entirely different reaction.

But once again, "wasp" describes a vast range of organisms.

Only a relatively small proportion are the familiar social wasps that build colonies and interact frequently with people.

There are many thousands of species with extraordinarily diverse lifestyles.

And ecologically they can be extremely important.

Wasps Are Predators

One of the most important things wasps do is eat other animals.

Or feed other animals to their larvae.

Many wasps hunt insects and other invertebrates.

That means they can contribute to regulating populations.

Consider a simple food chain:

Plant -> caterpillar -> wasp

The caterpillar consumes plant tissue.

The wasp consumes the caterpillar.

If predators disappear, prey populations can sometimes increase significantly.

That can produce a trophic cascade, where changes at one level of a food web influence other trophic levels.

So the wasp hovering around the garden may also belong to a group of organisms helping to regulate herbivorous insect populations.

Wasps as Natural Pest Control

Imagine a crop containing large numbers of insect herbivores.

If those insects reproduce rapidly, crop damage may increase.

One solution is pesticide.

But another form of control already exists in nature:

predators and parasitoids.

Wasps are particularly important here.

Some species hunt prey directly.

Others have an even more remarkable reproductive strategy.

The Extraordinary World of Parasitoid Wasps

Parasitoid wasps provide one of biology's more dramatic life histories.

A female may lay an egg inside or on another arthropod.

The wasp larva then develops using the host as a source of nutrients.

Eventually the host dies.

This differs from ordinary parasitism.

A parasite normally benefits from keeping its host alive for some time.

A parasitoid ultimately kills its host.

It sounds like something invented for a science-fiction film.

It is actually happening in gardens, fields and ecosystems around us.

Parasitoids and Biological Control

Parasitoid wasps can have considerable importance in agriculture because many attack insects regarded as crop pests.

Instead of relying entirely on chemical pesticides, growers can sometimes exploit natural enemies.

That leads directly into A-level discussions of biological control.

Suppose a crop pest increases rapidly.

A parasitoid species that attacks that pest may reduce its population.

Potential advantages include reduced pesticide use and greater specificity.

But biological control must also be approached carefully.

Introducing organisms into ecosystems can have unexpected consequences if their interactions with native species are poorly understood.

Once again, a small wasp opens the door to a much bigger ecological discussion.

Wasps Pollinate Too

Although predation is perhaps their most interesting ecological role, many adult wasps also visit flowers.

They require carbohydrates for energy and may feed on nectar.

As they travel between flowers they can carry pollen.

So wasps may function simultaneously as:

predators;
pollinators;
prey;
competitors;
hosts or parasites themselves.

This illustrates something fundamental about ecosystems.

Organisms rarely have only one relationship.

A Food Chain Is Too Simple

GCSE Biology often introduces ecology through food chains.

Grass -> caterpillar -> bird

They are useful models.

But real ecosystems do not contain isolated food chains.

They contain food webs.

Add flies and wasps and the network quickly becomes complicated.

Imagine:

Flowering plant -> hoverfly

Plant -> caterpillar -> wasp

Dead caterpillar -> fly larvae

Fly -> spider

Fly -> bird

Wasp -> bird

Nutrients from decomposition -> plant

Already the neat linear chain has become a network.

And even that is an extreme simplification.

Ecosystems Are Networks, Not Lists

This is one reason biodiversity matters.

Imagine removing one species from an ecosystem.

Another species may consume the same food, so perhaps very little initially changes.

Ecologists sometimes refer to this as functional redundancy.

But remove enough species performing similar ecological roles and the resilience of the system may decline.

It is rather like removing components from a complicated machine.

One missing component may not stop it.

Several missing components might.

The problem is that we often do not know exactly where that threshold lies.

"What Would Happen If All the Flies Disappeared?"

This makes an excellent classroom question.

At first, students may answer:

"Brilliant."

Then start exploring the consequences.

There could be less pollination by fly species.

Decomposition pathways could change.

Animals that eat flies would lose part of their food supply.

Competition between decomposers might change.

Nutrient cycling could be affected.

Plants dependent upon particular pollinators could decline.

Predators dependent upon those insects might then decline.

We should be careful not to claim that ecosystems would instantly collapse because every ecological system is different and other organisms can perform overlapping roles.

But it is equally wrong to imagine that removing an enormous and diverse group of organisms would have no consequences.

And What If the Wasps Disappeared?

Initially, picnics might become considerably more relaxing.

But ecological consequences could include reduced predation on other insects.

Some prey populations could increase.

Plant damage from herbivorous insects might therefore increase.

Certain pollination relationships could disappear.

Animals that eat wasps would lose a food source.

Parasitoid-host relationships would vanish.

Food-web structure would change.

The very creature we regard as a pest may itself be controlling organisms that would otherwise become pests.

The Ecological Importance of Being Eaten

This may sound a little harsh, but another important function of both flies and wasps is simply that other organisms eat them.

Birds eat flies.

Spiders eat flies.

Amphibians eat flies.

Fish eat aquatic fly larvae.

Dragonflies eat flying insects.

Other insects eat insects.

Wasps themselves are eaten by various predators.

An organism does not need to perform some sophisticated ecosystem service to matter.

Sometimes being food is enough.

Energy captured originally by photosynthetic organisms moves through ecosystems via feeding relationships.

Ultimately:

Sunlight -> producer -> consumer -> consumer

The insect buzzing around the window represents stored chemical energy.

To another organism, it is lunch.

The Energy Eventually Came From the Sun

We can take this idea further for A-level Biology.

Plants capture light energy during photosynthesis.

Some of that energy becomes chemical energy stored in organic molecules.

Herbivores consume plants.

Predators consume herbivores.

Detritivores consume dead material.

At each trophic transfer, some energy is lost from the biological system, largely through respiration and heat.

So a wasp catching a caterpillar represents energy moving through trophic levels.

A fly larva feeding upon decomposing material represents another route through the ecosystem.

Ecology becomes much more interesting when we stop thinking of organisms independently and start following energy and matter through the system.

A Useful Practical Investigation: Who Visits the Flowers?

This can make a surprisingly good ecological investigation.

Choose several flowering plant species and observe them for fixed periods.

For example:

10 minutes per plant.

Record visitors as categories such as:

bees;
hoverflies;
other flies;
wasps;
butterflies;
beetles;
other insects.

Repeat observations at different times of day.

You could investigate whether flower colour, shape or location affects visitor frequency.

Students could calculate:

Number of visits per minute = Total visits / Observation time

They could then compare different flowers.

Immediately you have:

sampling;
quantitative data;
behavioural ecology;
pollination;
biodiversity;
experimental controls;
reliability.

And students may discover that flowers are being visited by far more than bees.

Another Investigation: Decomposition

A second area worth exploring is decomposition.

This needs sensible hygiene precautions, and I would avoid experimenting with rotting animal material.

Plant material provides a much safer alternative.

Students could compare decomposition of leaves under different conditions.

Variables might include:

temperature;
moisture;
oxygen availability;
surface area.

One prediction might be:

Increasing temperature increases decomposition rate up to an optimum because decomposer enzymes operate more rapidly.

But at sufficiently high temperatures, enzymes can denature and organisms may die.

That links ecology beautifully with enzyme kinetics.

Temperature Matters to Insects Too

Flies and wasps are ectothermic.

Their body temperature and metabolic activity are strongly influenced by environmental temperature.

That helps explain why insects often become much more obvious during warm weather.

It also raises interesting questions about climate change.

If average temperatures change, then:

geographical distributions may change;
breeding seasons may change;
emergence dates may shift;
interactions between predators and prey may change;
interactions between pollinators and flowering plants may become mismatched.

Ecology is full of timing.

If two interacting species respond differently to environmental change, their relationship may be disrupted.

Population Control Is Never Quite as Simple as It Looks

Consider a hypothetical insect pest.

Its population increases.

Predatory wasps now have more food.

Their population may subsequently increase.

That could reduce the prey population.

As prey becomes scarce, wasp numbers may then decline.

This creates the possibility of population cycles.

A simplified model might be:

More prey -> more predators -> fewer prey -> fewer predators -> prey recovers

Real ecosystems are vastly more complicated, but this provides an introduction to predator-prey dynamics.

Students encountering population curves should remember that behind every line on the graph are real organisms interacting with one another.

Natural Selection Has Produced Some Extraordinary Adaptations

Both flies and wasps provide superb examples of adaptation.

Flies may possess:

compound eyes;
rapid visual processing;
highly manoeuvrable flight;
specialised mouthparts;
remarkable reproductive rates.

Wasps may possess:

venom;
ovipositors;
powerful sensory systems;
specialised hunting behaviour;
complex social organisation;
precise host recognition in parasitoid species.

Each characteristic exists within an evolutionary context.

Variation occurs.

Selection acts.

Individuals possessing advantageous characteristics may leave more offspring.

Allele frequencies can therefore change between generations.

The irritating insect buzzing around the room is also the product of an enormous evolutionary history.

Why Does a Fly Seem So Good at Avoiding Me?

There is a nice biological question hidden here too.

A fly does not experience its environment exactly as we do.

Its visual system is particularly effective at detecting movement.

When a large object moves towards it, sensory information triggers rapid escape behaviour.

Natural selection strongly favours this ability.

A fly that responds slowly to approaching predators is unlikely to enjoy a particularly successful reproductive career.

Generation after generation, successful escape responses are selected.

So each time I unsuccessfully attempt to persuade a fly to leave my workspace, I am essentially encountering the consequences of natural selection.

Why Do Wasps Become So Noticeable in Late Summer?

The familiar social wasps we encounter around food have seasonal colony cycles.

Earlier in the season, workers are heavily involved in obtaining resources for the developing colony.

Later, colony organisation changes and workers may become much more noticeable around sugary foods.

From our point of view, it can seem as though wasps have suddenly decided to ruin August.

From their perspective, they are simply responding to biological needs and available resources.

Behaviour that seems irrational or aggressive to us may make considerably more sense once examined through ecology and evolution.

This Raises an Interesting Question About "Pests"

What exactly is a pest?

Biologically, there is no taxonomic category called "pest".

It is largely a human label.

An organism becomes a pest when its activities conflict with ours.

A fly decomposing animal waste somewhere in the countryside may be performing an ecological service.

The same species walking across our food becomes a hygiene concern.

A wasp hunting caterpillars among plants may be welcome.

The same wasp entering a bedroom may not be.

Context changes our judgement.

The organism has not changed.

That Does Not Mean We Should Ignore the Risks

Ecological importance does not mean every interaction with an organism is desirable.

Some flies can transmit pathogens.

Some fly species bite.

Mosquitoes, themselves members of Diptera, are vectors for major human diseases.

Wasps can sting, and for someone with a severe allergy that can become medically serious.

Understanding an organism's ecological importance does not require pretending that it can never cause problems.

Biology is more interesting than that.

An organism can be simultaneously:

ecologically valuable and inconvenient to humans.

A-Level Biology Is Full of These Apparent Contradictions

Predators kill other animals but may stabilise ecosystems.

Parasites harm hosts but influence population dynamics.

Bacteria cause disease but also drive nutrient cycles.

Fungi spoil food but make decomposition possible.

Wasps sting us but kill crop pests.

Flies contaminate food but recycle nutrients and pollinate plants.

Nature rarely divides itself neatly into "good" and "bad".

Those are human categories.

Ecology is about relationships and consequences.

A Challenge for Students: Build the Fly-and-Wasp Food Web

Start with these organisms:

flowering plant;
caterpillar;
aphid;
hoverfly;
predatory wasp;
parasitoid wasp;
spider;
bird;
dead organic material;
bacteria;
fungi.

Now connect them.

Who eats whom?

Who pollinates whom?

What decomposes what?

What competes with what?

Which organisms could occupy several roles?

Then remove one organism.

What happens?

Remove another.

What happens now?

This is much closer to real ecological thinking than memorising a single food chain.

It Also Changes the Way We Think About Conservation

Conservation naturally attracts attention towards charismatic organisms.

Tigers.

Elephants.

Whales.

Pandas.

They are important and compelling.

But ecosystems also depend on an immense number of much smaller organisms that rarely appear on conservation posters.

Insects are part of the biological infrastructure of the planet.

Pollination, decomposition, predation and food-web interactions often depend on them.

Saving biodiversity therefore cannot simply mean protecting the species we happen to like.

Perhaps We Need to Protect the Irritating Ones Too

This is probably the part I find most interesting.

I do not particularly want a fly circling my head while I am trying to work.

Nor do I especially want a wasp crawling into my drink.

But I can simultaneously recognise that both belong to functioning ecosystems.

Perhaps that is one of the marks of understanding biology.

We stop asking whether nature is convenient for us and start asking how it works.

And sometimes the answer is:

It works partly because of the organisms we wish would go away.

A Different Way of Looking at the Next Fly

So the next time that familiar buzzing starts beside my computer, I doubt I shall suddenly welcome the visitor enthusiastically.

I will probably still encourage it towards the open window.

But perhaps with slightly more respect.

Because somewhere beyond my annoyance is an animal belonging to one of the most successful groups of organisms on Earth.

Its relatives pollinate flowers.

Others recycle dead material.

Others feed birds, spiders, amphibians and fish.

Others have transformed our understanding of genetics.

Wasps deserve a similar reassessment.

They are predators, parasitoids, pollinators and prey, forming part of complex ecological networks that regulate populations and transfer energy through ecosystems.

They may be irritating.

They may occasionally be dangerous.

But useless?

Definitely not.

Conclusion: Ecology Does Not Care Whether We Like Something

Perhaps one of the most important lessons we can teach through A-level Biology is that ecosystems are not collections of species living independently.

They are networks.

Remove one connection and perhaps very little happens.

Remove hundreds or thousands and eventually the network becomes weaker.

That is why biodiversity matters.

Not simply because having lots of different species is interesting, but because those organisms perform countless interacting ecological roles.

So perhaps we should reserve the word "useless" rather carefully.

The fly buzzing around my desk may still irritate me.

The wasp arriving at the picnic may still encourage everyone to move their drinks.

But both remind us of an important biological truth:

Nature is full of organisms whose importance becomes obvious only when we stop looking at them from a purely human point of view.

And the creature we would most happily remove from the ecosystem might turn out to be one of the creatures quietly helping to keep it functioning.