Resources need to be limited for competition to occur.

When resources are limited the competitively superior species will eventually eliminate the other species, but evidence for such competitive exclusion occurring in nature is not always conclusive. 

Connell’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone.

Competition might evolve mechanisms that promote co-existence rather than exclusion. One such mechanism is ‘resource partitioning.