Answer: Resources are always limited.
Exponential growth can be defined as the increase in the number of members in a population, at a constantly growing rate. In nature population may grow exponentially for some period but ultimately decline due to limitation in resource availability. Resources are available to population according to the carrying capacity of the region or ecosystem. Carrying capacity can be defined as the maximum number of individuals that an ecosystem can sustain on the basis of availability of resources like food, shelter and others.
The natural populations cannot keep growing exponentially because the increase in the size of the population may exceed the carrying capacity of the region or ecosystem. This has resulted due to the limitation of resources.