F. Population Dynamics · Population Dynamics

Symbiosis: Mutualism, Commensalism and Parasitism

F3.1 F2.1 A1.2 A1.4About 70 minutes

Learning goals

I can define symbiosis and classify relationships as mutualism, commensalism or parasitism using + / − / 0 notation.
I can distinguish obligate from facultative mutualism, ectoparasites from endoparasites, and parasites from parasitoids.
I can explain how parasites and pathogens affect host populations, using Canadian examples.
I can explain why some relationships are hard to classify and can change with conditions.

Words to know

symbiosis
A close, usually long-term association between individuals of two different species.

mutualism
A symbiosis in which both species benefit (+/+).

obligate mutualism
A mutualism in which at least one partner cannot survive without the other (for example the fungus and alga of many lichens).

facultative mutualism
A mutualism that benefits both partners but is not essential to either.

commensalism
A symbiosis in which one species benefits and the other is neither helped nor harmed (+/0).

parasitism
A symbiosis in which the parasite benefits by living on or in the host and harming it, usually without killing it quickly (+/−).

ectoparasite / endoparasite
A parasite that lives on the outside of its host (ticks, lice) / inside the host (tapeworms, roundworms).

parasitoid
An insect whose larvae develop inside or on a host, eventually killing it (many small wasps and some flies).

intermediate host / definitive host
A host in which a parasite develops but does not reproduce sexually / the host in which it becomes adult and reproduces sexually.

brood parasitism
Laying eggs in another species' nest so that the host raises the young (for example the brown-headed cowbird).

Living together

Symbiosis means "living together": a close, usually long-term association between two species. The curriculum asks you to explain three kinds:

Mutualism (+/+): both benefit.
Commensalism (+/0): one benefits, the other is unaffected.
Parasitism (+/−): the parasite benefits, the host is harmed.

You already know the other main interactions: competition (−/−) and predation (+/−). The chart below puts them all together.

Every two-species interaction can be described by its effect on each species. The cells are symmetric; for example, predation is +/− whichever species you call A.

Mutualism

Lichens: a fungus partnered with green algae or cyanobacteria. The photosynthetic partner makes sugars; the fungus provides shelter, water and minerals. Together they grow on bare rock, tree bark and Arctic tundra, where neither could live alone. Many lichens are obligate mutualisms. (Caribou rely heavily on ground lichens in winter.)
Mycorrhizae: fungi that grow on or into plant roots. The fungal threads act like extra root hairs, bringing water and phosphorus to the plant; the plant feeds the fungus sugars. Most land plants form mycorrhizae, including the trees of Canada's boreal forest.
Legumes and Rhizobium: bacteria in root nodules of clover, peas, beans and soybeans fix nitrogen from the air into a form the plant can use; the plant supplies sugars. (You met this in Grade 9.)
Pollination: bees, flies, moths and hummingbirds get nectar or pollen; flowers get their pollen carried to other plants.
Ants and aphids: some ants "farm" aphids, protecting them from predators in exchange for sweet honeydew.
Your gut microbiome: trillions of bacteria in your large intestine help digest fibre and make some vitamins, such as vitamin K, while living in a warm, food-rich home.

Mutualisms can be obligate (essential to at least one partner) or facultative (helpful but not essential: a bee can visit many flower species).

Commensalism

In commensalism, one species benefits and the other is (as far as we can tell) unaffected:

Epiphytes: mosses and some lichens growing on tree trunks get a place in the light; the tree is usually unaffected.
Hitchhiking seeds: burdock burrs cling to a passing animal's fur and get carried to new places (this is called phoresy).
Nesting: birds nesting in the branches of a tree.

Commensalism is the hardest relationship to prove, because a tiny cost or benefit to the second species is hard to measure. Heavy loads of epiphytes might shade leaves or break branches; birds nesting in a tree might fertilize it with droppings. Many supposed examples turn out, on closer study, to be slightly mutualistic or slightly parasitic.

Parasitism

A parasite lives on or in a host, takes nutrients from it and harms it, but usually does not kill it quickly (a dead host is a lost home). Pathogens (disease-causing bacteria, viruses, protists and fungi) are microscopic parasites.

Ectoparasites live on the outside: ticks, fleas, lice, mites, and leeches. The blacklegged tick carries the bacterium that causes Lyme disease, and its range has been spreading north in Ontario as winters warm.
Endoparasites live inside: tapeworms, roundworms, flukes.
Sea lampreys attach to lake trout and other fish with a sucking disc and feed on their blood and body fluids. After invading the upper Great Lakes through shipping canals, they helped collapse lake trout populations in the mid-1900s. Canada and the United States now control them through the Great Lakes Fishery Commission, using barriers and lampricides in spawning streams.
Parasitoids blur the line with predators: a small wasp lays eggs inside a caterpillar; the larvae eat it from the inside and eventually kill it. Many are used as biological control agents against pests.
Brood parasites: the brown-headed cowbird lays its eggs in other birds' nests (such as yellow warblers); the hosts raise the cowbird chicks, often at the expense of their own.
Plant parasites: dwarf mistletoe on spruce and pine, and dodder, a leafless vine that taps into other plants.

Hosts and life cycles. Many parasites need more than one host. The definitive host is where the parasite becomes adult and reproduces sexually; an intermediate host carries a developing stage.

The meningeal worm (brainworm) cycles between white-tailed deer (definitive host) and snails or slugs (intermediate hosts). Deer usually tolerate it, but when moose swallow infected snails, the worm can damage their nervous system.

Worked example

Let's reason about a parasite and two host populations

The meningeal worm (Parelaphostrongylus tenuis) normally lives in white-tailed deer, which usually show few symptoms. Moose that swallow infected snails can develop serious nerve damage, sometimes fatal. Using the life cycle, explain why moose populations in parts of Ontario where deer are abundant may be lower than where deer are scarce.

Source of infection: only deer spread larvae in their droppings; moose are a dead-end host that does not usually pass the worm on.
More deer → more larvae → more infected snails on the vegetation that moose also eat.
So the risk to moose rises with deer density, not moose density: for moose, the worm is a limiting factor driven by another species.
Result: in areas with many deer, more moose are infected and die, which helps explain why moose are often scarce where deer are abundant (though habitat and other factors also play a role).

Deer are the reservoir host. Where deer are dense, infected snails are common, so more moose become infected; the parasite indirectly links the two host populations. This is called apparent competition: deer harm moose through a shared parasite.

Parasites and population size

Parasites and pathogens are usually density-dependent limiting factors: they spread more easily when hosts are crowded and stressed. In some years, moose in parts of eastern Canada and the northeastern United States carry tens of thousands of winter ticks each; heavily infested moose rub off much of their hair trying to relieve the itching and can lose so much blood that many calves die. Tick numbers tend to rise after mild autumns and when moose are dense.

Because hosts evolve defences (immune systems, grooming) and parasites evolve ways around them, host-parasite relationships are a continuing coevolutionary arms race. Very harmful parasites can drive host populations down, which then reduces the parasite's own success. You will model a host-parasite pair in Investigation: Modelling Predator-Prey and Host-Parasite Populations.

Classify by evidence, not by appearance

Ask: does each species gain or lose survival or reproduction because of the other? A relationship can shift with conditions: mycorrhizal fungi help plants in poor soil, but in very rich soil they may take more sugar than they give back, becoming slightly parasitic.

Just for fun

A lichen is a fungus that has taken up farming. The alga does the cooking (photosynthesis), the fungus builds the house, and together they can move onto bare rock. Biologists like to say a lichen is a fungus that discovered agriculture.

Hands-on activity

Find symbioses in your neighbourhood

You need: a notebook, a hand lens or a phone camera's zoom, and (for Part B) an adult with a small knife and a cutting board.

Part A: lichen and moss survey (any season). On a walk, examine the bark of 10 trees. For each tree, note the species if you know it, whether there are lichens (crusty, leafy or shrubby growths, often grey-green) or mosses, and which side of the trunk they grow on. Classify each as mutualism (inside the lichen itself) or commensalism (lichen or moss on the tree).

Part B: goldenrod galls (late autumn or winter). In an old field, look for round swellings on the stems of dead goldenrod plants. These ball galls are made by the plant in response to a goldenrod gall fly larva. With permission (not in a protected park), collect 5 galls. An adult cuts each gall in half on the cutting board. Inside you may find: a living fly larva (the parasite of the goldenrod); a smaller larva or pupa of a parasitoid wasp that ate the fly larva; or an empty tunnel with a large hole, where a downy woodpecker or chickadee pecked in to eat the larva (predation).

What to look for: lichens are often more common on older, rougher bark and in cleaner air (many lichens are sensitive to air pollution). In the galls you will see three interactions in one tiny system: plant-parasite, parasite-parasitoid and predator-prey. Safety: an adult handles the knife; wash hands after handling plants and galls; stay on paths and do not collect lichens from protected areas.

Your turn

Classify each relationship and give its symbols: (a) bees pollinating apple blossoms, (b) a tapeworm in a wolf's intestine, (c) burdock burrs carried in a dog's fur, (d) Rhizobium in soybean roots.

What is the difference between obligate and facultative mutualism?

Is a tick an ectoparasite or an endoparasite?

How does a parasitoid differ from a typical parasite?

Why is commensalism hard to prove?

In the meningeal worm life cycle, which animal is the definitive host and which are intermediate hosts?

Explain why parasites are usually density-dependent limiting factors.

More practice: warm-up

What does symbiosis mean?

Give the symbols for mutualism.

Give an example of commensalism.

Which bird is a brood parasite?

What two organisms make up a lichen?

More practice: on your own

Explain why a parasite that kills its host very quickly may be less successful than one that does not.

Explain how sea lampreys could reduce lake trout populations even though each lamprey does not always kill its host.

Give an example of a relationship that can shift between mutualism and parasitism, and explain what causes the shift.

Why might a warmer climate increase Lyme disease risk in Ontario?

A field has many goldenrod ball galls. A biologist finds that 30% of larvae were killed by parasitoid wasps and 20% eaten by birds. What percentage survived to become adult flies, and what does this suggest about the fly's limiting factors?

Challenge

Moose winter tick loads are higher after mild autumns and when moose density is high. (a) Is the tick a density-dependent or density-independent factor for moose? (b) Explain how climate (density-independent) and density (density-dependent) interact here. (c) Predict the effect on moose calves of several mild autumns in a row.

Some biologists argue that the human gut microbiome shows mutualism, commensalism and parasitism all at once. Explain.

The meningeal worm makes deer and moose interact even though they eat somewhat different foods. Explain the term apparent competition and predict what would happen to moose in a region where deer numbers increase because of mild winters and more young forest.

Check your understanding

Quiz

A relationship in which one species benefits and the other is unaffected is:

Nitrogen-fixing bacteria in the root nodules of peas are an example of:

A wasp larva that develops inside a caterpillar and eventually kills it is a:

In the meningeal worm life cycle, snails are:

Which is an ectoparasite?

Why are lichens able to grow on bare rock?

For parents and teachers

Big idea (F3.1): symbiosis is a close association of two species: mutualism (+/+), commensalism (+/0) or parasitism (+/−). Parasites and pathogens are usually density-dependent limiting factors, and some, like the meningeal worm, link the populations of different host species.

How to help: the lichen survey works on any walk. In late autumn or winter, goldenrod galls are easy to find in old fields; an adult must cut them open. Ask your teen to give one Canadian example of each kind of symbiosis.

Safety: when walking in fields or woods in Ontario, wear long pants, check for ticks afterward, and learn to recognize poison ivy and giant hogweed (avoid touching).

Common difficulties:

Calling any close relationship "symbiosis = mutualism": in this course symbiosis includes parasitism and commensalism.
Confusing intermediate and definitive hosts.

Sources: Ontario Ministry of Natural Resources and Public Health Ontario information on meningeal worm, winter ticks and blacklegged ticks; Great Lakes Fishery Commission on sea lamprey control; standard ecology texts.