Competition within and between species
Competition happens when two living things need the same limited thing, such as food, water, light, space or a mate. Both are harmed because each gets less than it would alone (−, −).
- Within one species (intraspecific): two tigers fighting for one territory, or tree seedlings of the same kind crowding for light.
- Between different species (interspecific): lions and hyenas competing for the same prey, or two plant species taking the same nutrients from the soil.
In the end one competitor may win, or both may change what they eat so that they compete less.
Parasitism, predation and herbivory
In all three, one organism gains and the other loses (+, −).
- Predation: a predator kills and eats another animal, the prey. Example: a hawk and a mouse. The predator and prey numbers rise and fall together over time.
- Parasitism: a parasite lives on or in a host and takes food from it, and usually does not kill it quickly. Examples: ticks and lice (outside the host), tapeworm and malaria germs (inside), Cuscuta (dodder, a plant parasite).
- Herbivory: an animal eats parts of a plant. Example: goat eating leaves, caterpillar on a cabbage. The plant is harmed but often survives and regrows.
Plants can fight back with thorns, bitter taste or poison.
Mutualism and commensalism
Some relationships do not harm anyone.
- Mutualism (+, +): both partners gain. Examples: bees and flowers (food for the bee, pollination for the plant), root nodule bacteria in peas and beans (the plant gets nitrogen, the bacteria get sugar), and lichen (a fungus and an alga living together).
- Commensalism (+, 0): one partner gains and the other is not affected. Examples: egrets following cattle to eat insects, a remora fish riding on a shark, an orchid growing on a tree branch.
A relationship can be hard to classify, because the effect may change with time. Always say what you observed: who gains, who loses and who is not affected.
Key formulas and definitions
- Competition: (−, −)
- Predation, parasitism, herbivory: (+, −)
- Mutualism: (+, +)
- Commensalism: (+, 0)
Worked examples
1. Two kinds of birds feed on the same fruit tree and the fruit runs out. Name the interaction and signs.
Competition between species. Both get less food: (−, −).
2. A frog catches and eats a fly. Name the interaction and signs.
Predation. The frog (predator) gains, the fly (prey) loses: (+, −).
3. A leech sucks blood from a buffalo and the buffalo gets weak. Which interaction is this?
Parasitism: the leech gains and the buffalo (host) is harmed (+, −).
4. Why is a cow eating grass herbivory and not predation?
In herbivory the eaten organism is a plant, which is often not killed and may regrow. In predation an animal is killed for food.
5. Bacteria in the root nodules of a bean plant fix nitrogen, and the plant gives them sugar. Name the interaction.
Mutualism. Both gain (+, +).
6. A small bird rests on a rhino's back and the rhino is unaffected. Name the interaction.
Commensalism: the bird gains and the rhino is unaffected (+, 0). If the bird also removed ticks, it would become mutualism.
Common mistakes
- Thinking competition happens only between different species. It happens inside one species too.
- Mixing parasitism and predation. A predator kills and eats its prey; a parasite usually feeds on a living host over time.
- Calling every helpful relationship mutualism. In commensalism only one partner gains.
- Forgetting that herbivory also counts as a +/− relationship, because the plant is harmed.