What does carrying capacity refer to in relation to populations?

Carrying capacity refers to the maximum number of individuals of a particular species that an environment can support sustainably without degradation of that environment. This concept is crucial in understanding population dynamics and ecological balance. When a population exceeds its carrying capacity, it can lead to resource depletion, habitat destruction, and ultimately, population decline or collapse due to the inability of the environment to provide adequate resources such as food, water, and shelter.

In contrast, the other choices focus on different aspects of populations and ecosystems. The capacity of a city to support humans is a different concept concerned with urban planning and infrastructure rather than ecological balance. The minimum number of species required for biodiversity is about ensuring the diversity of life rather than the sustainability of a single population. Lastly, the average lifespan of a species in an ecosystem relates to individual species' life histories but does not address the collective limits of a population within its environment.