Permaculture promotes the use of biological resources as a replacement for fossil-fuel based systems. Long term, biological resources can be a long term investment, enabling sustainability for the land.
An example of biological resources - "Animal Tractor - chickens and pigs are well-known for scratching and digging up the ground in search of worms, insects and roots." Mollison and Slay.
Using fossil-fueled resources may be ok for the short term, as long as the long term agenda is to create a sustainable system dependent on biological resources.