Species Interactions and Ecological Niches

Five interactions, classified by who gains and who loses

Do not memorise definitions — memorise the sign pair.

• Competition $(-/-)$ — both species are worse off. Interspecific competition is between species; intraspecific is within one species and is usually the stronger of the two.
• Predation, including herbivory and parasitoidism $(+/-)$ — one gains, the other typically dies.
• Parasitism $(+/-)$ — the host is harmed but usually kept alive, because a dead host is a dead habitat. Ticks, tapeworms, mistletoe.
• Mutualism $(+/+)$ — pollinators and flowers, Rhizobium and legumes, mycorrhizae and roots.
• Commensalism $(+/0)$ — barnacles on a whale, epiphytes on a branch. The clean $0$ is rare in nature, which is why the exam usually signals it with the phrase "no measurable effect".

Niche and the competitive exclusion principle

A niche is the full set of conditions and resources a species uses — not merely where it lives (that is its habitat). The fundamental niche is what a species could occupy with no competitors; the realised niche is what it actually occupies. The realised niche is always a subset of the fundamental one.

The competitive exclusion principle says two species cannot occupy identical niches indefinitely in the same place: one is eliminated. The common outcome instead is resource partitioning, where the species diverge in time, space, or food size — which is why they coexist.