• Weak sustainability:
   
  Manufactured capital of equal value can take the place of natural capital
   
  
• Strong sustainability:
   
  The existing stock of natural capital must be maintained and enhanced because the functions it performs cannot be duplicated by manufactured capital

• Natural resources provide material and services
   
  
• Weak sustainability means we can replace or duplicate natural materials and services with manufactured goods and services
   
  
• Strong sustainability means that natural materials and services cannot be duplicated

There are two different levels of sustainability: weak and strong. Weak sustainability is the idea that natural capital can be used up as long as it is converted into manufactured capital of equal value.

The problem with weak sustainability is that, while we can assign a monetary value to manufactured goods and capital, it can be very difficult to assign a monetary value to natural materials and services. How much is a forest full of trees worth? A value can be calculated if you assume that all the trees are cut down and turned into furniture or paper. However, the forest provides a home for wildlife that provides food for hunters. It also provides a place for hikers to enjoy the natural environment.

Weak sustainability does not take into account the fact that some natural material and services can not be replaced by manufactured goods and services. (Other questions to ask participants are: What is the dollar value of the ozone layer? A wetland? An ocean fishery? An aquifer? A river full of salmon?)

Strong sustainability is the idea that there are certain functions that the environment performs that cannot be duplicated by humans. The ozone layer is one example of an ecosystem service that is difficult for humans to duplicate.