Critical Minimums and Extinction

By Dave Hanks

Maintaining the diversity of life-forms on the earth is extremely important! The loss of habitat (favorable living space) is the major reason a species becomes extinct. Also, over hunting or poaching can be especially devastating on large animals like elephants, rhinos, etc. The introduction, by man, of a new predator to an area can be very harmful because the prey species haven’t had the chance to adapt to the new enemy. Predators that have naturalized in an area are not a problem. They even help their prey species maintain their numbers at optimum levels by weeding out old or diseased, unproductive members, and thus reduce the stress on the food supplies. I know this sounds unfeeling, but this is how the natural world works.

In existing habitats it is necessary to maintain a species’ CRITICAL MINIMUM. Nothing lives forever, so reproduction is necessary to keep a population viable. Rodents have a low critical minimum, which means that only a few surviving individuals can easily repopulate an area because they can reproduce fast, often, and with big litters. Large, slow reproducers have a high critical minimum. Disease, predation, and old age will always reduce the numbers in a population; and if not enough young are born each year to off-set those losses, extinction is assured even if a few individuals still exist.

Pictured is a NORTHERN FOX SQUIRREL whose low critical minimum keeps this species abundant – especially in our yard!

Also pictured is a mother GRIZZLY and her cub at an Alaskan water hole (Ursines at a snow-melt pond in June). She matures late sexually – has a cub or twins only about every 4 or 5 years which means only about 5 or 6 sets in her lifetime, and not all make it to adulthood. Her species minimum is therefore high.