No species lives alone. Every population is tangled up with the other species around it — eating them, being eaten, competing with them, or quietly doing deals with them. There are six named types to know, and one quick trick that sorts them all.
📘 What you need to know
A community is multiple populations of different species living and interacting in the same area.
Inter means between, so interspecific relationships are between different species.
The six types: herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity.
In mutualism both species benefit. It is the only one of the six where nothing gets harmed.
Three mutualisms to name: root nodules in legumes, mycorrhizae in orchids, and zooxanthellae in hard corals.
You get credit for Latin names or common names, so do not lose sleep over spelling.
The six interactions
Interaction
What happens
Examples
Herbivory
An organism feeds on a plant
Cattle grazing grass; sea turtles feeding on sea grass; honeybees taking nectar and pollen
Predation
An organism catches and eats an animal, or eats one that recently died
Lions hunting zebra; dolphins catching fish; red kites feeding on roadkill
Interspecific competition
Different species compete for the same resource
Oak and beech competing for light; lions and hyenas competing for prey; red and grey squirrels competing for food
Mutualism
Different species work together and both gain
Pistol shrimp and goby fish sharing a burrow; oxpecker birds cleaning parasites off large mammals
Parasitism
A parasite lives on or in a host and harms it
Mistletoe taking water and nutrients from trees; fleas feeding on mammal blood
Pathogenicity
A microorganism lives inside a host and causes disease
Mycobacterium tuberculosis causing TB; the fungal pathogen behind Dutch elm disease
The quickest way to tell them apart
Forget memorising six definitions. Ask two questions instead: does species A gain or lose? and does species B gain or lose? Almost every interaction sorts itself out from there.
Parasitism and pathogenicity share a pattern. The difference is that a pathogen is a microorganism that causes disease, while a parasite simply feeds off its host.
Competition is the odd one out and worth pausing on. Nothing is eaten and nothing is infected, yet both species end up worse off, because each one uses resources the other needed.
Mutualism in detail
Three examples come up again and again. In each one, ask what each partner supplies and what it receives — that is the answer the mark scheme wants.
1. Root nodules in legumes
The legume family (Fabaceae) includes peas, beans and clover. Their roots carry small, round nodules containing bacteria such as Rhizobium.
The bacteria carry out nitrogen fixation: they convert nitrogen gas from the air into ammonia (NH3), which can then be converted into nitrates. Plants cannot use nitrogen gas at all, but they can use nitrate, and they need it to build proteins and nucleic acids.
In exchange the bacteria receive carbohydrates made by the plant in photosynthesis. Both sides get something they could not have produced alone.
This is why clover is grown in crop rotations. The bacteria in its nodules leave the soil richer in nitrogen for the next crop.
2. Mycorrhizae in orchids
Many plants form partnerships with fungi. The fungus grows long, thin threads called hyphae that weave among the plant’s roots. Because hyphae are so fine and so numerous, they hugely increase the surface area available for absorption, so the plant takes up more water and mineral ions. In return the fungus receives organic compounds such as glucose from the plant. These partnerships are called mycorrhizae.
The orchid family (Orchidaceae) takes this further. Orchid seeds are tiny and often rely on a fungal partner for the nutrients they need to germinate. A few unusual orchids cannot photosynthesise at all, and depend on their fungus to break down dead matter in the soil and supply everything — making the orchid a heterotroph rather than an autotroph. In that particular arrangement the fungus gets nothing back until the orchid dies and is decomposed.
3. Zooxanthellae in hard corals
Coral reefs are built by tiny animals called coral polyps, relatives of jellyfish and sea anemones. Their tentacles carry stinging cells (nematocysts) — and also house single-celled algae called zooxanthellae.
The trade works like this: the polyp’s body gives the algae shelter and protection, and the algae photosynthesise and pass carbon compounds such as carbohydrates to the polyp. The polyp uses that energy to secrete calcium carbonate, which forms the hard skeleton the whole reef is built from.
Why this one matters beyond the exam. When water gets too warm the polyps expel their zooxanthellae. Without their algal partners the corals lose both their colour and their main energy supply — this is coral bleaching, and it is mutualism breaking down in real time.
WORKED EXAMPLE
Naming interactions
Name the type of interspecific relationship in each case: (a) a tapeworm living in the gut of a dog, (b) oxpecker birds removing ticks from a buffalo, (c) red and grey squirrels feeding on the same nuts, (d) a fungus causing leaf loss and death in elm trees.
(a) tapeworm and dogparasitismthe worm gains, the dog is harmed, and the worm lives in the host(b) oxpecker and buffalomutualismthe bird gets food, the buffalo loses its parasites — both gain(c) red and grey squirrelsinterspecific competitiondifferent species, same resource, both worse off(d) fungus and elmpathogenicitya microorganism inside a host causing disease
WORKED EXAMPLE
Explaining a mutualism
Explain how both partners benefit from the relationship between a clover plant and the Rhizobium bacteria in its root nodules. [4]
What the bacteria dothey fix nitrogen gas from the air into ammonia, which becomes nitrateWhy the plant needs thatthe plant cannot use nitrogen gas, but it can use nitrate to build proteins and nucleic acidsWhat the plant suppliescarbohydrates made in photosynthesisWhy the bacteria need thatthe bacteria use those carbohydrates as an energy source, so both partners gaina 4-mark mutualism question is nearly always two benefits, each with a reason
💡 Exam tip
For mutualism, state the benefit to each partner separately. Two named benefits, two marks.
Use the plus and minus test to sort any unfamiliar example you are handed in a data question.
Say nitrogen fixation by name when writing about root nodules.
For mycorrhizae, the phrase examiners look for is increased surface area for absorption.
Distinguish parasite from pathogen: the pathogen is a microorganism causing disease.
Common names are fine. Write “the legume family” if Fabaceae will not come to you.
⚠ Common mix-up
Mutualism is not cooperation. Mutualism is between species; cooperation is within one.
Parasites do not usually kill the host quickly. A dead host is a dead home.
Herbivory is not predation. Feeding on plants has its own name.
The plant does not fix nitrogen. The bacteria do. The plant just gets the product.
Zooxanthellae are algae, not the coral. The coral is an animal; the algae are its lodgers.
Both species lose in competition. Do not write that the stronger one benefits.
Up next: Interspecific Competition — niches, competitive exclusion, and how you actually test whether two species are competing.
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