Final answer:
The relationship necessary for a variety of species to exist on the planet is environmental sustainability, which includes ecosystem stability and sustainable resource management.
Step-by-step explanation:
The relationship that must exist between the spheres for a variety of species to exist on the planet is environmental sustainability. This concept addresses the capability of the environment to support diverse forms of life while preserving the ecosystems' integrity for future generations. Factors such as sustainable development and the balance between renewable and non-renewable resources are crucial in maintaining biodiversity and the stability of ecosystems.
Human ecology theory tackles the interaction between humans and their environments, which includes the impact of human activities on the diversity of species within ecosystems. The interconnectedness of biological systems suggests that community ecology and ecosystem interactions are key determinants in the variety and stability of species present. This is evident when evaluating the economic and political relationships between settlements and the environment, a process that requires consideration of sustainability in both resource use and development practices.