In all of my teaching, one of the most important ecological concepts I believe that I shared with students is the concept that a population of organisms lives within a fairly specific range of environmental conditions.
The illustration below is a page from my wildlife textbook Wa-Maka-Skan. It is a simplified depiction that shows the general theoretical concept for one environmental factor. The bottom portion shows that for various fish species, one of the important environmental factors affecting their distribution and survival is water temperature.
This concept is relevant today, especially as human-caused global change alters the living conditions of species. Environments are shifted out from under the adaptations of the species. The faster the change and the larger the animal, the more problematic the situation becomes.
Range of Tolerance from Wa-Maka-Skan. © Fred Montague