"Don't put all your eggs in one basket"
Diversity is the very essence and joy of life. Maintaining and enhancing the diversity of existing eco-systems is essential for so many reasons. Preserving and protecting what little wilderness we have left is one of most important tasks. But in permaculture we don't stop there.
Wherever possible our designs always incorporate a 'zone 5' area which is left for nature to do as it will. We also try to make use of nature friendly techniques and technologies to minimise harm.
Permaculture designs should always try to incorporate a wide variety of plants, animals and approaches. This is not just for the sake of it, but because diversity can act like an insurance policy - if one crop fails, another may succeed. Even within an orchard there will be a diversity of different varieties. Take apples as an example. A healthy diverse orchard will contain early flowering, late flowering, eaters, and cookers. If an early frost gets some, others will be popping out flowers later on.
Polycultures (agriculturral systems with many plants), are now proven to be more productive overall and resilient to weather, pests and other factors, than monocultures (agricultural systems with only one plant species.) Another good reason to understand and use diversity...
Plant diversity is also key to many useful techniques such as 'integrated pest management'.
Human diversity is also key to creativity and a vibrant, healthy human society.