Food webs show who eats whom. But species meet in other ways too. Ecologists name each relationship by asking a simple question: does each species gain (+), lose (−) or stay about the same (0)? It builds on Food Chains and Food Webs.
| Relationship | Effect | Example |
|---|---|---|
| Competition | − / − | Lions and hyenas competing for the same zebras; two kinds of plants competing for light |
| Predation | + / − | An owl catches and eats a mouse |
| Mutualism | + / + | Bees get nectar; the flowers get pollinated |
| Commensalism | + / 0 | Barnacles ride on a whale and reach food; the whale is barely affected |
| Parasitism | + / − | A tick feeds on a deer's blood for days; the deer is harmed but not killed right away |
Key idea
Symbiosis means two species living closely together for a long time. It comes in three kinds: mutualism (both benefit), commensalism (one benefits, the other is unaffected) and parasitism (one benefits, the host is harmed).
Why it works
Predation and parasitism both have the pattern + / −, so the difference is in time and size. A predator kills and eats its prey, usually quickly. A parasite is usually much smaller than its host and lives on or in it for a long time, feeding without killing it right away. A dead host would end the parasite's food supply.
Worked example: Name it
A flea lives in a dog's fur and feeds on its blood for weeks. A wolf kills and eats a rabbit. Which one is predation?
- Score each pair: flea + / dog −; wolf + / rabbit −. Both are + / −.
- The wolf kills and eats the rabbit in one meal: predation.
- The flea feeds on a living host over a long time without killing it: parasitism.
Answer: The wolf and the rabbit. The flea is a parasite.
Common mistake
Students call every close relationship helpful, or call predation parasitism. Score both sides with +, − or 0 first. If one side is harmed, it is not mutualism. If the harmed one is killed and eaten, it is predation; if it is fed on over time, it is parasitism.