Individuals die, populations disappear, and species become extinct. That is one view of the world.
But another view of the world concentrates not so much on presence or absence as upon the numbers of organism and the degree of constancy of their numbers.
These are two very different ways of viewing the behavior of system and the usefulness of the view depends very much on the properties of the system concerned. If we are examining a particular device designed by the engineer to perform specific tasks under a rather narrow range of predictable external conditions, we are likely to be more concerned with consistent nonvariable performance in which slight departures from the performance goal are immediately counteracted.