Final answer:
Age structure is not an axis used by biologists to determine the rarity of a species; instead, local population size, habitat specificity, and geographic distribution are the key criteria used. The correct option is 3) Age structure.
Step-by-step explanation:
The axis that is not used by biologists to determine whether, or to what extent, a species is rare is age structure. Biologists use various criteria to assess the rarity and vulnerability of species, including local population size, habitat specificity, and geographic distribution. Age structure, while important for understanding the population dynamics of a species, is not typically used to determine the rarity of a species.
Ecologists who specialize in biogeography are particularly focused on patterns of species distribution (species distribution pattern) and how abiotic and biotic factors influence these patterns. Factors like range size, habitat specificity, and whether a species is endemic or a generalist, play vital roles in determining the distribution and abundance of species. Conservation biogeography additionally looks at how to protect and restore species based on historical and ecological data.