Niche: all ways a species utilizes the resources of its environment
Resources can be space, food, temperature/salinity range, mating conditions, other physiological requirements, predation refuge, etc.
Fundamental niche: entire niche a species is able to use
Realized niche: the actually utilized niche, which may be smaller than the fundamental niche due to
- competition for resources
- control by predators/grazers
- absence of other species (e.g. insects for pollination)
Competitive exclusion: best adapted species will outcompete less adapted species to extinction when niches overlap