Deer, for example, can jump over or wriggle underneath small barriers. The height of your fence or barrier depends mostly on what animals you're trying to keep out.
Deer, rabbits, woodchucks, and raccoons are all common garden pests that can be deterred with a fence. Garden fencing and barriers are the only sure-fire way to guarantee wild and domestic animals won't tamper with your garden. Why should I invest in garden fencing and barriers?