What is defined as the entire range of resource opportunities an organism can potentially occupy?

The concept you're looking for is the fundamental niche, which represents the total range of environmental conditions and resources that an organism can utilize if there are no limiting factors, such as competition or predation. It encompasses all the potential interactions an organism might have with abiotic (non-living) and biotic (living) factors in its environment.

Understanding the fundamental niche is essential in ecology because it helps scientists predict how organisms may respond to changes in their environment, such as habitat loss or climate change. It serves as a theoretical framework for determining the ecological possibilities for a species, demonstrating the maximum potential of their adaptations and behaviors.

In contrast, the realized niche describes the actual conditions and resources that an organism uses, which can be smaller than the fundamental niche due to ecological interactions. A habitat refers more specifically to the physical environment where an organism lives, while the ecological niche combines aspects of both the habitat and the role an organism plays in its ecosystem. Thus, the fundamental niche is critical for understanding an organism's ecological potential.