Holmgren's Proverb for this Principle: Don't put all your eggs in one basket.
Variety is the spice of life! It is also the key to sustainability. Nowhere in nature do you see just one thing, one plant, one environment. Our Permaculture Design should be no different.
Our plants should be diverse - this allows more variety in our diets which is healthy, more redundancy in our harvest which is prudent, less monoculture which is great for pest confusion, healthier soil, and creates habitats and microclimates which in turn lead to increased diversity of insects, birds, mammals, fungi, and all life in general. One way to utilize diversity in plants as a design element is with the concept of Permaculture Guilds.
Our plant species themselves should be diverse - besides diversity in flavors, different varieties also provide diversity in pest and disease resistance. Early season, mid season, and late season varieties also provides much needed and desired redundancy. It helps ensure a crop regardless of late frosts, droughts, pests, or any other unforeseen calamity that strikes at the perfect, or worst possible, time.
Our systems should be diverse - again this also allows for greater redundancy, which again provides more stability in our total system. Water harvesting is one of the most important systems that need to have diversification.
Finally, this diversity can also apply to many things non-agricultural: investing, skills, education, transportation, communication, communities, etc.
"Don't put all your eggs in one basket" reminds us that when we rely on only one method or system, if that system fails, then we have guaranteed failure instead of success and sustainability.
Diversity reduces vulnerability to a variety of threats and takes advantage of the unique nature of the environment in which it resides.
- David Holmgren