Two of the main ways that populations interact with one another is by competition and
by predation. In competition, individuals seek to obtain the same environmental resource.
In predation, one population is the resource of the other.
One can conceptualize competition as occurring horizonatally on the same resource
level, while predation takes place vertically between different resource levels.
Competition and predation comprise environmental pressures on the interacting populations. The
coexistence of populations under competition and predation indicates that these populations have
accomodated themselves to each other's presence and have evolved ways to survive in spite
of the pressures. In other words, they have coevolved.
In this section, we will see how competition and predation both hinder and benefit populations
and look at evidence for coevolution.