This is an example of multiple things. Natural selection is one. When two species feed in the same environment and one survives and the other doesn't, this is natural selection. It is also an example of competition. When two animals compete for a resource, or even if they both just use the resource, this is competition. The goats also must not need as much food to survive, or they must also have a readily available substitute, because otherwise both populations would've decreased, but it wouldn't be likely that one disappeared completely. This is also an example of overgrazing.