Got this definition from Investopedia.
"DEFINITION of 'Natural Capital'
A reference to the stock of natural resources, such as water and oil. Unlike other forms of equity (such as machines and buildings), which can be created on a regular basis, many natural resources are nonrenewable. Natural capital includes many resources that humans and other animals depend on to live and function, which leads to a dilemma between depleting and preserving those resources."
Simply put, Natural Resources are synonymous to Natural Capital.
The quote is saying that the assumption in modern economics fail to take into account the negative effect that their industry has on the environment.