In order to save a rare and endangered species, conservation biologists must know the critical size of that species. (Use only one word per blank.)

To answer your question effectively, it seems you are looking for a single word related to the concept of critical size in the context of conservation biology. Without additional context or specific blanks to fill, one possible word that fits this scenario is "population."

Explanation:

In conservation biology, knowing the critical "population" size of a rare and endangered species is crucial for ensuring its survival. The population size determines genetic diversity, reproductive success, and resilience against environmental changes. A small population size can lead to inbreeding, making the species susceptible to extinction. Understanding the minimum viable population (MVP) is essential for conservation programs aimed at protecting endangered species, as it helps in formulating strategies for habitat protection and restoration efforts.

If you have specific blanks or additional questions, feel free to provide more context!